Mott-Hubbard transition in V2O3 revisited - Hansmann, Philipp; Toschi, Alessandro; Sangiovanni, Giorgio; Saha-Dasgupta, T.; Lupi, S.; Marsi, Marino; Held, Karsten (physica status solidi (b), 2013)
The isostructural metal-insulator transition in Cr-doped V2O3 is the textbook example of a Mott-Hubbard transition between a paramagnetic metal (PM) and a paramagnetic insulator. We review recent theoretical calculations as well as experimental findings which shed new light on this famous transition. In particular, the old paradigm of a doping-pressure equivalence does not hold, and there is a microscale phase separation for Cr-doped V2O3. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
CRIS-Team WoS Importer:2020-02-19

Mott Insulating States with Competing Orders in the Triangular Lattice Hubbard Model - Wietek, Alexander; Rossi, Riccardo; Simkovic, Fedor; Klett, Marcel; Hansmann, Philipp; Ferrero, Michel; Stoudenmire, E. Miles; Schaefer, Thomas; Georges, Antoine (Physical Review X, 2021)
The physics of the triangular lattice Hubbard model exhibits a rich phenomenology, ranging from a metal-insulator transition, intriguing thermodynamic behavior, and a putative spin liquid phase at intermediate coupling, ultimately becoming a magnetic insulator at strong coupling. In this multimethod study, we combine a finite-temperature tensor network method, minimally entangled thermal typical states (METTS), with two Green-function-based methods, connected-determinant diagrammatic Monte Carlo and cellular dynamical mean-field theory, to establish several aspects of this model. We elucidate the evolution from the metallic to the insulating regime from the complementary perspectives brought by these different methods. We compute the full thermodynamics of the model on a width-four cylinder using METTS in the intermediate to strong coupling regime. We find that the insulating state hosts a large entropy at intermediate temperatures, which increases with the strength of the coupling. Correspondingly, and consistently with a thermodynamic Maxwell relation, the double occupancy has a minimum as a function of temperature which is the manifestation of the Pomeranchuk effect of increased localization upon heating. The intermediate coupling regime is found to exhibit both pronounced chiral as well as stripy antiferromagnetic spin correlations. We propose a scenario in which time-reversal symmetry-broken states compete with stripy-spin states at lowest temperatures.
CRIS-Team WoS Importer:2021-10-29

Mott insulators with boundary zeros - Wagner, N.; Crippa, L.; Amaricci, A.; Hansmann, Philipp; Klett, M.; König, E. J.; Schäfer, T.; Sante, D. Di; Cano, J.; Millis, A. J.; Georges, A.; Sangiovanni, G. (Nature Communications, 2023)
The topological classification of electronic band structures is based on symmetry properties of Bloch eigenstates of single-particle Hamiltonians. In parallel, topological field theory has opened the doors to the formulation and characterization of non-trivial phases of matter driven by strong electron-electron interaction. Even though important examples of topological Mott insulators have been constructed, the relevance of the underlying non-interacting band topology to the physics of the Mott phase has remained unexplored. Here, we show that the momentum structure of the Green’s function zeros defining the “Luttinger surface" provides a topological characterization of the Mott phase related, in the simplest description, to the one of the single-particle electronic dispersion. Considerations on the zeros lead to the prediction of new phenomena: a topological Mott insulator with an inverted gap for the bulk zeros must possess gapless zeros at the boundary, which behave as a form of “topological antimatter” annihilating conventional edge states. Placing band and Mott topological insulators in contact produces distinctive observable signatures at the interface, revealing the otherwise spectroscopically elusive Green’s function zeros.
CRIS-Team Scopus Importer:2023-12-15

Mott physics in the half-filled Hubbard model on a family of vortex-full square lattices - Ixert, D.; Assaad, F. F.; Schmidt, Kai Phillip (Physical Review B, 2014)
We study the half-filled Hubbard model on a one-parameter family of vortex-full square lattices ranging from the isotropic case to weakly coupled Hubbard dimers. The ground-state phase diagram consists of four phases: A semimetal and a band insulator which are connected to the weak-coupling limit, and a magnetically ordered Neel phase and a valence bond solid (VBS) which are linked to the strong-coupling Mott limit. The phase diagram is obtained by quantum Monte Carlo (QMC) and continuous unitary transformations (CUTs). The CUT is performed in a two-step process: Nonperturbative graph-based CUTs are used in the Mott insulating phase to integrate out charge fluctuations. The resulting effective spin model is tackled by perturbative CUTs about the isolated dimer limit yielding the breakdown of the VBS by triplon condensation. We find three scenarios when varying the interaction for a fixed anisotropy of hopping amplitudes: (i) one direct phase transition from Neel to semimetal, (ii) two phase transitions VBS to Neel and Neel to semimetal, or (iii) a smooth crossover from VBS to the band insulator. Our results are consistent with the absence of spin-liquid phases in the whole phase diagram.

Mukhanov-Sasaki equation in manifestly gauge-invariant linearized cosmological perturbation theory with dust reference fields - Giesel, Kristina; Herold, Laura; Li, Bao-Fei; Singh, Parampreet (Physical Review D - Particles, Fields, Gravitation and Cosmology, 2020)
The aim of this article is to understand the role of dust reference fields, often also called clocks, on cosmological perturbations around a classical spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) universe. We derive the Mukhanov-Sasaki equation for the Brown-Kuchař and Gaussian dust models, which both consider four dust fields as reference fields. The reduced phase space of Dirac observables, that is the gauge-invariant part of the theory, is constructed by means of an observable map applied to all elementary phase space variables of the coupled system, consisting of gravity, a massive scalar field and the dust degrees of freedom and automatically yields the set of independent physical variables. The evolution of these observables is governed by a so called physical Hamiltonian which can be derived once the set of reference fields are chosen and differs for each model. First, the reduced phase space for full general relativity as well as the corresponding equations of motion are derived for full general relativity. Then from this, the gauge-invariant versions of the equations of motion for the background are derived which contain a fingerprint of the dust reference fields. Afterwards we study linear cosmological perturbations around a FLRW metric using the scalar-vector-tensor decomposition and derive the equation of motion for the Mukhanov-Sasaki variable in this formalism for a chosen set of variables on the reduced phase space and expressed in terms of Dirac observables. The Mukhanov-Sasaki equation involves additional contributions that can be understood as back reactions from the dust reference fields. These additional dust contributions to the Mukhanov-Sasaki equation were absent if the dust energy and momentum density as well as their perturbations are vanishing. The nature of the correction terms suggests that Brown-Kuchař and Gaussian dust reference fields contribute differently. We numerically study the behavior of the dust contributions to the Mukhanov-Sasaki equation during inflation.<br
Keywords: Cosmological perturbation theory; Relational Formalism; Dirac Observables

Multiband nonequilibrium GW+EDMFT formalism for correlated insulators - Golez, Denis; Eckstein, Martin; Werner, Philipp (Physical Review B, 2019)
We study the dynamics of charge-transfer insulators after a photoexcitation using the three-band Emery model which is relevant for the description of cuprate superconductors. We provide a detailed derivation of the nonequilibrium extension of the multiband GW+EDMFT formalism and the corresponding downfolding procedure. The Peierls construction of the electron-light coupling is generalized to the multiband case resulting in a gauge invariant combination of the Peierls intraband acceleration and dipolar interband transitions. We use the formalism to study momentum-dependent (inverse) photoemission spectra and optical conductivities. The time-resolved spectral function shows a strong renormalization of the charge-transfer gap and a substantial broadening of some of the bands. While the upper Hubbard band exhibits a momentum-dependent broadening, an almost rigid band shift is observed for the ligand bands. The inverse photoemission spectrum reveals that the inclusion of nonlocal and interband charge fluctuations results in a very fast relaxation of the holes to the top of the Zhang-Rice singlet band. Consistent with the changes in the spectral function, the optical conductivity shows a renormalization of the charge-transfer gap, which is proportional to the photodoping. The details of the photoinduced changes strongly depend on the dipolar matrix elements, which calls for an ab initio determination of these parameters.
CRIS-Team Scopus Importer:2019-12-27

Multilayer Multiconfiguration Time-Dependent Hartree Study of Vibrationally Coupled Electron Transport Using the Scattering-State Representation - Wang, Haobin; Thoss, Michael (Journal of Physical Chemistry A, 2013)
The multilayer multiconfiguration time-dependent Hartree method is employed to study vibrationally coupled electron transport in models of single molecule junctions. To increase the efficiency of the simulation method, a representation of the Hamiltonian in terms of the scattering states of the underlying electronic Hamiltonian is used. It is found that with an appropriate choice of the scattering states the artificial electron correlation present in the original representation of the model is greatly reduced. This allows efficient simulation of the steady-state currents in a wide physical parameter space, which is demonstrated by several numerical examples. © 2013 American Chemical Society.
UnivIS-Import:2015-03-09:Pub.2013.nat.dphy.ITP.ppem.multil

Multi-particle excitations and spectral densities in quantum spin-systems - Schmidt, Kai Phillip; et al. (Physica B-Condensed Matter, 2002)
The excitation spectrum of the 2-leg S = 1/2 Heisenberg ladder is examined perturbatively. Using an optimally chosen continuous unitary transformation, we expand the Hamiltonian and the Raman operator about the limit of isolated rungs leading to high order series expansions allowing to calculate spectral densities quantitatively. The 2-particle sector is examined for total momentum k = 0. We show that triplet-triplet interaction gives rise to a band splitting. (C) 2002 Elsevier Science B.V. All rights reserved.
Keywords: quantum spin-systems; multi-particle excitations; spectral densities

Multiphonon quantum dynamics in cavity optomechanical systems - Macovei, Mihai; Palffy-Buß, Adriana (Physical Review A, 2022)
The multiphonon quantum dynamics in laser-pumped cavity optomechanical samples containing a vibrating mirror is investigated. Especially, we focus on dispersive interaction regimes where the externally applied coherent field frequency detuning from the optical resonator frequency is not equal to the mirror's oscillating frequency or to its multiples. As a result, for moderately strong couplings among the involved subsystems, the quantum dynamics of this complex system is described by multiphonon absorption or emission processes, respectively. Particularly, we demonstrate efficient ways to monitor the phonon quantum dynamics via photon detection. The possibility to extract the relevant sample parameters, for instance, the coupling strength between the mechanical mirror and the electromagnetic field, is also discussed.
CRIS-Team Scopus Importer:2022-03-25

Multiple reentrant glass transitions of soft spheres at high densities: Monotonicity of the curves of constant relaxation time in jamming phase diagrams depending on temperature over pressure and pressure - Schmiedeberg, Michael (Physical Review E, 2013)
By using molecular-dynamics simulations, we determine the jamming phase diagrams at high densities for a bidisperse mixture of soft spheres that interact according to repulsive power-law pair potentials. We observe that the relaxation time varies nonmonotonically as a function of density at constant temperature. Therefore, the jamming phase diagrams contain multiple reentrant glass transitions if temperature and density are used as control parameters. However, if we consider a new formulation of the jamming phase diagrams where temperature over pressure and pressure are employed as control parameters, no nonmonotonic behavior is observed.

Multiscale approaches to protein-mediated interactions between membranes-relating microscopic and macroscopic dynamics in radially growing adhesions - Bihr, Timo; Seifert, Udo; Smith, Ana-Suncana (New Journal of Physics, 2015)
Macromolecular complexation leading to coupling of two or more cellular membranes is a crucial step in a number of biological functions of the cell. While other mechanisms may also play a role, adhesion always involves the fluctuations of deformable membranes, the diffusion of proteins and the molecular binding and unbinding. Because these stochastic processes couple over a multitude of time and length scales, theoretical modeling of membrane adhesion has been a major challenge. Here we present an effective Monte Carlo scheme within which the effects of the membrane are integrated into local rates for molecular recognition. The latter step in the Monte Carlo approach enables us to simulate the nucleation and growth of adhesion domains within a system of the size of a cell for tens of seconds without loss of accuracy, as shown by comparison to 10(6) times more expensive Langevin simulations. To perform this validation, the Langevin approach was augmented to simulate diffusion of proteins explicitly, together with reaction kinetics and membrane dynamics. We use the Monte Carlo scheme to gain deeper insight to the experimentally observed radial growth of micron sized adhesion domains, and connect the effective rate with which the domain is growing to the underlying microscopic events. We thus demonstrate that our technique yields detailed information about protein transport and complexation in membranes, which is a fundamental step toward understanding even more complex membrane interactions in the cellular context.
Keywords: theory; modeling and simulations; protein-ligand interactions; membranes

Mutually attracting spin waves in the square-lattice quantum Antiferromagnet - Powalski, Michael; Schmidt, Kai Phillip; Uhrig, Goetz S. (SciPost Physics, 2018)
Spin waves (magnons) in two dimensions are the potential glue in high-temperature superconductors so that their quantitative understanding is mandatory. Yet even for the fundamental case of the undoped Heisenberg model on the square lattice a consistent picture is still lacking. Significant spectral continua are taken as evidence of the existence of fractional excitations (spinons), but descriptions in terms of spinons fail to show the established absence of an energy gap. Here a fully consistent picture of the dynamics in the square-lattice quantum antiferromagnet is provided which agrees with the experimental findings. The key step is to capture (i) the strong attractive interaction between the spin waves and (ii) the vertex corrections of the observables.©
CRIS-Team Scopus Importer:2023-03-27

“Mythology on the Move: Narrative Archetypes in Framing and Worldview.” - Sinding, Michael (2015)
Keywords: Northrop Frye; framing; narrative; archetype; worldview

N=1 Supergravity with loop quantum gravity methods and quantization of the SUSY constraint - Eder, Konstantin; Sahlmann, Hanno (Physical Review D, 2021)
In this paper, the classical and quantum theory of N=1 supergravity in four spacetime dimensions will be studied in the framework of loop quantum gravity. We discuss the canonical analysis of the supergravity Holst action as first introduced by Tsuda. In this way, we also derive a compact expression of the supersymmetry constraint, which plays a crucial role in canonical supergravity theories, akin to the role of the Hamiltonian constraint in nonsupersymmetric generally covariant theories. The resulting theory is then quantized using loop quantum gravity methods. In particular, we propose and discuss a quantization of the supersymmetry constraint and derive explicit expressions of the action of the resulting operator. This is important as it is the first step on the way to analyzing the Dirac algebra generated by supersymmetry and Hamiltonian constraint in the quantum theory and for finding physical states. We also discuss some qualitative properties of such solutions of the SUSY constraint.
CRIS-Team Scopus Importer:2021-03-05

Na clusters on metal supported Ar layers - Faber, Bernd; Dinh, P. M.; Reinhard, Paul-Gerhard; Suraud, E. (European Physical Journal D, 2012)
We investigate from a theoretical perspective structure and dynamics of Na clusters on a surface built from Ar layers grown on a metal support. The system is modeled by a hierarchical quantummechanical/ molecular-mechanical (QM/MM) approach treating the cluster electrons with time-dependent density-functional theory, the Ar atoms classically, and the metal support as a continuous dielectric medium. Caution has been taken to describe properly the dynamical polarizability of the Ar substrate. We study the effect of the Ar substrate and particularly of the metal support on the cluster structure and dynamics. The binding of Na6 and Na8 to the Ar surface is found to by very weak and the effect of the dielectric response of the metal (DRM) turns out to be negligible. The global properties of the optical response of the Na clusters are slightly changed by the Ar substrate and the DRM while the detailed spectral fragmentation depends sensitively on any change of the environment. The deposition dynamics of small Na clusters is crucially influenced by the mechanical hardness of the metal support while the DRM makes little effect. We also study the dependence on the number of Ar layers. For the first few layers (from two to four), the deposition dynamics changes dramatically with the number of layers. The results stabilize from six layers on upwards. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2012.
CRIS-Team Scopus Importer:2023-07-24

Nanometric thermal fluctuations of weakly confined biomembranes measured with microsecond time-resolution - Monzel, Cornelia; Schmidt, Daniel; Seifert, Udo; Smith, Ana-Suncana; Merkel, Rudolf; Sengupta, Kheya (Soft Matter, 2016)
We probe the bending fluctuations of bio-membranes using highly deflated giant unilamellar vesicles (GUVs) bound to a substrate by a weak potential arising from generic interactions. The substrate is either homogeneous, with GUVs bound only by the weak potential, or is chemically functionalized with a micro-pattern of very strong specific binders. In both cases, the weakly adhered membrane is seen to be confined at a well-defined distance above the surface while it continues to fluctuate strongly. We quantify the fluctuations of the weakly confined membrane at the substrate proximal surface as well as of the free membrane at the distal surface of the same GUV. This strategy enables us to probe in detail the damping of fluctuations in the presence of the substrate, and to independently measure the membrane tension and the strength of the generic interaction potential. Measurements were done using two complementary techniques - dynamic optical displacement spectroscopy (DODS, resolution: 20 nm, 10 ms), and dual wavelength reflection interference contrast microscopy (DW-RICM, resolution: 4 nm, 50 ms). After accounting for the spatio-temporal resolution of the techniques, an excellent agreement between the two measurements was obtained. For both weakly confined systems we explore in detail the link between fluctuations on the one hand and membrane tension and the interaction potential on the other hand.
Keywords: FLUORESCENCE CORRELATION SPECTROSCOPY; INTERFERENCE CONTRAST MICROSCOPY; RED-BLOOD-CELLS; FLUID MEMBRANES; LIPID-BILAYERS; BENDING STIFFNESS; MECHANICAL FLUCTUATIONS; PHOSPHOLIPID-VESICLES; UNILAMELLAR VESICLES; GIANT VESICLES

Natural-orbital impurity solver and projection approach for Green's functions - Lu, Y.; Cao, X.; Hansmann, Philipp; Haverkort, M. W. (Physical Review B, 2019)
We extend a previously proposed rotation and truncation scheme to optimize quantum Anderson impurity calculations with exact diagonalization [Y. Lu, M. Höppner, O. Gunnarsson, and M. W. Haverkort, Phys. Rev. B 90, 085102 (2014)PRBMDO1098-012110.1103/PhysRevB.90.085102] to density-matrix renormalization group (DMRG) calculations. The method reduces the solution of a full impurity problem with virtually unlimited bath sites to that of a small subsystem based on a natural impurity orbital basis set. The later is solved by DMRG in combination with a restricted-active-space truncation scheme. The method allows one to compute Green's functions directly on the real frequency or time axis. We critically test the convergence of the truncation scheme using a one-band Hubbard model solved in the dynamical mean-field theory. The projection is exact in the limit of both infinitely large and small Coulomb interactions. For all parameter ranges, the accuracy of the projected solution converges exponentially to the exact solution with increasing subsystem size.
CRIS-Team Scopus Importer:2020-02-19

NESSi: The Non-Equilibrium Systems Simulation package - Schüler, Michael; Golež, Denis; Murakami, Yuta; Bittner, Nikolaj; Herrmann, Andreas; Strand, Hugo U.R.; Werner, Philipp; Eckstein, Martin (Computer Physics Communications, 2020)
The nonequilibrium dynamics of correlated many-particle systems is of interest in connection with pump–probe experiments on molecular systems and solids, as well as theoretical investigations of transport properties and relaxation processes. Nonequilibrium Green's functions are a powerful tool to study interaction effects in quantum many-particle systems out of equilibrium, and to extract physically relevant information for the interpretation of experiments. We present the open-source software package NESSi (The Non-Equilibrium Systems Simulation package) which allows to perform many-body dynamics simulations based on Green's functions on the L-shaped Kadanoff–Baym contour. NESSi contains the library libcntr which implements tools for basic operations on these nonequilibrium Green's functions, for constructing Feynman diagrams, and for the solution of integral and integro-differential equations involving contour Green's functions. The library employs a discretization of the Kadanoff–Baym contour into time N points and a high-order implementation of integration routines. The total integrated error scales up to O(N−7), which is important since the numerical effort increases at least cubically with the simulation time. A distributed-memory parallelization over reciprocal space allows large-scale simulations of lattice systems. We provide a collection of example programs ranging from dynamics in simple two-level systems to problems relevant in contemporary condensed matter physics, including Hubbard clusters and Hubbard or Holstein lattice models. The libcntr library is the basis of a follow-up software package for nonequilibrium dynamical mean-field theory calculations based on strong-coupling perturbative impurity solvers. Program summary: Program Title: NESSi CPC Library link to program files: http://dx.doi.org/10.17632/973crf9hgd.1 Licensing provisions: MPL v2.0 Programming language: C++, python External routines/libraries: cmake, eigen3, hdf5 (optional), mpi (optional), omp (optional) Nature of problem: Solves equations of motion of time-dependent Green's functions on the Kadanoff–Baym contour. Solution method: Higher-order solution methods of integral and integro-differential equations on the Kadanoff–Baym contour.
CRIS-Team Scopus Importer:2020-07-31
Keywords: Kadanoff–Baym equations; Keldysh formalism; Nonequilibrium dynamics of quantum many-body problems; Numerical simulations

Networklike Propagation of Cell-Level Stress in Sheared Random Foams - Evans, Myfanwy; Kraynik, Andrew M.; Reinelt, Douglas A.; Mecke, Klaus; Schröder-Turk, Gerd (Physical Review Letters, 2013)
Quasistatic simple shearing flow of random monodisperse soap froth is investigated by analyzing SURFACE EVOLVER simulations of spatially periodic foams. Elastic-plastic behavior is caused by irreversible topological rearrangements (T1s) that occur when Plateau's laws are violated; the first T1 determines the elastic limit and frequent T1 avalanches sustain the yield-stress plateau at large strains. The stress and shape anisotropy of individual cells is quantified by Q, a scalar derived from an interface tensor that gauges the cell's contribution to the global stress. During each T1 avalanche, the connected set of cells with decreasing Q, called the stress release domain, is networklike and nonlocal. Geometrically, the networklike nature of the stress release domains is corroborated through morphological analysis using the Euler characteristic. The stress release domain is distinctly different from the set of cells that change topology during a T1 avalanche. Our results highlight the connection between the unique rheological behavior of foams and the complex large-scale cooperative rearrangements of foam cells that accompany distinctly local topological transitions.

Neural-Network Approach to Dissipative Quantum Many-Body Dynamics - Hartmann, Michael; Carleo, Giuseppe (Physical Review Letters, 2019)
In experimentally realistic situations, quantum systems are never perfectly isolated and the coupling to their environment needs to be taken into account. Often, the effect of the environment can be well approximated by a Markovian master equation. However, solving this master equation for quantum many-body systems becomes exceedingly hard due to the high dimension of the Hilbert space. Here we present an approach to the effective simulation of the dynamics of open quantum many-body systems based on machine-learning techniques. We represent the mixed many-body quantum states with neural networks in the form of restricted Boltzmann machines and derive a variational Monte Carlo algorithm for their time evolution and stationary states. We document the accuracy of the approach with numerical examples for a dissipative spin lattice system.
CRIS-Team Scopus Importer:2019-07-16

Neutron scattering evidence for isolated spin-1/2 ladders in (C5D12N)(2)CuBr4 - Savici, A. T.; Granroth, G. E.; Broholm, C. L.; Pajerowski, D. M.; Brown, C. M.; Talham, D. R.; Meisel, M. W.; Schmidt, Kai Phillip; Uhrig, G. S.; Nagler, S. E. (Physical Review B, 2009)
Inelastic neutron scattering was used to determine the spin Hamiltonian for the singlet ground-state system of fully deuterated BPCB, (C5D12N)(2)CuBr4. A two-leg spin-1/2 ladder model, with J(perpendicular to) = (1.09 +/- 0.01) meV and J(parallel to) = (0.296 +/- 0.005) meV, accurately describes the data. The experimental limit on the effective interladder exchange constant is vertical bar J(int)(eff)vertical bar less than or similar to 0.006 meV, and the limit on total diagonal, intraladder exchange is vertical bar J(F)+J(F')vertical bar <= 0.1 meV. Including the effects of copper to bromide covalent spin transfer on the magnetic form factor, the experimental ratios of intraladder bond energies are consistent with the predictions of continuous unitary transformation.

Neutron scattering from a coordination polymer quantum paramagnet - Schmidt, Kai Phillip; et al. (Physical Review B, 2006)
Inelastic neutron scattering measurements are reported for a powder sample of the spin-1/2 quantum paramagnet Cu(quinoxaline)Br-2. Magnetic neutron scattering is identified above an energy gap of 1.9 meV. Analysis of the sharp spectral maximum at the onset indicates that the material is magnetically quasi-one-dimensional. Consideration of the wave vector dependence of the scattering and polymeric structure further identifies the material as a two-legged spin-1/2 ladder. Detailed comparisons of the data to various models of magnetism in this material based on the single-mode approximation and the continuous unitary transformation are presented. The latter theory provides an excellent account of the data with leg exchange J(parallel to)=2.0 meV and rung exchange J(perpendicular to)=3.3 meV.

New insights in quantum geometry - Sahlmann, Hanno (Journal of Physics : Conference Series, 2012)
Quantum geometry, i.e., the quantum theory of intrinsic and extrinsic spatial geometry, is a cornerstone of loop quantum gravity. Recently, there have been many new ideas in this field, and I will review some of them. In particular, after a brief description of the main structures and results of quantum geometry, I review a new description of the quantized geometry in terms of polyhedra, new results on the volume operator, and a way to incorporate a classical background metric into the quantum description. Finally I describe a new type of exponentiated flux operator, and its application to Chern-Simons theory and black holes. © Published under licence by IOP Publishing Ltd.

Newton's constant from a minimal length: Additional models - Sahlmann, Hanno (Classical and Quantum Gravity, 2011)
We follow arguments of Verlinde (2010 arXiv:1001.0785 [hep-th]) and Klinkhamer (2010 arXiv:1006.2094 [hep-th]), and construct two models of the microscopic theory of a holographic screen that allow for the thermodynamical derivation of Newton's law, with Newton's constant expressed in terms of a minimal length scale l contained in the area spectrum of the microscopic theory. One of the models is loosely related to the quantum structure of surfaces and isolated horizons in loop quantum gravity. Our investigation shows that the conclusions reached by Klinkhamer regarding the new length scale l seem to be generic in all their qualitative aspects. © 2011 IOP Publishing Ltd.

New variables for classical and quantum gravity in all dimensions: I. Hamiltonian analysis - Bodendorfer, Norbert; Thiemann, Thomas; Thurn, Andreas (Classical and Quantum Gravity, 2013)
Loop quantum gravity (LQG) relies heavily on a connection formulation of general relativity such that (1) the connection Poisson commutes with itself and (2) the corresponding gauge group is compact. This can be achieved starting from the Palatini or Holst action when imposing the time gauge. Unfortunately, this method is restricted to D + 1 = 4 spacetime dimensions. However, interesting string theories and supergravity theories require higher dimensions and it would therefore be desirable to have higher dimensional supergravity loop quantizations at one's disposal in order to compare these approaches. In this series of papers we take first steps toward this goal. The present first paper develops a classical canonical platform for a higher dimensional connection formulation of the purely gravitational sector. The new ingredient is a different extension of the ADM phase space than the one used in LQG which does not require the time gauge and which generalizes to any dimension D > 1. The result is a Yang-Mills theory phase space subject to Gauss, spatial diffeomorphism and Hamiltonian constraint as well as one additional constraint, called the simplicity constraint. The structure group can be chosen to be SO(1, D) or SO(D + 1) and the latter choice is preferred for purposes of quantization.

New variables for classical and quantum gravity in all dimensions: III. Quantum theory - Bodendorfer, Norbert; Thiemann, Thomas; Thurn, Andreas (Classical and Quantum Gravity, 2013)
We quantize the new connection formulation of (D + 1)-dimensional general relativity developed in our companion papers by loop quantum gravity (LQG) methods. It turns out that all the tools prepared for LQG straightforwardly generalize to the new connection formulation in higher dimensions. The only new challenge is the simplicity constraint. While its 'diagonal' components acting at edges of spin-network functions are easily solved, its 'off-diagonal' components acting at vertices are non-trivial and require a more elaborate treatment.

New variables for classical and quantum gravity in all dimensions: II. Lagrangian analysis - Bodendorfer, Norbert; Thiemann, Thomas; Thurn, Andreas (Classical and Quantum Gravity, 2013)
We rederive the results of our companion paper, for matching space-time and internal signature, by applying in detail the Dirac algorithm to the Palatini action. While the constraint set of the Palatini action contains second class constraints, by an appeal to the method of gauge unfixing, we map the second class system to an equivalent first class system which turns out to be identical to the first class constraint system obtained via the extension of the ADM phase space performed in our companion paper. Central to our analysis is again the appropriate treatment of the simplicity constraint. Remarkably, the simplicity constraint invariant extension of the Hamiltonian constraint, that is a necessary step in the gauge unfixing procedure, involves a correction term which is precisely the one found in the companion paper and which makes sure that the Hamiltonian constraint derived from the Palatini Lagrangian coincides with the ADM Hamiltonian constraint when Gauss and simplicity constraints are satisfied. We therefore have rederived our new connection formulation of general relativity from an independent starting point, thus confirming the consistency of this framework.

New variables for classical and quantum gravity in all dimensions: IV. Matter coupling - Bodendorfer, Norbert; Thiemann, Thomas; Thurn, Andreas (Classical and Quantum Gravity, 2013)
We employ the techniques introduced in the companion papers (Bodendorfer et al 2011 arXiv: 1105.3703 [gr-qc]; arXiv: 1105.3704 [gr-qc]; arXiv: 1105.3705 [gr-qc]) to derive a connection formulation of Lorentzian general relativity coupled to Dirac fermions in dimensions D + 1 >= 3 with a compact gauge group. The technique that accomplishes that is similar to the one that has been introduced in 3+1 dimensions already. First one performs a canonical analysis of Lorentzian general relativity using the time gauge and then introduces an extension of the phase space analogous to the one employed in [1] to obtain a connection theory with SO(D + 1) as the internal gauge group subject to additional constraints. The success of this method rests heavily on the strong similarity of the Lorentzian and Euclidean Clifford algebras. A quantization of the Hamiltonian constraint is provided.

New variables for classical and quantum gravity in all dimensions: V. Isolated horizon boundary degrees of freedom - Bodendorfer, Norbert; Thiemann, Thomas; Thurn, Andreas (Classical and Quantum Gravity, 2014)
In this paper, we generalize the treatment of isolated horizons in loop quantum gravity, resulting in a Chern-Simons theory on the boundary in the four-dimensional case, to non-distorted isolated horizons in 2(n + 1)-dimensional spacetimes. The key idea is to generalize the four-dimensional isolated horizon boundary condition by using the Euler topological density E-(2n) of a spatial slice of the black hole horizon as a measure of distortion. The resulting symplectic structure on the horizon coincides with the one of higher-dimensional SO(2(n + 1))-Chern-Simons theory in terms of a Peldan-type hybrid connection Gamma(0) and resembles closely the usual treatment in (3 + 1) dimensions. We comment briefly on a possible quantization of the horizon theory. Here, some subtleties arise since higher-dimensional non-Abelian Chern-Simons theory has local degrees of freedom. However, when replacing the natural generalization to higher dimensions of the usual boundary condition by an equally natural stronger one, it is conceivable that the problems originating from the local degrees of freedom are avoided, thus possibly resulting in a finite entropy.
Keywords: loop quantum gravity; higher dimensions; black holes; Chern-Simons theory

No-boundary measure and preference for large e-foldings in multi-field inflation - Hwang, Dong-il; Kim, Soo A; Lee, Bum-Hoon; Sahlmann, Hanno; Yeom, Dong-han (Classical and Quantum Gravity, 2013)
The no-boundary wavefunction of quantum gravity usually assigns only very small probability to long periods of inflation. This was a reason to doubt about the no-boundary wavefunction to explain the observational universe. We study the no-boundary proposal in the context of multi-field inflation to see whether the number of fields changes the situation. For a simple model, we find that indeed the no-boundary wavefunction can give higher probability for sufficient inflation, but the number of fields involved N has to be very high, e.g., N ≃ m. © 2013 IOP Publishing Ltd.

No-Collapse Accurate Quantum Feedback Control via Conditional State Tomography - Borah, Sangkha; Sarma, Bijita (Physical Review Letters, 2023)
The effectiveness of measurement-based feedback control protocols is hampered by the presence of measurement noise, which affects the ability to accurately infer the underlying dynamics of a quantum system from noisy continuous measurement records to determine an accurate control strategy. To circumvent such limitations, this Letter explores a real-time stochastic state estimation approach that enables noise-free monitoring of the conditional dynamics including the full density matrix of the quantum system using noisy measurement records within a single quantum trajectory - a method we name as "conditional state tomography."This, in turn, enables the development of precise measurement-based feedback control strategies that lead to effective control of quantum systems by essentially mitigating the constraints imposed by measurement noise and has potential applications in various feedback quantum control scenarios. This approach is particularly useful for reinforcement-learning-(RL) based control, where the RL-agent can be trained with arbitrary conditional averages of observables, and/or the full density matrix as input (observation), to quickly and accurately learn control strategies.
CRIS-Team Scopus Importer:2023-12-15

No invariant perfect qubit codes - Mansuroglu, Refik; Sahlmann, Hanno (Journal of High Energy Physics, 2023)
Perfect tensors describe highly entangled quantum states that have attracted particular attention in the fields of quantum information theory and quantum gravity. In loop quantum gravity, the natural question arises whether SU(2) invariant tensors, which are fundamental ingredients of the basis states of spacetime, can also be perfect. In this work, we present a number of general constraints for the layout of such invariant perfect tensors (IPTs) and further describe a systematic and constructive approach to check the existence of an IPT of given valence. We apply our algorithm to show that no qubit encoding of valence 6 can be described by an IPT and close a gap to prove a no-go theorem for invariant perfect qubit encodings. We also provide two alternative proofs for the non-existence of 4-valent qubit IPTs which has been shown in [1, 2].
CRIS-Team Scopus Importer:2023-02-17
Keywords: Gauge-Gravity Correspondence; Holography and Condensed Matter Physics (AdS/CMT); Models of Quantum Gravity

Noise correlations in time- and angle-resolved photoemission spectroscopy - Stahl, Christopher; Eckstein, Martin (Physical Review B, 2019)
In time-resolved photoemission experiments, more than one electron can be emitted from the solid by a single ultrashort pulse. We theoretically demonstrate how correlations between the momenta of outgoing electrons relate to time-dependent two-particle correlations in the solid. This can extend the scope of time-and angle-resolved photoemission spectroscopy to probe superconducting and charge density fluctuations in systems without long-range order, and to reveal their dynamics independent of the electronic gap and thus unrestricted by the energy-time uncertainty. The proposal is illustrated for superconductivity in a BCS model. An impulsive perturbation can quench the gap on ultrafast timescales, while nonequilibrium pairing correlations persist much longer, even when electron-electron scattering beyond mean-field theory is taken into account. There is thus a clear distinction between a dephasing of the Cooper pairs and the thermalization into the normal state. While a measurement of the gap would be blind to such pairing correlations, they can be revealed by the angular correlations in photoemission.
CRIS-Team WoS Importer:2019-07-12

Nonequilibrium-DMFT based RIXS investigation of the two-orbital Hubbard model - Werner, Philipp; Johnston, Steven; Eckstein, Martin (EPL - Europhysics Letters, 2021)
Resonant inelastic X-ray scattering (RIXS) detects various types of high- and low-energy elementary excitations in correlated solids, and this tool will play an increasingly important role in investigations of time-dependent phenomena in photo-excited systems. While theoretical frameworks for the computation of equilibrium RIXS spectra are well established, the development of appropriate methods for nonequilibrium simulations is an active research field. Here, we apply a recently developed nonequilibrium dynamical mean field theory (DMFT) based approach to compute the RIXS response of photo-excited two-orbital Mott insulators. The results demonstrate the feasibility of multi-orbital nonequilibrium RIXS calculations and the sensitivity of the quasi-elastic fluorescence-like features and d-d excitation peaks on the nonequilibrium population of the Hubbard bands.
CRIS-Team Scopus Importer:2021-05-14

Nonequilibrium evolution of the optical conductivity of the weakly interacting Hubbard model: Drude response and pi-ton type vertex corrections - Simard, Olivier; Eckstein, Martin; Werner, Philipp (Physical Review B, 2021)
The optical conductivity contains information about energy absorption and the underlying physical processes. In finite-dimensional systems, vertex corrections to the bare bubble need to be considered, which is a computationally challenging task. Recent numerical studies showed that in the weak-coupling limit, near an ordering instability with wave-vector pi, the vertical ladder describing particle-hole pairs interacting via the exchange of this wave vector becomes the dominant vertex correction. The corresponding Maki-Thompson-like diagram has been dubbed pi-ton. Here we add the pi-ton ladder vertex correction to dynamical mean-field theory estimates of the optical conductivity. By performing calculations on the Kadanoff-Baym contour, we reveal the characteristic spectral signatures of the pi-tons and their evolution under nonequilibrium conditions. We consider interaction quenches of the weakly correlated Hubbard model near the antiferromagnetic phase boundary and analyze the evolution of the Drude and pi-ton features. While the bubble contribution to the optical conductivity is found to thermalize rapidly, after some oscillations with frequencies related to the local spectral function, the pi-ton contribution exhibits a slower evolution. We link this observation to the prethermalization phenomenon which has been previously studied in weakly interacting, quenched Hubbard models.
CRIS-Team WoS Importer:2022-01-07

Nonequilibrium information erasure below kTln2 - Konopik, M.; Friedenberger, Alexander; Kiesel, N.; Lutz, E. (EPL - Europhysics Letters, 2020)
Landauer's principle states that information erasure requires heat dissipation. While Landauer's original result focused on equilibrium memories, we here investigate the reset of information stored in a nonequilibrium state of a symmetric two-state memory. We derive a nonequilibrium generalization of the erasure principle and demonstrate that the corresponding bounds on heat and work may be reduced to zero. We further introduce reset protocols that harness energy and entropy of the initial preparation and so allow to reach these nonequilibrium bounds. We finally provide numerical simulations with realistic parameters of an optically levitated nanosphere memory that support these findings. Our results indicate that local dissipation-free information reset is possible away from equilibrium.
CRIS-Team Scopus Importer:2020-11-20

Nonequilibrium resonant inelastic x-ray scattering study of an electron-phonon model - Werner, Philipp; Eckstein, Martin (Physical Review B, 2021)
We use the nonequilibrium dynamical mean-field theory formalism to compute the equilibrium and nonequilibrium resonant inelastic x-ray scattering (RIXS) signal of a strongly interacting fermionic lattice model with a coupling of dispersionless phonons to the total charge on a given site. In the atomic limit, this model produces phonon subbands in the spectral function, but not in the RIXS signal. Electron hopping processes however result in phonon-related modifications of the charge excitation peak. We discuss the equilibrium RIXS spectra and the characteristic features of nonequilibrium states induced by photo-doping and by the application of a static electric field. The latter produces features related to Wannier-Stark states, which are dressed with phonon sidebands. Thanks to the effect of field-induced localization, the phonon features can be clearly resolved even in systems with weak electron-phonon coupling.
CRIS-Team Scopus Importer:2021-09-10

Nonequilibrium steady states, coexistence, and criticality in driven quasi-two-dimensional granular matter - Schindler, Thomas; Kapfer, Sebastian (Physical Review E, 2019)
Nonequilibrium steady states of vibrated inelastic frictionless spheres are investigated in quasi-twodimensional confinement via molecular dynamics simulations. The phase diagram in the density-amplitude plane exhibits a fluidlike disordered and an ordered phase with threefold symmetry, as well as phase coexistence between the two. A dynamical mechanism exists that brings about metastable traveling clusters and at the same time stable clusters with anisotropic shapes at low vibration amplitude. Moreover, there is a square bilayer state which is connected to the fluid by BKTHNY-type two-step melting with an intermediate tetratic phase. The critical behavior of the two continuous transitions is studied in detail. For the fluid-tetratic transition, critical exponents of (gamma) over tilde = 1.73, eta(4)approximate to 1/4, and z = 2.05 are obtained.
CRIS-Team WoS Importer:2019-02-21

Nonequilibrium steady-state theory of photodoped Mott insulators - Li, Jiajun; Eckstein, Martin (Physical Review B, 2021)
Photodoped states are widely observed in laser-excited Mott insulators, in which charge excitations are quickly created and can exist beyond the duration of the external driving. Despite the fruitful experimental explorations, theoretical studies on the microscopic models face the challenge to simultaneously deal with exponentially separated time scales, especially in multiband systems, where the longtime behaviors are often well beyond the reach of state-of-the-art numerical tools. Here, we address this difficulty by introducing a steady-state description of photodoped Mott insulators using an open-system setup, where the photodoped system is stabilized as a nonequilibrium steady state by a weak external driving. Taking advantage of the stationarity, we implement and discuss the details of an efficient numerical tool using the steady-state dynamical mean-field theory combined with the noncrossing approximation. We demonstrate that these stationary photodoped states exhibit the same properties of their transient counterparts, while being solvable with reasonable computational efforts. Furthermore, they can be parametrized by just a few physical quantities, including the effective temperature and the density of charge excitations, which confirms the universal nature of photodoped states indeed independent of the excitation protocols. As a first application, we consider the stationary photodoped states in a two-band Hubbard model with intertwined spin-and-orbital ordering and find a family of hidden phases unknown from the previous studies, implying an apparently unexplored time regime of the relaxation of the intertwined orders.
CRIS-Team Scopus Importer:2021-02-12

Nonergodic diffusion of single atoms in a periodic potential - Kindermann, Farina; Dechant, Andreas; Hohmann, Michael; Lausch, Tobias; Mayer, Daniel; Schmidt, Felix; Lutz, Eric; Widera, Artur (Nature Physics, 2017)
Diffusion can be used to infer the microscopic features of a system from the observation of its macroscopic dynamics. Brownian motion accurately describes many diffusive systems, but non-Brownian and nonergodic features are often observed on short timescales. Here, we trap a single ultracold caesium atom in a periodic potential and measure its diffusion. We engineer the particle-environment interaction to fully control motion over a broad range of diffusion constants and timescales. We use a powerful stroboscopic imaging method to detect single-particle trajectories and analyse both non-equilibrium diffusion properties and the approach to ergodicity. Whereas the variance and two-time correlation function exhibit apparent Brownian motion at all times, higher-order correlations reveal strong non-Brownian behaviour. We additionally observe the slow convergence of the exponential displacement distribution to a Gaussian and-unexpectedly-a much slower approach to ergodicity, in perfect agreement with an analytical continuous-time random-walk model. Our experimental system offers an ideal testbed for the detailed investigation of complex diffusion processes.
CRIS-Team Scopus Importer:2023-03-27

Non-Gaussian morphology on large scales: Minkowski functionals of the REFLEX cluster catalogue - Mecke, Klaus; et al. (Astronomy & Astrophysics, 2001)
In order to quantify higher-order correlations of the galaxy cluster distribution we use a complete family of additive measures which give scale-dependent morphological information. Minkowski functionals can be expressed analytically in terms of integrals of n-point correlation functions. They can be compared with measured Minkowski functionals of volume limited samples extracted from the Reflex survey. We find significant non-Gaussian features in the large-scale spatial distribution of galaxy clusters. A Gauss-Poisson process can be excluded as a viable model for the distribution of galaxy clusters at the significance level of 95%.
Keywords: large-scale structure of Universe; galaxies : clusters : general; cosmology : observation; cosmology : theory

Nonlinear bond-operator theory and 1/d expansion for coupled-dimer magnets. I. Paramagnetic phase - Joshi, Darshan G.; Coester, Kris; Schmidt, Kai Phillip; Vojta, Matthias (Physical Review B, 2015)
For coupled-dimer Heisenberg magnets, a paradigm of magnetic quantum phase transitions, we develop a systematic expansion in 1/d, the inverse number of space dimensions. The expansion employs a formulation of the bond-operator technique and is based on the observation that a suitably chosen product-state wave function yields exact zero-temperature expectation values of local observables in the d -> infinity limit, with corrections vanishing as 1/d. We demonstrate the approach for a model of dimers on a hypercubic lattice, which generalizes the square-lattice bilayer Heisenberg model to arbitrary d. In this paper, we use the 1/d expansion to calculate static and dynamic observables at zero temperature in the paramagnetic singlet phase, up to the quantum phase transition, and compare the results with numerical data available for d = 2. Contact is also made with previously proposed refinements of bond-operator theory as well as with a perturbative expansion in the interdimer coupling. In a companion paper, the present 1/d expansion will be extended to the ordered phase, where it is shown to consistently describe the entire phase diagram including the quantum critical point.

Nonlinear dynamics of weakly dissipative optomechanical systems - Roque, Thales Figueiredo; Marquardt, Florian; Yevtushenko, Oleg M. (New Journal of Physics, 2020)
Optomechanical systems attract a lot of attention because they provide a novel platform for quantum measurements, transduction, hybrid systems, and fundamental studies of quantum physics. Their classical nonlinear dynamics is surprisingly rich and so far remains underexplored. Works devoted to this subject have typically focussed on dissipation constants which are substantially larger than those encountered in current experiments, such that the nonlinear dynamics of weakly dissipative optomechanical systems is almost uncharted waters. In this work, we fill this gap and investigate the regular and chaotic dynamics in this important regime. To analyze the dynamical attractors, we have extended the 'generalized alignment index' method to dissipative systems. We show that, even when chaotic motion is absent, the dynamics in the weakly dissipative regime is extremely sensitive to initial conditions. We argue that reducing dissipation allows chaotic dynamics to appear at a substantially smaller driving strength and enables various routes to chaos. We identify three generic features in weakly dissipative classical optomechanical nonlinear dynamics: the Neimark-Sacker bifurcation between limit cycles and limit tori (leading to a comb of sidebands in the spectrum), the quasiperiodic route to chaos, and the existence of transient chaos.
CRIS-Team WoS Importer:2020-02-04

Non-oscillating power spectra in loop quantum cosmology - Elizaga de Navascués, Beatriz; Mena Marugán, Guillermo A.; Prado, Santiago (Classical and Quantum Gravity, 2021)
We characterize in an analytical way the general conditions that a choice of vacuum state for the cosmological perturbations must satisfy to lead to a power spectrum with no scale-dependent oscillations over time. In particular, we pay special attention to the case of cosmological backgrounds governed by effective loop quantum cosmology and in which the Einsteinian branch after the bounce suffers a pre-inflationary period of decelerated expansion. This is the case more often studied in the literature because of the physical interest of the resulting predictions. In this context, we argue that non-oscillating power spectra are optimal to gain observational access to those regimes near the bounce where loop quantum cosmology effects are non-negligible. In addition, we show that non-oscillatory spectra can indeed be consistently obtained when the evolution of the perturbations is ruled by the hyperbolic equations derived in the hybrid loop quantization approach. Moreover, in the ultraviolet regime of short wavelength scales we prove that there exists a unique asymptotic expansion of the power spectrum that displays no scale-dependent oscillations over time. This expansion would pick out the natural Poincaré and Bunch-Davies vacua in Minkowski and de Sitter spacetimes, respectively, and provides an appealing candidate for the choice of a vacuum for the perturbations in loop quantum cosmology based on physical motivations.
CRIS-Team Scopus Importer:2021-01-22
Keywords: Cosmological perturbations; Loop quantum cosmology; Power spectrum; Vacuum state

Nonperturbative linked-cluster expansions for the trimerized ground state of the spin-one kagome Heisenberg model - Ixert, Dominik; Tischler, Tobias; Schmidt, Kai Phillip (Physical Review B, 2015)
We use nonperturbative linked-cluster expansions to determine the ground-state energy per site of the spin-one Heisenberg model on the kagome lattice. To this end, a parameter is introduced allowing us to interpolate between a fully trimerized state and the isotropic model. The ground-state energy per site of the full graph decomposition up to graphs of six triangles (18 spins) displays a complex behavior as a function of this parameter close to the isotropic model which we attribute to divergencies of partial series in the graph expansion of quasi-1D unfrustrated chain graphs. More concretely, these divergencies can be traced back to a quantum critical point of the one-dimensional unfrustrated chain of coupled triangles. Interestingly, the reorganization of the nonperturbative linked-cluster expansion in terms of clusters with enhanced symmetry yields a ground-state energy per site of the isotropic two-dimensional model that is in quantitative agreement with other numerical approaches in favor of a spontaneous trimerization of the system. Our findings are of general importance for any nonperturbative linked-cluster expansion on geometrically frustrated systems.

Nonperturbative linked-cluster expansions in long-range ordered quantum systems - Ixert, Dominik; Schmidt, Kai Phillip (Physical Review B, 2016)
We introduce a generic scheme to perform nonperturbative linked cluster expansions in long-range ordered quantum phases. Clusters are considered to be surrounded by an ordered reference state leading to effective edge fields in the exact diagonalization on clusters, which break the associated symmetry of the ordered phase. Two approaches, based either on a self-consistent solution of the order parameter or on minimal sensitivity with respect to the ground-state energy per site, are formulated to find the optimal edge field in each NLCE order. Furthermore, we investigate the scaling behavior of the NLCE data sequences towards the infinite-order limit. We apply our scheme to gapped and gapless ordered phases of XXZ Heisenberg models on various lattices and for spins 1/2 and 1 using several types of cluster expansions ranging from a full-graph decomposition, rectangular clusters, up to more symmetric square clusters. It is found that the inclusion of edge fields allows to regularize nonperturbative linked-cluster expansions in ordered phases yielding convergent data sequences. This includes the long-range spin-ordered ground state of the spin-1/2 and spin-1 Heisenberg model on the square and triangular lattice as well as the trimerized valence bond crystal of the spin-1 Heisenberg model on the kagome lattice.

Nonperturbative theory of effective Hamiltonians for deformations in two-dimensional materials: Moire systems and dislocations - Rost, Fabian; Gupta, R.; Fleischmann, Maximilian; Weckbecker, Dominik; Ray, Nicolas; Olivares, J.; Vogl, Michael; Sharma, S.; Pankratov, Oleg; Shallcross, Sam (Physical Review B, 2019)
We demonstrate that there exists a continuum Hamiltonian H(r, p) that is formally the operator equivalent of the general tight-binding method, inheriting the associativity and Hermiticity of the latter operator. This provides a powerful and controlled method of obtaining effective Hamiltonians via Taylor expansion with respect to momentum and, optionally, deformation fields. In particular, for fundamentally nonperturbative defects, such as twist faults and partial dislocations, the method allows the deformation field to be retained to all orders, providing an efficient scheme for the generation of transparent and compact Hamiltonians for such defects. We apply the method to a survey of incommensurate physics in twist bilayers of graphene, graphdiyne, MoS2, and phosphorene. For graphene we are able to reproduce the "reflected Dirac cones" of the 30 degrees quasicrystalline bilayer found in a recent angle-resolved photoemission spectroscopy experiment, and we show that it is an example of a more general phenomenon of coupling by the moire momentum. We show that incommensurate physics is governed by the decay of the interlayer interaction on the scale of the single-layer reciprocal lattices, and demonstrate that incommensurate scattering effects lead to a very rapid broadening of band manifolds as the twist angle is tuned through commensurate values.
CRIS-Team WoS Importer:2021-10-18

Nonsingular quantum gravitational dynamics of an Lemaître-Tolman-Bondi dust shell model: The role of quantization prescriptions - Giesel, Kristina; Li, Bao-Fei; Singh, Parampreet (Physical Review D - Particles, Fields, Gravitation and Cosmology, 2021)
We study some consequences of the loop quantization of the outermost dust shell in the Lemaître-Tolman-Bondi spacetime with a homogeneous dust density using different quantization strategies motivated by loop quantum gravity. Prior work has dealt with loop quantizing this model by employing holonomies and the triads, following the procedure in standard loop quantum cosmology. In this work we compare this quantization with the one in which holonomies and gauge-covariant fluxes are used. While both of the quantization schemes resolve the central singularity, they lead to different mass gaps at which a trapped surface forms. This trapped surface which is matched to an exterior generalized Vaidya spacetime disappears when the density of the dust cloud is in the Planck regime. We find that the quantization based on holonomies and gauge-covariant fluxes generically results in an asymmetric evolution of the dust shell in which the Vaidya mass associated with the white hole as seen by an external observer is 2/π of the one for the black hole. This effective difference in masses results from difference in the classical limits in pre- and postbounce regimes in the two quantizations. This distinctive feature rules out formation of any black-hole-white-hole twins in presence of gauge-covariant flux modifications, which is in contrast to the quantization using holonomies and triads where the gravitational collapse always leads to black hole–white hole twins. Another striking difference lies in the fact that for the quantization based on holonomies and gauge-covariant fluxes there can be situations in which during a nonsingular collapse only a black hole forms without a white hole.<br

Non-universal Voronoi cell shapes in amorphous ellipsoid packs - Schröter, Matthias; Schaller, Fabian; Kapfer, Sebastian; Hilton, James E.; Cleary, Paul W.; Mecke, Klaus; de Michele, Christiano; Schilling, Tanja; Saadatfar, Mohammad; Delaney, Gary; Schröder-Turk, Gerd (EPL - Europhysics Letters, 2015)
In particulate systems with short-range interactions, such as granular matter or simple fluids, local structure determines the macroscopic physical properties. We analyse local structure metrics derived from the Voronoi diagram of oblate ellipsoids, for various aspect ratios and global packing fractions φg. We focus on jammed static configurations of frictional ellipsoids, obtained by tomographic imaging and by discrete element method simulations. The rescaled distribution of local packing fractions φl, defined as the ratio of particle volume and its Voronoi cell volume, is found to be independent of the particle aspect ratio, and coincide with results for sphere packs. By contrast, the typical Voronoi cell shape, quantified by the Minkowski tensor anisotropy index β = β02,0, points towards a difference between random packings of spheres and those of oblate ellipsoids. While the average cell shape β of all cells with a given value of is similar in dense and loose jammed sphere packings, the structure of dense and loose ellipsoid packings differs substantially such that this does not hold true.

Novel extrapolation for strong coupling expansions - Schmidt, Kai Phillip; et al. (ACTA PHYSICA POLONICA B, JAGELLONIAN UNIV, INST PHYSICS, 2003)
We present a novel extrapolation scheme for high order series expansions. The idea is to express the series, obtained in orders of an external variable, in terms of an internal parameter of the system. Here we apply this method to the 1-triplet dispersion in an antiferromagnetic S = 1/2 Heisenberg ladder. By the use of the internal parameter the accuracy of the truncated series is enhanced tremendously.

Novel Growth Regime of MDCK II Model Tissues on Soft Substrates - Kaliman, Sara; Jayachandran, Christina; Rehfeldt, Florian; Smith, Ana-Suncana (Biophysical Journal, 2014)
It is well established that MDCK II cells grow in circular colonies that densify until contact inhibition takes place. Here, we show that this behavior is only typical for colonies developing on hard substrates and report a new growth phase of MDCK II cells on soft gels. At the onset, the new phase is characterized by small, three-dimensional droplets of cells attached to the substrate. When the contact area between the agglomerate and the substrate becomes sufficiently large, a very dense monolayer nucleates in the center of the colony. This monolayer, surrounded by a belt of three-dimensionally packed cells, has a well-defined structure, independent of time and cluster size, as well as a density that is twice the steady-state density found on hard substrates. To release stress in such dense packing, extrusions of viable cells take place several days after seeding. The extruded cells create second-generation clusters, as evidenced by an archipelago of aggregates found in a vicinity of mother colonies, which points to a mechanically regulated migratory behavior.
Keywords: COLLECTIVE CELL-MIGRATION; STIFFNESS; APOPTOSIS

Novel input polarisation independent endoscopic cross-polarised optical coherence tomography probe - Blessing, Katharina; Schirmer, Judith; Sharma, Gargi; Singh, Kanwarpal (Journal of Biophotonics, 2020)
Lead by the original idea to perform noninvasive optical biopsies of various tissues, optical coherence tomography found numerous medical applications within the last two decades. The interference based imaging technique opens the possibility to visualise subcellular morphology up to an imaging depth of 3 mm and up to micron level axial and lateral resolution. The birefringence properties of the tissue are visualised with enhanced contrast using polarisation sensitive or cross-polarised optical coherence tomography (OCT) techniques. Although, it requires strict control over the polarisation states, resulting in several polarisation controlling elements. In this work, we propose a novel input-polarisation independent endoscopic system based on cross-polarised OCT. We tested the feasibility of our approach by measuring the polarisation change from a quarter-wave plate for different rotational angles. Further performance tests reveal a lateral resolution of 30 μm and a sensitivity of 103 dB. Images of the human nail bed and cow muscle tissue demonstrate the potential of the system to measure structural and birefringence properties of the tissue endoscopically.
CRIS-Team Scopus Importer:2020-10-02
Keywords: endoscopy; medical imaging; optical biopsy; optical coherence tomography; polarisation sensitive methods

Nuclear clocks for testing fundamental physics - Peik, E.; Schumm, T.; Safronova, M. S.; Pálffy, Adriana; Weitenberg, J.; Thirolf, P. G. (Quantum Science and Technology, 2021)
The low-energy, long-lived isomer in 229Th, first studied in the 1970s as an exotic feature in nuclear physics, continues to inspire a multidisciplinary community of physicists. It has stimulated innovative ideas and studies that expand the understanding of atomic and nuclear structure of heavy elements and of the interaction of nuclei with bound electrons and coherent light. Using the nuclear resonance frequency, determined by the strong and electromagnetic interactions inside the nucleus, it is possible to build a highly precise nuclear clock that will be fundamentally different from all other atomic clocks based on resonant frequencies of the electron shell. The nuclear clock will open opportunities for highly sensitive tests of fundamental principles of physics, particularly in searches for violations of Einstein's equivalence principle and for new particles and interactions beyond the standard model. It has been proposed to use the nuclear clock to search for variations of the electromagnetic and strong coupling constants and for dark matter searches. The 229Th nuclear optical clock still represents a major challenge in view of the tremendous gap of nearly 17 orders of magnitude between the present uncertainty in the nuclear transition frequency (about 0.2 eV, corresponding to ∼48 THz) and the natural linewidth (in the mHz range). Significant experimental progress has been achieved in recent years, which will be briefly reviewed. Moreover, a research strategy will be outlined to consolidate our present knowledge about essential 229mTh properties, to determine the nuclear transition frequency with laser spectroscopic precision, realize different types of nuclear clocks and apply them in precision frequency comparisons with optical atomic clocks to test fundamental physics. Two avenues will be discussed: laser-cooled trapped 229Th ions that allow experiments with complete control on the nucleus-electron interaction and minimal systematic frequency shifts, and Th-doped solids enabling experiments at high particle number and in different electronic environments.
CRIS-Team Scopus Importer:2021-05-14
Keywords: dark matter searches; fundamental physics; nuclear clock; thorium-229 isomer

Nuclear coherent population transfer to the Th 229m isomer using X-ray pulses - Kirschbaum, Tobias; Minkov, Nikolay; Pálffy, Adriana (Physical Review C - Nuclear Physics, 2022)
Population of the 8 eV Th229m isomer via the second nuclear excited state at 29.19 keV by means of coherent X-ray pulses is investigated theoretically. We focus on two nuclear coherent population transfer schemes using partially overlapping X-ray pulses known from quantum optics: Stimulated Raman adiabatic passage, and successive π pulses. Numerical results are presented for three possible experimental setups. Our results identify the Gamma Factory as the most promising scenario, where two ultraviolet pulses combined with relativistically accelerated ions deliver the required intensities for efficient isomer population. Our simulations require knowledge of the in-band and cross-band nuclear transition probabilities. We give theoretically predicted values for the latter and discuss them in the context of recent experiments.
CRIS-Team Scopus Importer:2022-07-22

Nuclear Pasta at Finite Temperature with the Time-Dependent Hartree-Fock Approach - Schuetrumpf, B.; Klatt, Michael; Iida, K.; Maruhn, J. A.; Mecke, Klaus; Reinhard, Paul-Gerhard (Journal of Physics: Conference Series, 2016)
We present simulations of neutron-rich matter at sub-nuclear densities, like supernova matter. With the time-dependent Hartree-Fock approximation we can study the evolution of the system at temperatures of several MeV employing a full Skyrme interaction in a periodic three-dimensional grid [1]. The initial state consists of α particles randomly distributed in space that have a Maxwell-Boltzmann distribution in momentum space. Adding a neutron background initialized with Fermi distributed plane waves the calculations reflect a reasonable approximation of astrophysical matter. The matter evolves into spherical, rod-like, connected rod-like and slab-like shapes. Further we observe gyroid-like structures, discussed e.g. in [2], which are formed spontaneously choosing a certain value of the simulation box length. The ρ-T-map of pasta shapes is basically consistent with the phase diagrams obtained from QMD calculations [3]. By an improved topological analysis based on Minkowski functionals [4], all observed pasta shapes can be uniquely identified by only two valuations, namely the Euler characteristic and the integral mean curvature. In addition we propose the variance in the cell-density distribution as a measure to distinguish pasta matter from uniform matter.
CRIS-Team Scopus Importer:2023-07-24

Nucleation of Ligand-Receptor Domains in Membrane Adhesion - Bihr, Timo; Seifert, Udo; Smith, Ana-Suncana (Physical Review Letters, 2012)
We present a comprehensive model for the nucleation of domains in membrane adhesion. We determine the critical number of bonds in a nucleus and calculate the probability distribution of nucleation time from a discrete master equation. The latter is characterized by only four effective rates, which account for cooperative effects between bonds. We validate our model by finding excellent agreement with extensive Langevin simulations. In the range of parameters typical for cell adhesion, we find the critical number of bonds to be small. Furthermore, we find a characteristic separation between the bonds at which nucleation is particularly fast, pointing to potential regulatory mechanisms that could be used to control the cell recognition processes. DOI: 10.1103/PhysRevLett.109.258101
Keywords: GIANT VESICLES; CELLS; KINETICS; MECHANICS; DIFFUSION; STIFFNESS; DYNAMICS; GROWTH; SHAPE

Numerical computations of next-to-leading order corrections in spinfoam large-j asymptotics - Han, Muxin; Huang, Zichang; Liu, Hongguang; Qu, Dongxue (Physical Review D, 2020)
We numerically study the next-to-leading order corrections of the Lorentzian Engle-Pereira-RovelliLivine (EPRL) 4-simplex amplitude in the large- j expansions. We perform large- j expansions of Lorentzian EPRL 4-simplex amplitudes with two different types of boundary states, the coherent intertwiners and the coherent spin-network, and numerically compute the leading-order and the next-to-leading order O(1/j) contributions of these amplitudes. We also study the dependences of these O(1/j) corrections on the Barbero-Inunirzi parameter gamma. We show that they, as functions of gamma, stabilize to finite real constants as gamma -> infinity. Lastly, we obtain the quantum corrections to the Regge action because of the O(1/j) contribution to the spinfoam amplitude.
CRIS-Team WoS Importer:2021-01-01

Numerically exact, time-dependent study of correlated electron transport in model molecular junctions - Wang, Haobin; Thoss, Michael (Journal of Chemical Physics, 2013)
The multilayer multiconfiguration time-dependent Hartree theory within second quantization representation of the Fock space is applied to study correlated electron transport in models of single-molecule junctions. Extending previous work, we consider models which include both electron-electron and electronic-vibrational interaction. The results show the influence of the interactions on the transient and the stationary electrical current. The underlying physical mechanisms are analyzed in conjunction with the nonequilibrium electronic population of the molecular bridge. © 2013 American Institute of Physics.
UnivIS-Import:2015-03-09:Pub.2013.nat.dphy.ITP.ppem.numeri

Observational exclusion of a consistent loop quantum cosmology scenario - Bolliet, Boris; Barrau, Aurelien; Grain, Julien; Schander, Susanne (Physical Review D, 2016)
It is often argued that inflation erases all the information about what took place before it started. Quantum gravity, relevant in the Planck era, seems therefore mostly impossible to probe with cosmological observations. In general, only very ad hoc scenarios or hyper fine-tuned initial conditions can lead to observationally testable theories. Here we consider a well-defined and well-motivated candidate quantum cosmology model that predicts inflation. Using the most recent observational constraints on the cosmic microwave background B-modes, we show that the model is excluded for all its parameter space, without any tuning. Some important consequences are drawn for the deformed algebra approach to loop quantum cosmology. We emphasize that neither loop quantum cosmology in general nor loop quantum gravity are disfavored by this study but their falsifiability is established.

Observation of optical activity in dielectric biomimetic 8-srs networks - Turella, Fabio; Cumming, Benjamin P.; Schröder-Turk, Gerd; Gu, Min (Optics Letters, 2015)
We demonstrate the first experimental realization of the 8-srs network by direct laser writing in a dielectric medium. The optical properties of this photonic crystal are both numerically simulated and experimentally measured. The numerical simulations indicate a strong optical activity (OA) for regions of the spectrum close to the unit cell size (3 extgreekmm) and a high sensitivity to angular resolution. Experimentally, we observe 40° of OA for a region of the spectrum between 3 and 3.5 extgreekmm, confirming both simulation predictions.
EAM Import::2019-03-08

Observation of two-magnon bound states in the two-leg ladders of (Ca, La)(14)Cu24O41 - Schmidt, Kai Phillip; et al. (Physical Review Letters, 2001)
Phonon-assisted two-magnon absorption is studied in the spin(7)1/2 two-leg ladders of (Ca, La) 14Cu24O41 for E parallel to c (legs) and E parallel to a (rungs). We verify the theoretically predicted existence of two-magnon singlet bound states, which give rise to peaks at approximate to 2140 and 2800 cm(-1). The two-magnon continuum is observed at approximate to 4000 cm(-1). Two different theoretical approaches (Jordan-Wigner fermions and perturbation theory) describe the data very well for J(parallel to) approximate to 1020-1100 cm(-1), J(parallel to)/J(perpendicular to) approximate to 1-1.2. At high energies, the magnetic contribution to sigma(omega) is strikingly similar in the ladders and in the undoped high-T-c cuprates, which emphasizes the importance of strong quantum fluctuations in the latter.

Observing Membrane and Cell Adhesion via Reflection Interference Contrast Microscopy - Abdelrahman, Ahmed; Smith, Ana-Suncana; Sengupta, Kheya (Humana Press Inc., 2023)
Reflection interference contrast microscopy (RICM) is an optical microscopy technique ideally suited for imaging adhesion. While RICM (and the closely related interference reflection microscopy (IRM)) has been extensively used qualitatively or semiquantitatively to image cells, including immune cells, it can also be used quantitatively to measure membrane to surface distance, especially for model membranes. Here, we present a protocol for RICM and IRM imaging and the details of semiquantitative and quantitative analysis.
CRIS-Team Scopus Importer:2023-05-19
Keywords: Adhesion; Antiflex; Fluctuations; Interferometry; Membranes

Observing polarization patterns in the collective motion of nanomechanical arrays - Doster, Juliane; Shah, Tirth; Foesel, Thomas; Paulitschke, Philipp; Marquardt, Florian; Weig, Eva M. (Nature Communications, 2022)
In recent years, nanomechanics has evolved into a mature field, and it has now reached a stage which enables the fabrication and study of ever more elaborate devices. This has led to the emergence of arrays of coupled nanomechanical resonators as a promising field of research serving as model systems to study collective dynamical phenomena such as synchronization or topological transport. From a general point of view, the arrays investigated so far can be effectively treated as scalar fields on a lattice. Moving to a scenario where the vector character of the fields becomes important would unlock a whole host of conceptually interesting additional phenomena, including the physics of polarization patterns in wave fields and their associated topology. Here we introduce a new platform, a two-dimensional array of coupled nanomechanical pillar resonators, whose orthogonal vibration directions encode a mechanical polarization degree of freedom. We demonstrate direct optical imaging of the collective dynamics, enabling us to analyze the emerging polarization patterns, follow their evolution with drive frequency, and identify topological polarization singularities.
CRIS-Team Scopus Importer:2022-05-20

Occurrence of connected clusters in motility-induced phase-separated states of persistent active particles at zero temperature - Schmiedeberg, Michael (EPL - Europhysics Letters, 2023)
To study the interplay of the formation of clusters of overlapping particles and motility-induced phase separation in the zero temperature limit in two dimensions, we consider a simple model system consisting of a bidisperse mixture of disks that are only subject to a repulsion force in case of overlaps and an active force. The orientation of the disks is chosen randomly in the beginning and does not change anytime during the simulation thus corresponding to an infinite persistence length. Simulations with our model system reveal that clusters of overlapping particles occur in the dense phase of a phase-separated state in case of intermediate values of the ratio of active to repulsive force. However, for smaller activities there are only a few overlaps between the particles in the dense phase and the coexistence region ends at a packing fraction below the onset of the formation of clusters with overlapping particles. For large activities small clusters corresponding to small patches of the dense phase are found that are unstable due to the activity. Finally, the occurrence of connected clusters affects the phase separation. Our findings on how cluster formation and phase separation are related are relevant to many active particle systems as after adding thermal noise our model system corresponds to a system of active Brownian particles that have been intensely studied in recent years. Further extensions by adding friction, inertia, or more complex interactions are possible.<br
Keywords: active particles; phase separation; clustering; non-equilibrium

On (cosmological) singularity avoidance in loop quantum gravity - Thiemann, Thomas; Brunnemann, Johannes (Classical and Quantum Gravity, 2006)
Loop quantum cosmology (LQC), mainly due to Bojowald, is not the cosmological sector of loop quantum gravity (LQG). Rather, LQC consists of a truncation of the phase space of classical general relativity to spatially homogeneous situations which is then quantized by the methods of LQG. Thus, LQC is a quantum-mechanical toy model (finite number of degrees of freedom) for LQG (a genuine QFT with an infinite number of degrees of freedom) which provides important consistency checks. However, it is a non-trivial question whether the predictions of LQC are robust after switching on the inhomogeneous fluctuations present in full LQG. Two of the most spectacular findings of LQC are that: (1) the inverse scale factor is bounded from above on zero-volume eigenstates which hints at the avoidance of the local Curvature singularity and (2) the quantum Einstein equations are non-singular which hints at the avoidance of the global initial singularity. This rests on (1) a key technique developed for LQG and (2) the fact that there are no inhomogeneous excitations. We display the result of a calculation for LQG which proves that the (analogon of the) inverse scale factor, while densely defined, is not bounded from above on zero-volume eigenstates. Thus, in full LQG, if curvature singularity avoidance is realized, then not in this simple way. In fact, it turns out that the boundedness of the inverse scale factor is neither necessary nor sufficient for the curvature singularity avoidance and that non-singular evolution equations are neither necessary nor sufficient for initial singularity avoidance because none of these criteria are formulated in terms of observable quantities. After outlining what would be required, we present the results of a calculation for LQG which could be a first indication that our criteria at least for curvature singularity avoidance are satisfied in LQG.

One- and two-triplon spectra of a cuprate ladder - Schmidt, Kai Phillip; et al. (Physical Review Letters, 2007)
We have performed inelastic neutron scattering on the near ideal spin-ladder compound La4Sr10Cu24O41 as a starting point for investigating doped ladders and their tendency toward superconductivity. A key feature was the separation of one-triplon and two-triplon scattering. Two-triplon scattering is observed quantitatively for the first time and so access is realized to the important strong magnetic quantum fluctuations. The spin gap is found to be 26.4 +/- 0.3 meV. The data are successfully modeled using the continuous unitary transformation method, and the exchange constants are determined by fitting to be J(leg) = 186 meV and J(rung) = 124 meV along the leg and rung, respectively; a substantial cyclic exchange of J(cyc) = 31 meV is confirmed.

One-bead microrheology with rotating particles - Schmiedeberg, Michael; Stark, Holger (EPL - Europhysics Letters, 2005)
We lay the theoretical basis for one-bead microrheology with rotating particles, i.e., a method where colloids are used to probe the mechanical properties of viscoelastic media. Based on a two-fluid model. we calculate the compliance and discuss it for two cases. We first assume that the elastic and the fluid component exhibit both stick boundary conditions at the particle surface. Then, the compliance fulfills a generalized Stokes law with a complex shear modulus whose validity is only limited by inertial effects, in contrast to translational motion. Secondly, we find that the validity of the Stokes regime is reduced when the elastic network is not coupled to the particle.

On loop quantum gravity kinematics with a non-degenerate spatial background - Sahlmann, Hanno (Classical and Quantum Gravity, 2010)
In a remarkable paper (Koslowski T A 2007 arXiv:0709.3465[gr-qc]), Koslowski introduced kinematical representations for loop quantum gravity in which a non-degenerate spatial background metric is present. He also considered their properties and showed that Gauß and diffeomorphism constraints can be implemented. With this paper, we streamline and extend his treatment. In particular, we show that the standard regularization of the geometric operators leads to well-defined operators in the new representations, and we work out their properties fully. We also give details on the implementation of the constraints. All of this is done in such a way as to show that the standard representation is a particular (and in some ways exceptional) case of the more general constructions. This does not mean that these new representations are as fundamental as the standard one. Rather, we believe they might be useful to find some form of effective theory of loop quantum gravity on large scales. © 2010 IOP Publishing Ltd.

On moments and scaling regimes in anomalous random walks - Schmiedeberg, Michael; Zaburdaev, Vasily; Stark, Holger (Journal of Statistical Mechanics-Theory and Experiment, 2009)
Anomalous diffusion is commonly characterized by an exponent in the power law of the mean square displacement as a function of time < r(2)(t)> proportional to t(2 nu). On the other hand, it is known that the probability distribution function of diffusing particles can be approximated by (1/t(alpha))Phi(r/t(alpha)). While for classical normal diffusion this scaling relation is exact, it may not be valid globally for anomalous diffusion. In general, the exponent a obtained from the scaling of the central part of the probability distribution function differs from the exponent nu given by the mean square displacement. In this paper we systematically study how the scaling of different moments and parts of the probability distribution function can be determined and characterized even when no global scaling exists. We consider three rigorous methods for finding, respectively, the mean square displacement exponent nu, the scaling exponent alpha and the profile of the scaling function Phi. We also show that alternatively the scaling exponent alpha can be determined by analyzing fractional moments with q << 1. All analytical results are obtained in the framework of continuous-time random walks. For a wide class of coupled random walks, including the famous Levy walk model, we introduce a new unifying description which allows straightforward generalizations to other systems. Finally, we show how fractional moments help to analyze experimental or simulation data consistentl
Keywords: transport properties (theory); dynamics (theory); diffusion

On rotating black holes in DHOST theories - Ben Achour, Jibril; Liu, Hongguang; Motohashi, Hayato; Mukohyama, Shinji; Noui, Karim (Journal of Cosmology and Astroparticle Physics, 2020)
Using the disformal solution-generating method, we construct new axisymmetric solutions in Degenerate Higher Order Scalar Tensor (DHOST) theories. The method consists in first considering a "seed"known solution in DHOST theories and then performing a disformal transformation of the metric to obtain a new solution. In vacuum, the two solutions are equivalent but they become physically inequivalent when one considers coupling to matter. In that way, we "disform"the stealth Kerr black hole solution and we obtain a first analytic rotating non-stealth solution in DHOST theories, while the associated scalar field is time-dependent with a constant kinetic density. The new solution is characterized by three parameters: the mass, the spin and the disformal parameter which encodes the deviation with respect to the Kerr geometry. We explore some geometrical properties of the novel disformed Kerr geometry which is no more Ricci flat but has the same singularity as the Kerr metric, admits an ergoregion, and is asymptotically flat. Moreover, the hidden symmetry of the Kerr solution is broken, providing an example of a non-circular geometry in a higher order theory of gravity. We also discuss geodesic motions and compute its (disformed) null directions which are interesting tools to understand the causal structure of the geometry. In addition, to illustrate again the potentiality of the disformal solution-generating method, we present another axisymmetric solution for DHOST theories obtained from a disformal transformation of the generalized Kerr solution of Einstein-Scalar gravity.
CRIS-Team Scopus Importer:2020-12-04
Keywords: astrophysical black holes; GR black holes; modified gravity

On spatially diffeomorphism invariant quantizations of the bosonic string - Wolz, Florian; Sahlmann, Hanno (Friedrich-Alexander-Universität Erlangen-Nürnberg, 2015)
String theory is one of the candidates for a theory that not only describes the nature of gravity at microscopic scales but also unites all fundamental forces into one common framework. This is possible by the simple assumption that all matter is given by small one-dimensional objects – strings – that may be open or closed. Yet, an analysis of the theory suggests that in the quantum formulation the diffeomorphism in- variance of the theory seems to be broken in the sense that the constraints that generate the diffeomorphisms are only implemented weakly. Since the constraint algebra is the 1+1 dimen- sional version of the Dirac hypersurface deformation algebra of general relativity, the idea is that the physical subspace that corresponds to the kernel of the constraints are the diffeomorphism invariant states. We therefore analyze the role of the hypersurface deformation algebra in the standard quantization of the bosonic string in further detail. It turns out that the spatial diffeomorphism constraint is anomaly-free, however, one also obtains that neither the physical states of the theory lie in the kernel of these constraints nor do they create physical states. We analyze the action of the constraints in terms of the oscillations on the strings with an eye towards implementing spatial diffeos strongly. It turns out that the action is so complicated that we were unable to nd solutions. Therefore, in a second part of the work we focus on a construction of a manifest diffeomorphism invariant quantization of the string in the spirit of previous work by Thiemann, i.e. the states are independent of the chosen parametrization. With the help of the Gelfand-Naimark-Segal construc- tion we obtain a Hilbert space and discuss a concrete representation that excites distinct points on the string. In the end we obtain a Fock space for a subalgebra of the mentioned representation and start a discussion of the diffeomorphism constraint and the mass within our framework. The bosonic string also is a model for the hypersurface deformation algebra in loop quantum gravity and the issues of anomaly freeness. Thus our work might shed light on associated questions

On the control of dispersion interactions between biological membranes and protein coated biointerfaces - Blackwell, Robert; Hemmerle, Arnaud; Baer, Andreas; Späth, Matthias; Peukert, Wolfgang; Parsons, Drew; Sengupta, Kheya; Smith, Ana-Suncana (Journal of Colloid and Interface Science, 2021)
Hypothesis: Interaction of cellular membranes with biointerfaces is of vital importance for a number of medical devices and implants. Adhesiveness of these surfaces and cells is often regulated by depositing a layer of bovine serum albumin (BSA) or other protein coatings. However, anomalously large separations between phospholipid membranes and the biointerfaces in various conditions and buffers have been observed, which could not be understood using available theoretical arguments. Methods: Using the Lifshitz theory, we here evaluate the distance-dependent Hamaker coefficient describing the dispersion interaction between a biointerface and a membrane to understand the relative positioning of two surfaces. Our theoretical modeling is supported by experiments where the biointerface is represented by a glass substrate with deposited BSA and protein layers. These biointerfaces are allowed to interact with giant unilamellar vesicles decorated with polyethylene glycol (PEG) using PEG lipids to mimic cellular membranes and their pericellular coat. Results: We demonstrate that careful treatment of the van der Waals interactions is critical for explaining the lack of adhesiveness of the membranes with protein-decorated biointerfaces. We show that BSA alone indeed passivates the glass, but depositing an additional protein layer on the surface BSA, or producing multiple layers of proteins and BSA results in repulsive dispersion forces responsible for 100 nm large equilibrium separations between the two surfaces.
CRIS-Team Scopus Importer:2021-05-14
Keywords: Biointerfaces; Dispersion forces; Lifshitz theory; Reflection interference contrast microscopy (RICM); Van der Waals interactions

On the Embedding of Quantum Field Theory on Curved Spacetimes into Loop Quantum Gravity - Stottmeister, Alexander; Thiemann, Thomas (Friedrich-Alexander-Universität Erlangen-Nürnberg, 2015)

On the Hartle-Hawking state for loop quantum gravity - Dhandhukiya, Satya; Sahlmann, Hanno (Friedrich-Alexander-Universität Erlangen-Nürnberg, 2016)
The dynamical laws of an evolving system determine how the system will look at later times, given some initial state the system started from. In quantum cosmology, one ap- plies this idea to the earliest moments of the universe, which requires one to have some idea about the intial conditions at the beginning of time. One proposal describing these initial conditions in the context of canonical quantum gravity is due to Hartle and Hawk- ing. Their proposal entails a preferred initial state for the universe which is based on a Euclidean path integral over all compact positive definite four-geometries which have a given three-geometry as a boundary. The wave function constructed this way satisfies the constraint equations of general relativity in ADM variables in a formal sense. Following the connection formulation, general relativity can be equivalently recast using the so-called Ashtekar-Barbero variables. The idea followed in this thesis is to mimic Hartle’s and Hawking’s procedure to construct an initial state in terms of these new variables. We observe that the wave function constructed this way does not satisfy the constraint equations of loop quantum gravity, even in a formal sense. We investigate this issue in the simple case of a relativistic particle. We finally suggest a modification of the proposal that does satisfy the constraints at least in a formal sense and start to consider its implications in quantum cosmology.  <attachment></attachment><attachment></attachment>

On the implementation of the canonical quantum simplicity constraint - Bodendorfer, Norbert; Thiemann, Thomas; Thurn, Andreas (Classical and Quantum Gravity, 2013)
In this paper, we discuss several approaches to solve the quadratic and linear simplicity constraints in the context of the canonical formulations of higher dimensional general relativity and supergravity developed in our companion papers. Since the canonical quadratic simplicity constraint operators have been shown to be anomalous in any dimension D >= 3 in Class. Quantum Grav. 30 045003, non-standard methods have to be employed to avoid inconsistencies in the quantum theory. We show that one can choose a subset of quadratic simplicity constraint operators which are non-anomalous among themselves and allow for a natural unitary map of the spin networks in the kernel of these simplicity constraint operators to the SU(2)-based Ashtekar-Lewandowski Hilbert space in D = 3. The linear constraint operators on the other hand are non-anomalous by themselves; however, their solution space is shown to differ in D = 3 from the expected Ashtekar-Lewandowski Hilbert space. We comment on possible strategies to make a connection to the quadratic theory. Also, we comment on the relation of our proposals to the existing work in the spin foam literature and how these works could be used in the canonical theory. We emphasize that many ideas developed in this paper are certainly incomplete and should be considered as suggestions for possible starting points for more satisfactory treatments in the future.

On the quantum origin of the seeds of cosmic structure - Perez, Alejandro; Sahlmann, Hanno; Sudarsky, Daniel (Classical and Quantum Gravity, 2006)
The current understanding of the quantum origin of cosmic structure is discussed critically. We point out that in the existing treatments a transition from a symmetric quantum state to an (essentially classical) non-symmetric state is implicitly assumed, but not specified or analysed in any detail. In facing this issue, we are led to conclude that new physics is required to explain the apparent predictive power of the usual schemes. Furthermore, we show that the novel way of looking at the relevant issues opens new windows from where relevant information might be extracted regarding cosmological issues and perhaps even clues about aspects of quantum gravity. © 2006 IOP Publishing Ltd.

On the relation between operator constraint, master constraint, reduced phase space and path integral quantization - Han, Muxin; Thiemann, Thomas (Classical and Quantum Gravity, 2010)
Path integral formulations for gauge theories must start from the canonical formulation in order to obtain the correct measure. A possible avenue to derive it is to start from the reduced phase space formulation. In this paper we review this rather involved procedure in full generality. Moreover, we demonstrate that the reduced phase space path integral formulation formally agrees with the Dirac's operator constraint quantization and, more specifically, with the master constraint quantization for first-class constraints. For first-class constraints with nontrivial structure functions the equivalence can only be established by passing to Abelian(ized) constraints which is always possible locally in phase space. Generically, the correct configuration space path integral measure deviates from the exponential of the Lagrangian action. The corrections are especially severe if the theory suffers from second-class secondary constraints. In a companion paper we compute these corrections for the Holst and Plebanski formulations of GR on which current spin foam models are based.

On the relation between rigging inner product and master constraint direct integral decomposition - Han, Muxin; Thiemann, Thomas (Journal of Mathematical Physics, 2010)
Canonical quantization of constrained systems with first-class constraints via Dirac's operator constraint method proceeds by the theory of Rigged Hilbert spaces, sometimes also called refined algebraic quantization. This method can work when the constraints form a Lie algebra. When the constraints only close with nontrivial structure functions, the Rigging map can no longer be defined. To overcome this obstacle, the master constraint method has been proposed which replaces the individual constraints by a weighted sum of absolute squares of the constraints. Now the direct integral decomposition (DID) methods, which are closely related to Rigged Hilbert spaces, become available and have been successfully tested in various situations. It is relatively straightforward to relate the rigging inner product to the path integral that one obtains via reduced phase space methods. However, for the master constraint, this is not at all obvious. In this paper we find sufficient conditions under which such a relation can be established. Key to our analysis is the possibility to pass to equivalent, Abelian constraints, at least locally in phase space. Then the master constraint DID for those Abelian constraints can be directly related to the rigging map and therefore has a path integral formulation. (C) 2010 American Institute of Physics. [doi:10.1063/1.3486359]
Keywords: Dirac equation; Hilbert spaces; integral equations; Lie algebras; master equation; quantisation (quantum theory)

On thermal fluctuations in thin film flow - Mecke, Klaus; et al. (IOP PUBLISHING LTD, 2005)
In bulk fluids hydrodynamic Navier-Stokes equations are proven to be valid down to the nanometre scale. However, during the dewetting process of thin liquid films of nanometre thickness the interplay of surface tension gamma, substrate potential and thermal noise can lead to qualitatively different behaviour on laterally much larger scales up to microns. By deriving a stochastic thin film equation with a conserved noise term we show that the spectrum of capillary waves changes from an exponential decay to a power law k(B)T/(gamma q(2)) for large wavevectors q due to thermal fluctuations at temperature T. Also the time evolution of film roughness sigma (t) and of the typical wavevector k(t) of unstable perturbations changes qualitatively. Whereas a deterministic Navier-Stokes equation in the lubrication approximation predicts in the linear regime a constant k(t) = k(0), one finds a coarsening k(2)(t) - k(0)(2) similar to k(B)T/gamma sigma(2)(t) due to thermal noise.

ON THE SOLUTION OF THE INITIAL-VALUE CONSTRAINTS FOR GENERAL-RELATIVITY COUPLED TO MATTER IN TERMS OF ASHTEKAR VARIABLES - Thiemann, Thomas (Classical and Quantum Gravity, 1993)
This new trick might also be of interest for Yang-Mills theories on curved backgrounds.

Open-loop linear control of quadratic Hamiltonians with applications - Johnsson, Mattias T.; Burgarth, Daniel (Physical Review A, 2024)
The quantum harmonic oscillator is one of the most fundamental objects in physics. We consider the case where it is extended to an arbitrary number modes and includes all possible terms that are bilinear in the annihilation and creation operators, and we assume that we also have an arbitrary time-dependent drive term that is linear in those operators. Such a Hamiltonian is very general, covering a broad range of systems, including quantum optics, superconducting circuit QED, quantum error correcting codes, Bose-Einstein condensates, atomic wave packet transport beyond the adiabatic limit, and many others. We examine this situation from the point of view of quantum control, making use of optimal control theory to determine what can be accomplished, both when the controls are arbitrary and when they must minimize some cost function. In particular we develop a class of analytical pulses. We then apply our theory to a number of specific topical physical systems to illustrate its use and provide explicit control functions, including the case of the continuously driven conditional displacement gate.
CRIS-Team Scopus Importer:2024-02-16

Optical control of competing exchange interactions and coherent spin-charge coupling in two-orbital Mott insulators - Barbeau, Marion M. S.; Eckstein, Martin; Katsnelson, Mikhail; Mentink, Johan H. (SciPost Physics, 2019)
In order to have a better understanding of ultrafast electrical control of exchange interactions in multi-orbital systems, we study a two-orbital Hubbard model at half filling under the action of a time-periodic electric field. Using suitable projection operators and a generalized time-dependent canonical transformation, we derive an effective Hamiltonian which describes two different regimes. First, for a wide range of non-resonant frequencies, we find a change of the bilinear Heisenberg exchange J(ex) that is analogous to the single-orbital case. Moreover we demonstrate that also the additional biquadratic exchange interaction B-ex can be enhanced, reduced and even change sign depending on the electric field. Second, for special driving frequencies, we demonstrate a novel spin-charge coupling phenomenon enabling coherent transfer between spin and charge degrees of freedom of doubly ionized states. These results are confirmed by an exact time-evolution of the full two-orbital Mott-Hubbard Hamiltonian.
CRIS-Team WoS Importer:2019-04-09

Optical signatures of the coupled spin-mechanics of a levitated magnetic microparticle - Wachter, Vanessa; Bittencourt, Victor A. S. V.; Xie, Shangran; Sharma, Sanchar; Joly, Nicolas; Russell, Philip St. John; Marquardt, Florian; Viola Kusminskiy, Silvia (Journal of the Optical Society of America B-Optical Physics, 2021)
We propose a platform that combines the fields of cavity optomagnonics and levitated optomechanics to control and probe the coupled spin-mechanics of magnetic dielectric particles. We theoretically study the dynamics of a levitated Faraday-active dielectric microsphere serving as an optomagnonic cavity, placed in an external magnetic field and driven by an external laser. We find that the optically driven magnetization dynamics induces angular oscillations of the particle with low associated damping. Further, we show that the magnetization and angular motion dynamics can be probed via the power spectrum of the outgoing light. Namely, the characteristic frequencies attributed to the angular oscillations and the spin dynamics are imprinted in the light spectrum by two main resonance peaks. Additionally, we demonstrate that a ferromagnetic resonance setup with an oscillatory perpendicular magnetic field can enhance the resonance peak corresponding to the spin oscillations and induce fast rotations of the particle around its anisotropy axis. (C) 2021 Optical Society of America
Created from Fastlane, WoS look-up

Optical spectroscopy of (La, Ca)(14)Cu24O41 spin ladders: comparison of experiment and theory - Schmidt, Kai Phillip; et al. (ELSEVIER SCIENCE BV, 2002)
Transmission and reflectivity of LaxCa14-xCu24O41 two-leg spin-(1)/(2) ladders were measured in the mid-infrared regime between 500 and 12 000 cm(-1). This allows us to determine the optical conductivity a, directly and with high sensitivity. Here we show data for x = 4 and 5 with the electrical field polarized parallel to the rungs (E a) and to the legs (E c). Three characteristic peaks are identified as magnetic excitations by comparison with two different theoretical calculations. (C) 2002 Elsevier Science B.V. All rights reserved.
Keywords: spin ladder; bound state; phonon-assisted 2-magnon absorption

Optical spectroscopy of low-dimensional quantum spin systems - Schmidt, Kai Phillip; et al. (2003)
Low-dimensional quantum spin systems display fascinating excitation spectra. In recent years, optical spectroscopy was shown to be a powerful tool for the study of these spectra by means of phonon-assisted infrared absorption. We discuss the results of antiferromagnetic S=1/2 cuprates with various topologies: the spinon continuum observed in the weakly coupled chains of CaCu2O3, two-triplet bound states and the continuum of the two-leg ladders in (La,Ca)(14)Cu24O41, and the bimagnon-plus-phonon spectrum of the bilayer YBa2Cu3O6, an undoped parent compound of the 2D high-T-c cuprates. Various theoretical approaches (dynamical DMRG, continuous unitary transformations (CUT), and spin-wave theory) are used for a quantitative analysis. Particular attention is paid to the role of the cyclic four-spin exchange.

Optimal motion of triangular magnetocapillary swimmers - Sukhov, Alexander; Ziegler, Sebastian; Xie, Qingguang; Trosman, Oleg; Pande, Jayant; Grosjean, Galien; Hubert, Maxime; Vandewalle, Nicolas; Smith, Ana-Suncana; Harting, Jens (Journal of Chemical Physics, 2019)
A system of ferromagnetic particles trapped at a liquid-liquid interface and subjected to a set of magnetic fields (magnetocapillary swimmers) is studied numerically using a hybrid method combining the pseudopotential lattice Boltzmann method and the discrete element method. After investigating the equilibrium properties of a single, two, and three particles at the interface, we demonstrate a controlled motion of the swimmer formed by three particles. It shows a sharp dependence of the average center-of-mass speed on the frequency of the time-dependent external magnetic field. Inspired by experiments on magnetocapillary microswimmers, we interpret the obtained maxima of the swimmer speed by the optimal frequency centered around the characteristic relaxation time of a spherical particle. It is also shown that the frequency corresponding to the maximum speed grows and the maximum average speed decreases with increasing interparticle distances at moderate swimmer sizes. The findings of our lattice Boltzmann simulations are supported by bead-spring model calculations.
CRIS-Team Scopus Importer:2019-10-08

Optimizing linked-cluster expansions by white graphs - Coester, K.; Schmidt, Kai Phillip (Physical Review E, 2015)
We introduce a white-graph expansion for the method of perturbative continuous unitary transformations when implemented as a linked-cluster expansion. The essential idea behind an expansion in white graphs is to perform an optimized bookkeeping during the calculation by exploiting the model-independent effective Hamiltonian in second quantization and the associated inherent cluster additivity. This approach is shown to be especially well suited for microscopic models with many coupling constants, since the total number of relevant graphs is drastically reduced. The white-graph expansion is exemplified for a two-dimensional quantum spin model of coupled two-leg XXZ ladders.

Optomechanical creation of magnetic fields for photons on a lattice - Schmidt, Manfred; Keßler, Stefan; Peano, Vittorio; Painter, Oskar; Marquardt, Florian (Optica, 2015)
Recently, there has been growing interest in the creation of artificial magnetic fields for uncharged particles, such as cold atoms or photons. These efforts are partly motivated by the resulting desirable features, such as transport along edge states that is robust against backscattering. We analyze how the optomechanical interaction between photons and mechanical vibrations can be used to create artificial magnetic fields for photons on a lattice. The ingredients required are an optomechanical crystal, i.e., a free-standing photonic crystal with localized vibrational and optical modes, and two laser beams with the right pattern of phases. One of the two schemes analyzed here is based on optomechanical modulation of the links between optical modes, while the other is a lattice extension of optomechanical wavelengthconversion setups. We analyze both schemes theoretically and present numerical simulations of the resulting optical spectrum, photon transport in the presence of an artificial Lorentz force, edge states, and the photonic Aharonov– Bohm effect. We discuss the requirements for experimental realizations. Finally, we analyze the completely general situation of an optomechanical system subject to an arbitrary optical phase pattern and conclude that it is best described in terms of gauge fields acting in synthetic dimensions. In contrast to existing nonoptomechanical approaches, the schemes analyzed here are very versatile, since they can be controlled fully optically, allowing for time-dependent in situ tunability without the need for individual electrical addressing of localized optical modes.
CRIS-Team Scopus Importer:2023-03-07
Keywords: Coupled resonators; Optical microelectromechanical devices; Photonic crystals

Orbital selective coupling in CeRh3 B2: Coexistence of high Curie and high Kondo temperatures - Amorese, Andrea; Hansmann, Philipp; Marino, Andrea; Körner, Peter; Willers, Thomas; Walters, Andrew; Zhou, Ke Jin; Kummer, Kurt; Brookes, Nicholas B.; Lin, Hong Ji; Chen, Chien Te; Lejay, Pascal; et al. (Physical Review B, 2023)
We investigated the electronic structure of the enigmatic CeRh3B2 using resonant inelastic scattering and x-ray absorption spectroscopy in combination with ab initio density functional calculations. We find that the Rh4d states are irrelevant for the high-temperature ferromagnetism and the Kondo effect. We also find that the Ce4f crystal-field strength is too small to explain the strong reduction of the Ce magnetic moment. The data revealed instead the presence of two different active Ce4f orbitals, with each coupling selectively to different bands in CeRh3B2. The intersite hybridization of the |J=52,Jz=±12) crystal-field state and Ce5d band combined with the intrasite Ce4f-5d exchange creates the strong ferromagnetism, while hybridization between the |J=52,Jz=±52) and the B sp in the ab-plane contributes to the Kondo interaction, which causes the moment reduction. This orbital selective coupling explains the unique and seemingly contradictory properties of CeRh3B2.
CRIS-Team Scopus Importer:2023-04-21

Orientation averaged angular distributions of photo-electrons from free Na clusters - Wopperer, Philipp; Faber, Bernd; Dinh, P. M.; Reinhard, Paul-Gerhard; Suraud, Eric (Physics Letters A, 2010)
We explore, from a theoretical perspective, photo-electron angular distributions (PAD) of small Na clusters using the time-dependent local-density approximation (TDLDA). We assume an isotropic ensemble of free clusters and develop, for the case of one-photon emission, analytical formulae for the orientation averaged PAD which require only six TDLDA calculations in properly chosen reference frames. All information in the averaged PAD is contained in one anisotropy parameter. This parameter varies little with system size, but is crucially influenced by the detailed ionic structure. © 2010 Elsevier B.V. All rights reserved.
CRIS-Team Scopus Importer:2023-07-24
Keywords: Angular distribution; Electron emission; Metal cluster; Orientation averaging

Origins of bond and spin order in rare-earth nickelate bulk and heterostructures - Lu, Yi; Zhong, Zhicheng; Haverkort, Maurits W.; Hansmann, Philipp (Physical Review B, 2017)
We analyze the charge- and spin-response functions of rare-earth nickelates RNiO3 and their heterostructures using random-phase approximation in a two-band Hubbard model. The interorbital charge fluctuation is found to be the driving mechanism for the rock-salt-type bond order in bulk RNiO3, and good agreement of the ordering temperature with experimental values is achieved for all RNiO3 using realistic crystal structures and interaction parameters. We further show that magnetic ordering in bulk is not driven by the spin fluctuation and should be instead explained as ordering of localized moments. This picture changes for low-dimensional heterostructures, where the charge fluctuation is suppressed and overtaken by the enhanced spin instability, which results in a spin-density-wave ground state observed in recent experiments. Predictions for spectroscopy allow for further experimental testing of our claims.
CRIS-Team WoS Importer:2020-02-19

Oscillating bound states for a giant atom - Guo, Lingzhen; Kockum, Anton Frisk; Marquardt, Florian; Johansson, Goran (Physical Review Research, 2020)
We investigate the relaxation dynamics of a single artificial atom interacting, via multiple coupling points, with a continuum of bosonic modes (photons or phonons) in a one-dimensional waveguide. In the nonMarkovian regime, where the traveling time of a photon or phonon between the coupling points is sufficiently large compared to the inverse of the bare relaxation rate of the atom, we find that a boson can be trapped and form a stable bound state. As a key discovery, we further find that a persistently oscillating bound state can appear inside the continuous spectrum of the waveguide if the number of coupling points is more than two since such a setup enables multiple bound modes to coexist. This opens up prospects for storing and manipulating quantum information in larger Hilbert spaces than available in previously known bound states.
CRIS-Team WoS Importer:2021-01-22

Oscillations due to time-delayed driving of a ball in a water jet - a challenging problem of the International Physicists' Tournament 2019 - Michalke, Simon; Fösel, Angela; Schmiedeberg, Michael (European Journal of Physics, 2020)
The International Physicists' Tournament (IPT) 2019 dealt with 17 challenging problems. In this article, we present experimental as well as theoretical approaches exemplarily to one of these tasks. A ball placed on a hard and flat surface can show oscillation movement when being hit by a jet of water from above. To explain the oscillations, a theoretical model is introduced and its predictions are compared to the experimental measurements. Furthermore, the structure of the IPT itself is characterized shortly in this article, as well as the idea of a new and innovative seminar which was set up at Erlangen University to prepare and assist students in taking part in International Physicists' Tournament. Furthermore, the educational relevance of physics tournaments like IPT for physics education at university is discussed in detail.<br
Keywords: Physics Competitions; International Physicists' Tournament; German Physicists' Tournament; oscillations; nonlinear dynamics; harmonic oscillator; time-delay

Overload wave-memory induces amnesia of a self-propelled particle - Hubert, Maxime; Perrard, Stephane; Vandewalle, Nicolas; Labousse, Matthieu (Nature Communications, 2022)
Information storage is a key element of autonomous, out-of-equilibrium dynamics, especially for biological and synthetic active matter. In synthetic active matter however, the implementation of internal memory in self-propelled systems is often absent, limiting our understanding of memory-driven dynamics. Recently, a system comprised of a droplet generating its guiding wavefield appeared as a prime candidate for such investigations. Indeed, the wavefield, propelling the droplet, encodes information about the droplet trajectory and the amount of information can be controlled by a single scalar experimental parameter. In this work, we show numerically and experimentally that the accumulation of information in the wavefield induces the loss of time correlations, where the dynamics can then be described by a memory-less process. We rationalize the resulting statistical behavior by defining an effective temperature for the particle dynamics where the wavefield acts as a thermostat of large dimensions, and by evidencing a minimization principle of the generated wavefield.
CRIS-Team Scopus Importer:2022-08-05

Parallel Simulations of Self-propelled Microorganisms - Pickl, Kristina; Hofmann, Matthias; Preclik, Tobias; Köstler, Harald; Smith, Ana-Suncana; Rüde, Ulrich (Parallel Computing: Accelerating Computational Science and Engineering (CSE), 2014)
The scalability of a parallel simulation of self-propelled microorganisms in a fluid in the regime of low Reynolds numbers is investigated on current HPC clusters. For this, the lattice Boltzmann solver from the WALBERLA framework is coupled with the rigid body physics engine pe. The latter is enhanced with spring-like pair-wise interactions that can not only span over neighboring process domains but as well act as long-range pair-wise interactions between distant process domains. The communication is designed in such a way that only neighboring processes and the processes hosting the objects attached to the springs communicate. The suitability and efficiency of this approach is analyzed by a weak scaling of a representative setup for a swarm simulation on two supercomputers currently in the top ten of the TOP500 list, namely JUQUEEN and SUPERMUC. © 2014 The authors and IOS Press.
UnivIS-Import:2015-04-20:Pub.2014.tech.IMMD.lsinfs.parall_9
Keywords: computational fluid dynamics; MPI parallel simulation; particulate flow simulation; self-propelled microorganism

Particle-conserving dynamics on the single-particle level - Schindler, Thomas; Wittmann, R.; Brader, J. M. (Physical Review E, 2019)
We generalize the particle-conserving dynamics method of de las Heras et al. [J. Phys.: Condens. Matter 28, 244024 (2016)JCOMEL0953-898410.1088/0953-8984/28/24/244024] to binary mixtures and apply this to hard rods in one dimension. Considering the case of one species consisting of only one particle enables us to address the tagged-particle dynamics. The time-evolution of the species-labeled density profiles is compared to exact Brownian dynamics and (grand-canonical) dynamical density functional theory. The particle-conserving dynamics yields improved results over the dynamical density functional theory and well reproduces the simulation data at short and intermediate times. However, the neglect of a strict particle order (due to the fundamental statistical assumption of ergodicity) leads to errors at long times for our one-dimensional setup. The isolated study of that error makes clear the fundamental limitations of (adiabatic) density-based theoretical approaches when applied to systems of any dimension for which particle caging is a dominant physical mechanism.
CRIS-Team Scopus Importer:2021-04-20

Particle segregation in a sedimenting bidisperse soft sphere system - Kohl, Matthias; Schmiedeberg, Michael (Soft Matter, 2014)
We study the sedimentation process of a binary colloidal soft sphere system where significant overlaps of the particles are possible. We employ estimates of the equation of states in the small and large pressure limit in order to predict the final states of the sedimentation process. Furthermore, Brownian dynamics simulations were performed in order to confirm the predictions and to explore the dynamics of the sedimentation. We observe that the segregation process due to gravity usually consists of multiple steps. Instead of single particles moving upwards or downwards we usually observe that first local segregation occurs, then clusters consisting of particles of one species are formed that finally sink towards their equilibrium position within the final sedimentation profile. The possible final states include complex phases like a phase consisting of large particles on the top and the bottom of the system with small particles in between. We also observe metastable network-like structures.

Passivity and microlocal spectrum condition - Sahlmann, Hanno; Verch, Rainer (Communications in Mathematical Physics, 2000)
In the setting of vector-valued quantum fields obeying a linear wave-equation in a globally hyperbolic, stationary spacetime, it is shown that the two-point functions of passive quantum states (mixtures of ground- or KMS-states) fulfill the microlocal spectrum condition (which in the case of the canonically quantized scalar field is equivalent to saying that the two-pnt function is of Hadamard form). The fields can be of bosonic or fermionic character. We also give an abstract version of this result by showing that passive states of a topological *-dynamical system have an asymptotic pair correlation spectrum of a specific type.

Path integral renormalization in loop quantum cosmology - Bodendorfer, Norbert; Han, Muxin; Haneder, Fabian; Liu, Hongguang (Physical Review D, 2021)
A coarse graining technique akin to block spin transformations that groups together fiducial cells in a homogeneous and isotropic universe has been recently developed in the context of loop quantum cosmology. The key technical ingredient was an SU(1,1) group and Lie algebra structure of the physical observables as well as the use of Perelomov coherent states for SU(1,1). It was shown that the coarse graining operation is completely captured by changing group representations. Based on this result, it was subsequently shown that one can extract an explicit renormalization group flow of the loop quantum cosmology Hamiltonian operator in a simple model with dust-clock. In this paper, we continue this line of investigation and derive a coherent state path integral formulation of this quantum theory and extract an explicit expression for the renormalization-scale dependent classical Hamiltonian entering the path integral for a coarse grained description at that scale. We find corrections to the nonrenormalized Hamiltonian that are qualitatively similar to those previously investigated via canonical quantization. In particular, they are again most sensitive to small quantum numbers, showing that the large quantum number (spin) description captured by so called "effective equations"in loop quantum cosmology does not reproduce the physics of many small quantum numbers (spins). Our results have direct impact on path integral quantization in loop quantum gravity, showing that the usually taken large spin limit should be expected not to capture (without renormalization, as mostly done) the physics of many small spins that is usually assumed to be present in physically reasonable quantum states.
CRIS-Team Scopus Importer:2021-07-09

Pattern-fluid interpretation of chemical turbulence - Scholz, Christian; Schröder-Turk, Gerd; Mecke, Klaus (Physical Review E, 2015)
The spontaneous formation of heterogeneous patterns is a hallmark of many nonlinear systems, from biological tissue to evolutionary population dynamics. The standard model for pattern formation in general, and for Turing patterns in chemical reaction-diffusion systems in particular, are deterministic nonlinear partial differential equations where an unstable homogeneous solution gives way to a stable heterogeneous pattern. However, these models fail to fully explain the experimental observation of turbulent patterns with spatio-temporal disorder in chemical systems. Here we introduce a pattern-fluid model as a general concept where turbulence is interpreted as a weakly interacting ensemble obtained by random superposition of stationary solutions to the underlying reaction-diffusion system. The transition from turbulent to stationary patterns is then interpreted as a condensation phenomenon, where the nonlinearity forces one single mode to dominate the ensemble. This model leads to better reproduction of the experimental concentration profiles for the "stationary phases" and reproduces the turbulent chemical patterns observed by Q. Ouyang and H. L. Swinney [Chaos 1, 411 (1991)].

Patterns of Polar Domains in a Spatiotemporal Model of Interacting Polarities - Aliee, Maryam; Boudaoud, Arezki (Physical Review Letters, 2019)
Polarity fields are known to exhibit long distance patterns, in both physical and biological systems. The mechanisms behind such patterns are poorly understood. Here, we describe the dynamics of polarity fields using an original physical model that generalizes classical spin models on a lattice by incorporating effective transport of polarity molecules between neighboring sites. We account for an external field and for ferromagnetic interactions between sites and prescribe the time evolution of the system using two distinct dissipative classes for nonconserved and conserved variables representing polarity orientation and magnitude, respectively. We observe two main types of steady-state configurations-disordered configurations and patterns of highly polar spots surrounded by regions with low polarity-and we characterize patterns and transitions between configurations. Our results may provide alternative pattern-generating mechanisms for materials endowed with polarity fields.
CRIS-Team WoS Importer:2019-07-26

Peakedness properties of SU(3) heat kernel coherent states - Lang, Thorsten; Thiemann, Thomas (Friedrich-Alexander-Universität Erlangen-Nürnberg, 2015)

Perfect and Controllable Nesting in Minimally Twisted Bilayer Graphene - Fleischmann, Maximilian; Gupta, Reena; Wullschläger, Florian; Theil, Simon; Weckbecker, Dominik; Meded, Velimir; Sharma, Sangeeta; Meyer, Bernd; Shallcross, Sam (Nano Letters, 2020)
Parallel ("nested") regions of a Fermi surface (FS) drive instabilities of the electron fluid, for example, the spin density wave in elemental chromium. In one-dimensional materials, the FS is trivially fully nested (a single nesting vector connects two "Fermi dots"), while in higher dimensions only a fraction of the FS consists of parallel sheets. We demonstrate that the tiny angle regime of twist bilayer graphene (TBLG) possesses a phase, accessible by interlayer bias, in which the FS consists entirely of nestable "Fermi lines", the first example of a completely nested FS in a two-dimensional (2D) material. This nested phase is found both in the ideal as well as relaxed structure of the twist bilayer. We demonstrate excellent agreement with recent STM images of topological states in this material and elucidate the connection between these and the underlying Fermiology. We show that the geometry of the Fermi lines network is controllable by the strength of the applied interlayer bias, and thus TBLG offers unprecedented access to the physics of FS nesting in 2D materials.
CRIS-Team Scopus Importer:2020-02-07
Keywords: Fermi surface nesting; Graphene; helical network; twist bilayer

Performance of Scientific Applications on Modern Supercomputers - Deserno, Frank; Hager, Georg; Brechtefeld, Frank; Wellein, Gerhard (High Performance Computing in Science and Engineering Munich 2004 Transactions of the Second Joint HLRB and KONWIHR Status and ResultWorkshop, March 2-3, 2004, Technical University of Munich, andLeibniz-Rechenzentrum Munich, Germany., 2005)
UnivIS-Import:2015-04-16:Pub.2005.tech.IMMD.lsinfs.perfor_0

Permeability of Porous Materials Determined from the Euler Characteristic - Scholz, Christian; Wirner, Frank; Götz, Jan; Rüde, Ulrich; Schröder-Turk, Gerd; Mecke, Klaus; Bechinger, Clemens (Physical Review Letters, 2012)
We study the permeability of quasi-two-dimensional porous structures of randomly placed overlapping monodisperse circular and elliptical grains. Measurements in microfluidic devices and lattice Boltzmann simulations demonstrate that the permeability is determined by the Euler characteristic of the conducting phase. We obtain an expression for the permeability that is independent of the percolation threshold and shows agreement with experimental and simulated data over a wide range of porosities. Our approach suggests that the permeability explicitly depends on the overlapping probability of grains rather than their shape. © 2012 American Physical Society.
UnivIS-Import:2015-03-09:Pub.2012.tech.IMMD.lsinfs.permea

Persistent Conductivity in n-type 3C-SiC Observed at Low Temperatures - Beljakowa, Svetlana; Hauck, Martin; Bockstedte, Michel Georg; Fromm, Felix; Hundhausen, Martin; Nagasawa, H.; Weber, Heiko B.; Pensl, Gerhard; Krieger, Michael (Materials Science Forum, 2014)
Persistent conductivity in n-type 3C-SiC is investigated in a wide temperature range down to 3 K by Hall effect, admittance spectroscopy, low temperature photoluminescence (LTPL) and Raman spectroscopy. We propose a model, which clearly explains the persistent behavior of the electron density η below 50 K. It is experimentally verified that the persistent conductivity results from doped SF bunches, which can be considered nano-polytype inclusions in 3C-SiC. © (2014) Trans Tech Publications, Switzerland.
UnivIS-Import:2015-04-14:Pub.2014.nat.dphy.IAP.LAP.persis
Keywords: 3C-SiC; Hall effect; Persistent conductivity; Raman spectroscopy; Stacking faults (SFs)

Persisting topological order via geometric frustration - Schmidt, Kai Phillip (Physical Review B, 2013)
We introduce a toric code model on the dice lattice which is exactly solvable and displays topological order at zero temperature. In the presence of a magnetic field, the flux dynamics is mapped to the highly frustrated transverse field Ising model on the kagome lattice. This correspondence suggests an intriguing disorder by disorder phenomenon in a topologically ordered system implying that the topological order is extremely robust due to the geometric frustration. Furthermore, a connection between fully frustrated transverse field Ising models and topologically ordered systems is demonstrated which opens an exciting physical playground due to the interplay of topological quantum order and geometric frustration.

Perturbation theory of nearly spherical dielectric optical resonators - Gohsrich, Julius; Shah, Tirth; Aiello, Andrea (Physical Review A, 2021)
Dielectric spheres of various sizes may sustain electromagnetic whispering-gallery modes resonating at optical frequencies with very narrow linewidths. Arbitrary small deviations from the spherical shape typically shift and broaden such resonances. Our goal is to determine these shifted and broadened resonances. A boundary-condition perturbation theory for the acoustic vibrations of nearly circular membranes was developed by Rayleigh more than a century ago. We extend this theory to describe the electromagnetic excitations of nearly spherical dielectric cavities. This approach permits us to avoid dealing with decaying quasinormal modes. We explicitly find the frequencies and the linewidths of the optical resonances for arbitrarily deformed nearly spherical dielectric cavities, as power series expansions by a small parameter, up to and including second-order terms. We thoroughly discuss the physical conditions for the applicability of perturbation theory.
CRIS-Team WoS Importer:2021-09-03

Perturbation theory of optical resonances of deformed dielectric spheres - Aiello, Andrea; Harris, Jack G. E.; Marquardt, Florian (Physical Review A, 2019)
We analyze the optical resonances of a dielectric sphere the surface of which has been slightly deformed in an arbitrary way. Setting up a perturbation series up to second order, we derive both the frequency shifts and the modified linewidths. Our theory is applicable, for example, to freely levitated liquid drops or solid spheres, which are deformed by thermal surface vibrations, centrifugal forces, or arbitrary surface waves. A dielectric sphere is effectively an open system the description of which requires the introduction of non-Hermitian operators characterized by complex eigenvalues and non-normalizable eigenfunctions. We avoid these difficulties using the Kapur-Peierls formalism, which enables us to extend the popular Rayleigh-Schrodinger perturbation theory to the case of electromagnetic Debye potentials describing the light fields inside and outside the near-spherical dielectric object. We find analytical formulas, valid within certain limits, for the deformation-induced first- and second-order corrections to the central frequency and bandwidth of a resonance. As an application of our method, we compare our results with preexisting ones, finding full agreement.
CRIS-Team WoS Importer:2019-09-10

Perturbative approach to an exactly solved problem: Kitaev honeycomb model - Vidal, Julien; Schmidt, Kai Phillip; Dusuel, Sebastien (Physical Review B, 2008)
We analyze the gapped phase of the Kitaev honeycomb model perturbatively in the isolated-dimer limit. Our analysis is based on the continuous unitary transformations method, which allows one to compute the spectrum as well as matrix elements of operators between eigenstates at high order. The starting point of our study consists of an exact mapping of the original honeycomb spin system onto a square-lattice model involving an effective spin and a hard-core boson. We then derive the low-energy effective Hamiltonian up to order 10 which is found to describe an interacting-anyon system, contrary to the order 4 result which predicts a free theory. These results give the ground-state energy in any vortex sector and thus also the vortex gap, which is relevant for experiments. Furthermore, we show that the elementary excitations are emerging free fermions composed of a hard-core boson with an attached spin- and phase-operator string. We also focus on observables and compute, in particular, the spin-spin correlation functions. We show that they admit a multiplaquette expansion that we derive up to order 6. Finally, we study the creation and manipulation of anyons with local operators, show that they also create fermions, and discuss the relevance of our findings for experiments in optical lattices.
Keywords: anyons; boson systems; fermion systems; optical lattices; perturbation techniques; spin systems

Perturbative study of the Kitaev model with spontaneous time-reversal symmetry breaking - Dusuel, Sebastien; Schmidt, Kai Phillip; Vidal, Julien; Zaffino, Rosa Letizia (Physical Review B, 2008)
We analyze the Kitaev model on the triangle-honeycomb lattice whose ground state has recently been shown to be a chiral spin liquid. We consider two perturbative expansions: the isolated-dimer limit containing Abelian anyons and the isolated-triangle limit. In the former case, we derive the low-energy effective theory and discuss the role played by multiplaquette interactions. In this phase, we also compute the spin-spin correlation functions for any vortex configuration. In the isolated-triangle limit, we show that the effective theory is, at lowest nontrivial order, the Kitaev honeycomb model at the isotropic point. We also compute the next-order correction which opens a gap and yields non-Abelian anyons.

Phase behaviour of a simple fluid confined in a periodic porous material - Stopper, Daniel; Schroder-Turk, Gerd E.; Mecke, Klaus; Roth, Roland (Molecular Physics, 2021)
Using density functional theory for a simple fluid of hard spheres with a square-well attraction, we study the phase behaviour of the fluid confined in triply periodic porous material, where the pore space is bounded by a bicontinuous minimal surface. We combine our numerical results with the morphometric thermodynamics, an ansatz that expresses thermodynamic quantities as a linear combination of four additive geometrical terms, and analyse the dependency of the transition point between a liquid and a gas on the geometrical properties of the confining space. By demonstrating that the morphometric approach can be employed in order to account for the thermodynamic behaviour of a simple fluid in a complex porous material, we make the first step towards a better understanding of experimental observations that can help to characterise the pore space.
CRIS-Team WoS Importer:2021-04-02

Phase field crystal model for particles with n-fold rotational symmetry in two dimensions - Weigel, Robert; Schmiedeberg, Michael (Modelling and Simulation in Materials Science and Engineering, 2022)
We introduce a phase field crystal (PFC) model for particles with n-fold rotational symmetry in two dimensions. Our approach is based on a free energy functional that depends on the reduced one-particle density, the strength of the orientation, and the direction of the orientation, where all these order parameters depend on the position. The functional is constructed such that for particles with axial symmetry (i.e. n = 2) the PFC model for liquid crystals as introduced by Lowen (2010 J. Phys.: Condens. Matter 22 364105) is recovered. We discuss the stability of the functional and explore phases that occur for 1 <= n <= 6. In addition to isotropic, nematic, stripe, and triangular order, we also observe cluster crystals with square, rhombic, honeycomb, and even quasicrystalline symmetry. The n-fold symmetry of the particles corresponds to the one that can be realized for colloids with symmetrically arranged patches. We explain how both, repulsive as well as attractive patches, are described in our model.
CRIS-Team WoS Importer:2022-09-16

Phase field crystal model for particles with n-fold rotational symmetry in two dimensions - Weigel, Robert; Schmiedeberg, Michael (Modelling and Simulation in Materials Science and Engineering, 2022)
We introduce a phase field crystal (PFC) model for particles with n-fold rotational symmetry in two dimensions. Our approach is based on a free energy functional that depends on the reduced one-particle density, the strength of the orientation, and the direction of the orientation, where all these order parameters depend on the position. The functional is constructed such that for particles with axial symmetry (i.e. n = 2) the PFC model for liquid crystals as introduced by Löwen (2010 J. Phys.: Condens. Matter 22 364105) is recovered. We discuss the stability of the functional and explore phases that occur for 1 ⩽ n ⩽ 6. In addition to isotropic, nematic, stripe, and triangular order, we also observe cluster crystals with square, rhombic, honeycomb, and even quasicrystalline symmetry. The n-fold symmetry of the particles corresponds to the one that can be realized for colloids with symmetrically arranged patches. We explain how both, repulsive as well as attractive patches, are described in our model.<br

Phase space quantization and loop quantum cosmology: A Wigner function for the Bohr-compactified real line - Sahlmann, Hanno; Fewster, Christopher (Classical and Quantum Gravity, 2008)
We give a definition for the Wigner function for quantum mechanics on the Bohr compactification of the real line and prove a number of simple consequences of this definition. We then discuss how this formalism can be applied to loop quantum cosmology. As an example, we use the Wigner function to give a new quantization of an important building block of the Hamiltonian constraint. © 2008 IOP Publishing Ltd.

Phasonic Diffusion and Self-confinement of Decagonal Quasicrystals in Hyperspace - Hielscher, Johannes; Martinsons, Miriam; Schmiedeberg, Michael; Kapfer, Sebastian (Journal of Physics: Conference Series, 2020)
We introduce a novel simulation method that is designed to explore fluctuations of the phasonic degrees of freedom in decagonal colloidal quasicrystals. Specifically, we attain and characterise thermal equilibrium of the phason ensemble via Monte Carlo simulations with particle motions restricted to elementary phasonic flips. We find that, at any temperature, the random tiling ensemble is strongly preferred over the minimum phason-strain quasicrystal. Phasonic flips are the dominant carriers of diffusive mass transport in physical space. Sub- diffusive transients suggest cooperative flip behaviour on short time scales. In complementary space, particle mobility is geometrically restricted to a thin ring around the acceptance domain, resulting in self-confinement and persistent phasonic orde
Keywords: quasicrystal; phason; simulation

Phason-induced dynamics of colloidal particles on quasicrystalline substrates - Kromer, Justus; Schmiedeberg, Michael; Roth, Johannes; Stark, Holger (European Physical Journal E, 2013)
Phasons are special hydrodynamic modes that occur in quasicrystals. The trajectories of particles due to a phasonic drift were recently studied by Kromer et al. (Phys. Rev. Lett. 108, 218301 (2012)) for the case where the particles stay in the minima of a quasicrystalline potential. Here, we study the mean motion of colloidal particles in quasicrystalline laser fields when a phasonic drift or displacement is applied and also consider the cases where the colloids cannot follow the potential minima. While the mean square displacement is similar to the one of particles in a random potential with randomly changing potential wells, there also is a net drift of the colloids that reverses its direction when the phasonic drift velocity is increased. Furthermore, we explore the dynamics of the structural changes in a laser-induced quasicrystal during the rearrangement process that is caused by a steady phasonic drift or an instantaneous phasonic displacement.

Phonon-based thermal configurational forces: Definitions and applications in rupture of semiconductors - Birang Oskouei, Seyedehelmira; Smith, Ana-Suncana; Steinmann, Paul (Engineering Fracture Mechanics, 2021)
A general framework encompassing spatial and material settings of atomistic crystalline systems is presented. Theoretical and computational analysis of configurational forces in the context of atomistic thermomechanics constitutes the central topic of the presented paper. We distinguish between atomistic configurational forces stemming from the changes in static energy density and the changes in vibrational energy density. To this end, we first propose a novel atomistic phonon-based thermal configurational forces rendering infinitesimal changes in vibrational energy density under configurational variations. Next, we employ pair and triplet configurational forces resulting from the changes in static energy density due to two-body stretch and three-body stretch-bending. Using thermal configurational forces, we study numerically the problem of temperature rise, bond stretching and bending occurring at the crack tip induced by brittle crack propagation in crystalline lattices. Energy release caused by large harmonic vibrations and deformations is estimated by the magnitude of configurational forces at the crack tip. The atomistic temperature is shown to be governed by atom-wise frequency through local entropy. Finally, we demonstrate suitability and versatility of atomistic configurational forces in analyzing fracture of silicon diamond lattice subjected to mode-I loading at different finite temperatures.
Created from Fastlane, Scopus look-up
Keywords: Atomistic configurational mechanics; Atomistic fracture mechanics; Configurational forces; Effective empirical potential; Spatial and material settings

Photoelectron spectra: A tool of analysis of irradiation dynamics - Bär, Matthias; Dinh, P. M.; Reinhard, Paul-Gerhard; Suraud, Eric (Journal of Physics: Conference Series, 2012)
We explore from a theoretical perspective photoelectron spectra emitted from a Na8 cluster irradiated by a short laser pulse. We use time-dependent density-functional theory (TDDFT) at the level of the local-density approximation (LDA). That includes a self-interaction correction (SIC) to recover the proper emission threshold. As environments are known to play an important role in irradiation dynamics, we consider the example of Na8 deposited on a MgO(001) surface and compare it to the case of a free cluster. Anisotropy of emission is also briefly discussed.
CRIS-Team Scopus Importer:2023-07-24

Photoenhanced excitonic correlations in a Mott insulator with nonlocal interactions - Bittner, Nikolaj; Golež, Denis; Eckstein, Martin; Werner, Philipp (Physical Review B, 2020)
We investigate the effect of nonlocal interactions on the photodoped Mott insulating state of the two-dimensional Hubbard model using a nonequilibrium generalization of the dynamical cluster approximation. In particular, we compare the situation where the excitonic states are lying within the continuum of doublon-holon excitations to a setup where the excitons appear within the Mott gap. In the first case, the creation of nearest-neighbor doublon-holon pairs by excitations across the Mott gap results in enhanced excitonic correlations, but these excitons quickly decay into uncorrelated doublons and holons. In the second case, photoexcitation results in long-lived excitonic states. While in a low-temperature equilibrium state, excitonic features are usually not evident in single-particle observables such as the photoemission spectrum, we show that the photoexcited nonequilibrium system can exhibit in-gap states associated with the excitons. The comparison with exact-diagonalization results for small clusters allows us to identify the signatures of the excitons in the photoemission spectrum.
CRIS-Team Scopus Importer:2020-03-06

Photoinduced strange metal with electron and hole quasiparticles - Dasari, Nagamalleswara Rao; Li, Jiajun; Werner, Philipp; Eckstein, Martin (Physical Review B, 2021)
Photodoping of Mott insulators or correlated metals can create an unusual metallic state which simultaneously hosts holelike and electronlike particles. We study the dynamics of this state up to long times, as it passes its kinetic energy to the environment. When the system cools down, it crosses over from a bad metal into a resilient quasiparticle regime, in which quasiparticle bands are formed with separate Fermi levels for electrons and holes, but quasiparticles do not yet satisfy the Fermi-liquid paradigm. Subsequently, the transfer of energy to the environment slows down significantly, and the system does not reach the Fermi-liquid state even on the timescale of picoseconds. The transient photodoped strange metal exhibits unusual properties of relevance for ultrafast charge and heat transport: In particular, there can be an asymmetry in the properties of electrons and holes, and strong correlations between electrons and holes, as seen in the spectral properties.
CRIS-Team Scopus Importer:2021-06-11

Photon channelling in foams - Schmiedeberg, Michael; Miri, MirFaez; Stark, Holger (European Physical Journal E, 2005)
Experiments by Gittings, Bandyopadhyay and Durian (Europhys. Lett. 65, 414 (2004)) demonstrate that light possesses a higher probability to propagate in the liquid phase of a foam due to total reflection. The authors term this observation photon channelling which we investigate in this article theoretically. We first derive a central relation in the work of Gitting et al. without any free parameters. It links the photon's path-length fraction f in the liquid phase to the liquid fraction epsilon. We then construct two-dimensional Voronoi foams, replace the cell edges by channels to represent the liquid films and simulate photon paths according to the laws of ray optics using transmission and reflection coefficients from Fresnel's formulas. In an exact honeycomb foam, the photons show superdiffusive behavior. It becomes diffusive as soon as disorder is introduced into the foams. The dependence of the diffusion constant on channel width and refractive index is explained by a one-dimensional random-walk model. It contains a photon channelling state that is crucial for the understanding of the numerical results. At the end, we shortly comment on the observation that photon channelling only occurs in a finite range of epsilon.

Physical properties of quantum field theory measures - Mourao, José Manuel; Thiemann, Thomas; Velhinho, Jose (Journal of Mathematical Physics, 1999)
Well known methods of measure theory on infinite dimensional spaces are used to study physical properties of measures relevant to quantum field theory. The difference of typical configurations of free massive scalar field theories with different masses is studied. We apply the same methods to study the Ashtekar-Lewandowski (AL) measure on spaces of connections. In particular we prove that the diffeomorphism group acts ergodically, with respect to the AL measure, on the Ashtekar-Isham space of quantum connections modulo gauge transformations. We also prove that a typical, with respect to the AL measure, quantum connection restricted to a (piecewise analytic) curve leads to a parallel transport discontinuous at every point of the curve. (C) 1999 American Institute of Physics. [S0022-2488(99)00404-1].

Physics and Literature. Concepts – Transfer – Aestheticization - Heydenreich, Aura; Mecke, Klaus (De Gruyter, 2021)
Physics and Literature is a unique collaboration between physicists, literary scholars, and philosophers, the first collection of essays to examine together how science and literature, beneath their practical differences, share core dimensions – forms of questioning, thinking, discovering and communicating insights.This book advances an in-depth exploration of relations between physics and literature from both perspectives. It turns around the tendency to discuss relations between literature and science in one-sided and polarizing ways. The collection is the result of the inaugural conference of ELINAS, the Erlangen Center for Literature and Natural Science, an initiative dedicated to building bridges between literary and scientific research. ELINAS revitalizes discussion of science-literature interconnections with new topics, ideas and angles, by organizing genuine dialogue among participants across disciplinary lines. The essays explore how scientific thought and practices are conditioned by narrative and genre, fiction, models and metaphors, and how science in turn feeds into the meaning-making of literary and philosophical texts. These interdisciplinary encounters enrich reflections on epistemology, cognition and aesthetic
Keywords: Interdisziplinärer Transfer; Konzeptbildung; Metaphernbildung; Naturwissenschaft in Literatur

Physics challenged by cells - Smith, Ana-Suncana (Nature Physics, 2010)
Cells are the building blocks of life. Ideas traditionally applied to physical problems are now helping to unravel their complex mysteries.
Keywords: FREE-ENERGY DIFFERENCES; FLUCTUATION THEOREM; ADHESION; RECONSTITUTION; EQUALITY; GROWTH; FORCE

Physics competitions in the time of a pandemic: 3D printing as a new approach to the quantitative investigation of Cartesian divers at home - Bley, Jonas; Pietz, Antony; Fösel, Angela; Schmiedeberg, Michael; Heusler, Stefan; Pusch, Alexander (European Journal of Physics, 2021)
Despite the difficult circumstances due to the COVID-19 pandemics, physics students can tackle interesting questions that are part of physics competitions as the German Physicists' Tournament (GPT) 2020. Due to the COVID-19 pandemics in 2020, many competitions such as the GPT are held online. Furthermore, the usual options of equipment offered by the supervising university institutions could not be used by the students. The problems of the GPT 2020 therefore had to be chosen in such a way that they could be examined at home using simple means. One of these supposedly simple but profound experiments—the Cartesian diver—is described in this article. The physics of the Cartesian diver has been discussed before Fakhruddin (2003 Phys. Teach. 41 53), Güémez et al (2002 Am. J. Phys. 70 710) and De Luca and Ganci (2011 Phys. Educ. 46 528), as well as various modifications (Ivanov and Nikolov 2019 Phys. Educ. 55 025006). We present a new way of investigating Cartesian divers quantitatively by using 3D printing and common household materials. The paper is addressed to undergraduate students and educators teaching physics at university.<br

Physics of cell adhesion: some lessons from cell-mimetic systems - Sackmann, Erich; Smith, Ana-Suncana (Soft Matter, 2014)
Cell adhesion is a paradigm of the ubiquitous interplay of cell signalling, modulation of material properties and biological functions of cells. It is controlled by competition of short range attractive forces, medium range repellant forces and the elastic stresses associated with local and global deformation of the composite cell envelopes. We review the basic physical rules governing the physics of cell adhesion learned by studying cell-mimetic systems and demonstrate the importance of these rules in the context of cellular systems. We review how adhesion induced micro-domains couple to the intracellular actin and microtubule networks allowing cells to generate strong forces with a minimum of attractive cell adhesion molecules (CAMs) and to manipulate other cells through filopodia over micrometer distances. The adhesion strength can be adapted to external force fluctuations within seconds by varying the density of attractive and repellant CAMs through exocytosis and endocytosis or protease-mediated dismantling of the CAM-cytoskeleton link. Adhesion domains form local end global biochemical reaction centres enabling the control of enzymes. Actin-microtubule crosstalk at adhesion foci facilitates the mechanical stabilization of polarized cell shapes. Axon growth in tissue is guided by attractive and repulsive clues controlled by antagonistic signalling pathways.
Keywords: INTERFERENCE CONTRAST MICROSCOPY; RECEPTOR-LIGAND BONDS; SUPPORTED MEMBRANES; GIANT VESICLES; FLUID MEMBRANES; IMMUNOLOGICAL SYNAPSE; GROWTH CONES; T-CELLS; IN-VIVO; ACTIVATION

Physics of Organelle Membrane Bridging via Cytosolic Tethers is Distinct From Cell Adhesion - Kamal, Mohammad Arif; Janes, Josip Augustin; Li, Long; Thibaudau, Franck; Smith, Ana-Suncana; Sengupta, Kheya (Frontiers in Physics, 2022)
Tremendous progress has been made recently in imaging the contacts between intra-cellular organelles, which are thought to be mediated by soluble tethers. However, they are still difficult to study in cellulo, and reconstituting them in vitro is a standing challenge. Here we take a mimetic approach to study Giant unilamellar vesicles (GUVs) and supported lipid bilayers (SLBs) interacting via single- (or double-) stranded DNA sequences of two different lengths. Like intra-cellular tethers which may reside in the cytosol when unbound, the DNA-tethers are soluble, but can insert into the membrane with the help of cholesterol moieties found at their extremities. Tether-exchange between the bulk “cytosol” and the GUV/SLB membrane leads to a novel statistical ensemble in which the entire system equilibrates together, rather than individual GUVs behaving as separate closed systems. As a consequence, adhesion between the GUV and the SLB is driven by collective entropic effects amplified by tether shape changes associated with membrane bridging. A direct experimental consequence is an unusual dependence on tether-concentration, which becomes an important control parameter at low concentrations, while tether length/flexibility are less important. The establishment of this fundamentally different interaction between two membranes suggests that in physiological conditions, the regulation of contact formation inside cells may be very different from the case of the much studied ligand-receptor mediated cell adhesion.
CRIS-Team Scopus Importer:2022-02-04
Keywords: biomembranes; biomimetic systems; cell adhesion; DNA tethers; GUV; organelle bridging; tethers; vesicles

Physics of Spatially Structured Materials - Mecke, Klaus (New Perspectives in Stochastic Geometry, 2009)
Modern imaging techniques with nanometre resolution open up the possibility to study the relationship between physical properties and geometric features of spatially structured materials.
EAM Import::2019-03-12

Physics Opportunities with the Gamma Factory - Budker, Dmitry; Gorchtein, Mikhail; Krasny, Mieczyslaw Witold; Pálffy, Adriana; Surzhykov, Andrey (Annalen Der Physik, 2022)
CRIS-Team Scopus Importer:2022-03-18

Physik und Ethik. Juli Zeh im Dialog zu »Schilf« - Heydenreich, Aura; Mecke, Klaus (Physik und Poetik. Produktionsästhetik und Werkgenese. Autorinnen und Autoren im Dialog, 2015)

Physik und Poetik. Produktionsästhetik und Werkgenese. Autorinnen und Autoren im Dialog. Interviews mit: Thomas Lehr, Durs Grünbein, Raoul Schrott, Ulrike Draesner, Jens Harder, Ulrich Woelk, Juli Zeh, Michael Hampe - Heydenreich, Aura; Mecke, Klaus (De Gruyter, 2015)
Viele Schriftsteller beschäftigen sich intensiv mit den weltbildprägenden Erkenntnissen der Physik, oft unbeachtet von Literaturkritik und Wissenschaft. Ihre Gründe sind dabei so vielfältig wie die Bücher, die sie schreiben. Sie in Interviews zu Wort kommen zu lassen, bereichert den Diskurs über die zwei Kulturen um eine Stimme, die beide kennt: Literatur und Physik. Physik begegnet uns in allen Bereichen des Lebens; angefangen von Geräten in der technisierten Zivilisation, über die Beschreibung von Phänomenen in der Natur bis zum grundlegenden Verständnis der Welt, das durch Quanten- und Relativitätstheorie ein anderes geworden ist. Es verwundert daher nicht, dass Schriftsteller physikalische Erkenntnisse verwenden, um von Menschen und der Welt, in der sie leben, zu erzählen. Experimentelle Literatur und Metaphern in physikalischen Theorien sind nur zwei Themen, die in den Interviews mit Ulrike Draesner, Durs Grünbein, Michael Hampe, Jens Harder, Reinhard Jirgl, Thomas Lehr, Ulrich Woelk und Juli Zeh zur Sprache kommen. Einig sind sich alle, dass Physik und Literatur zwei Erkenntnisweisen der Welt sind, die sich ergänzen und bedinge

Polarization Maintaining Fiber-Based Depth Encoded Cross-Polarized Optical Coherence Tomography - Romodina, Maria N.; Blessing, Katharina; Singh, Kanwarpal (IEEE Access, 2022)
Cross-polarized optical coherence tomography offers improved imaging contrast of birefringent tissue, such as collagen organization, compared to conventional optical coherence tomography systems. Therefore, it is a promising diagnostic tool for fibrosis and cancer. Existing cross-polarized optical coherence tomography systems require free-space polarization-controlling elements, which makes these systems bulky. In this work, we present a polarization-maintaining fiber-based minimalistic system for cross-polarization optical coherence tomography suitable for endoscopic applications. A long section of a polarization maintaining fiber is placed in the sample arm and acts as a delay line due to a large group-delay difference for two orthogonally polarized modes of the fiber. Co- and cross-polarized images appear at different depth positions within the image, providing a contrast enhancement due to visualization of the change of the light's polarization state in tissue. We demonstrate system designs for both benchtop and hand-held configurations. The images of a mouse's esophagus ex-vivo and the nailbed of a human volunteer are presented and reveal changes in the light's polarization state due to the presence of collagen in the tissue. The proposed all-fiber-based cross-polarized optical coherence tomography system has a high potential for endoscopy and can be translated to clinical applications.
CRIS-Team WoS Importer:2023-01-27

Polymer and Fock representations for a scalar field - Sahlmann, Hanno; Ashtekar, Abhay; Lewandowski, Jerzy (Classical and Quantum Gravity, 2003)
In loop quantum gravity, matter fields can have support only on the 'polymer-like' excitations of quantum geometry, and their algebras of observables and Hilbert spaces of states cannot refer to a classical, background geometry. Therefore, to adequately handle the matter sector, one has to address two issues already at the kinematic level. First, one has to construct the appropriate background-independent operator algebras and Hilbert spaces. Second, to make contact with low-energy physics, one has to relate this 'polymer description' of matter fields to the standard Fock description in Minkowski space. While this task has been completed for gauge fields, important gaps remained in the treatment of scalar fields. The purpose of this letter is to fill these gaps.

Position-Squared Coupling in a Tunable Photonic Crystal Optomechanical Cavity - Paraiso, Taofiq K.; Kalaee, Mahmoud; Zang, Leyun; Pfeifer, Hannes; Marquardt, Florian; Painter, Oskar (Physical Review X, 2015)
We present the design, fabrication, and characterization of a planar silicon photonic crystal cavity in which large position-squared optomechanical coupling is realized. The device consists of a double-slotted photonic crystal structure in which motion of a central beam mode couples to two high-Q optical modes localized around each slot. Electrostatic tuning of the structure is used to controllably hybridize the optical modes into supermodes that couple in a quadratic fashion to the motion of the beam. From independent measurements of the anticrossing of the optical modes and of the dynamic optical spring effect, a position-squared vacuum coupling rate as large as (g) over tilde'/2 pi = 245 Hz is inferred between the optical supermodes and the fundamental in-plane mechanical resonance of the structure at omega(m)/2 pi = 8.7 MHz, which in displacement units corresponds to a coupling coefficient of g'/2 pi = 1 THz/nm(2). For larger supermode splittings, selective excitation of the individual optical supermodes is used to demonstrate optical trapping of the mechanical resonator with measured (g) over tilde'/2 pi = 46 Hz.

Preparation of granulocyte concentrates by apheresis - Leitner, G.; Panzer, S.; Reesink, H. W.; Stiegler, G.; Fischer-Nielsen, A.; Dickmeiss, E.; Einsele, H.; Reinhard, Paul-Gerhard; Schrezenmeier, H.; Wiesneth, M.; Coluccia, P.; Nygell, U. Axdorph; et al. (Vox Sanguinis, 2019)
CRIS-Team Scopus Importer:2023-03-23

Pressure dependence of the magnetization plateaus of SrCu2(BO3)(2) - Schneider, David A.; Coester, Kris; Mila, Frederic; Schmidt, Kai Phillip (Physical Review B, 2016)
We show that the critical fields of the magnetization plateaus of the Shastry-Sutherland model decrease significantly upon increasing the ratio of inter-to intradimer coupling and accordingly that the magnetization plateaus of SrCu2(BO3)(2) shift to lower field under pressure, making the first two plateaus at 1/8 and 2/15 potentially accessible to neutron scattering experiments. These conclusions are based on the derivation of an effective classical model of interacting pinwheel-shaped spin-2 bound states using a combination of perturbative and graph-based continuous unitary transformations, showing that pinwheel crystals are indeed the lowest-energy plateau structures at low magnetization and that a simple model of intermediate-range two-body repulsion between pinwheels is able to account quantitatively for the plateau sequence.

Primordial scalar power spectrum from the Euclidean big bounce - Schander, Susanne; Barrau, Aurelien; Bolliet, Boris; Linsefors, Linda; Mielczarek, Jakub; Grain, Julien (Physical Review D, 2016)
In effective models of loop quantum cosmology, the holonomy corrections are associated with deformations of space-time symmetries. The most evident manifestation of the deformations is the emergence of a Euclidean phase accompanying the nonsingular bouncing dynamics of the scale factor. In this article, we compute the power spectrum of scalar perturbations generated in this model, with a massive scalar field as the matter content. Instantaneous and adiabatic vacuum-type initial conditions for scalar perturbations are imposed in the contracting phase. The evolution through the Euclidean region is calculated based on the extrapolation of the time direction pointed by the vectors normal to the Cauchy hypersurface in the Lorentzian domains. The obtained power spectrum is characterized by a suppression in the IR regime and oscillations in the intermediate energy range. Furthermore, the speculative extension of the analysis in the UV reveals a specific rise of the power leading to results incompatible with the data.

Probing the Fluctuations of Optical Properties in Time-Resolved Spectroscopy - Randi, Francesco; Esposito, Martina; Giusti, Francesca; Misochko, Oleg; Parmigiani, Fulvio; Fausti, Daniele; Eckstein, Martin (Physical Review Letters, 2017)
We show that, in optical pump-probe experiments on bulk samples, the statistical distribution of the intensity of ultrashort light pulses after interaction with a nonequilibrium complex material can be used to measure the time-dependent noise of the current in the system. We illustrate the general arguments for a photoexcited Peierls material. The transient noise spectroscopy allows us to measure to what extent electronic degrees of freedom dynamically obey the fluctuation-dissipation theorem, and how well they thermalize during the coherent lattice vibrations. The proposed statistical measurement developed here provides a new general framework to retrieve dynamical information on the excited distributions in nonequilibrium experiments, which could be extended to other degrees of freedom of magnetic or vibrational origin.

Problem-specific classical optimization of Hamiltonian simulation - Mansuroglu, Refik; Fischer, Felix; Hartmann, Michael (Physical Review Research, 2023)
Nonequilibrium time evolution of large quantum systems is a strong candidate for quantum advantage. Variational quantum algorithms have been put forward for this task, but their quantum optimization routines suffer from trainability and sampling problems. Here, we present a classical preprocessing routine for variational Hamiltonian simulation that circumvents the need for a quantum optimization by expanding rigorous error bounds in a perturbative regime for suitable time steps. The resulting cost function is efficiently computable on a classical computer. We show that there always exists potential for optimization with respect to a Trotter sequence of the same order and that the cost value has the same scaling as Trotter in simulation time and system size. Unlike previous work on classical preprocessing, the method is applicable to any Hamiltonian system independent of locality and interaction lengths. Via numerical experiments for spin-lattice models, we find that our approach significantly improves digital quantum simulation capabilities with respect to Trotter sequences for the same resources. For short times, we find accuracy improvements of more than three orders of magnitude for our method as compared to Trotter sequences of the same gate number. Moreover, for a given gate number and accuracy target, we find that the preoptimization we introduce enables simulation times that are consistently more than ten times longer for a target accuracy of 0.1%.
CRIS-Team Scopus Importer:2023-11-03

Progress in Mimetic Studies of Cell Adhesion and the Mechanosensing - Smith, Ana-Suncana; Sackmann, Erich (ChemPhysChem, 2009)
Vesicle-substrate adhesion has been studied for over two decades with the motivation to understand and mimic cell adhesion. In recent years, with progress in theoretical modelling, the development of experimental techniques, and improved data-analysis procedures, considerable advances hove been made in the understanding of the adhesion process. It is this progress which constitutes the focus of this review.
Keywords: adhesion; cells; mechanosensing; specific binding; vesicles

Projective cluster-additive transformation for quantum lattice models - Hörmann, Max; Schmidt, Kai Phillip (SciPost Physics, 2023)
We construct a projection-based cluster-additive transformation that block-diagonalizes wide classes of lattice Hamiltonians H = H0 + V. Its cluster additivity is an essential ingredient to set up perturbative or non-perturbative linked-cluster expansions for degenerate excitation subspaces of H0. Our transformation generalizes the minimal transformation known amongst others under the names Takahashi’s transformation, Schrieffer-Wolff transformation, des Cloiseaux effective Hamiltonian, canonical van Vleck effective Hamiltonian or two-block orthogonalization method. The effective cluster-additive Hamiltonian and the transformation for a given subspace of H, that is adiabatically connected to the eigenspace of H0 with eigenvalue e0n, solely depends on the eigenspaces of H connected to e0m with e0m ≤ e0n. In contrast, other cluster-additive transformations like the multi-block orthogonalization method or perturbative continuous unitary transformations need a larger basis. This can be exploited to implement the transformation efficiently both perturbatively and non-perturbatively. As a benchmark, we perform perturbative and non-perturbative linked-cluster expansions in the low-field ordered phase of the transverse-field Ising model on the square lattice for single spin-flips and two spin-flip bound-states.
Created from Fastlane, Scopus look-up

Projective limits of state spaces I. Classical formalism - Lanery, Suzanne; Thiemann, Thomas (Journal of Geometry and Physics, 2017)
The present first paper aims at clarifying the classical structures that underlies this formalism, namely projective limits of symplectic manifolds [27, subsection 2.1]. In particular, this allows us to discuss accurately the issues hindering an easy implementation of the dynamics in this context, and to formulate a strategy for overcoming them [27, subsection 4.1]. (C) 2016 Elsevier B.V. All rights reserved.
Keywords: Field theory; Projective limits; Symplectic geometry; Algebras of observables; Constrained Hamiltonian systems

Projective limits of state spaces II. Quantum formalism - Lanery, Suzanne; Thiemann, Thomas (Journal of Geometry and Physics, 2017)
After discussing the formalism at the classical level in a first paper (Lanery, 2017), the present second paper is devoted to the quantum theory. In particular, we inspect in detail how such quantum projective state spaces relate to inductive limit Hilbert spaces and to infinite tensor product constructions (Lanery, 2016, subsection 3.1) [1]. Regarding the quantization of classical projective structures into quantum ones, we extend the results by Okolow (2013), that were set up in the context of linear configuration spaces, to configuration spaces given by simply-connected Lie groups, and to holomorphic quantization of complex phase spaces (Lanky, 2016, subsection 2.2) [1]. (C) 2017 Elsevier B.V. All rights reserved.
Keywords: Quantum field theory; Projective limits; Algebras of observables; Geometric quantization; Position representation; Holomorphic quantization

Projective loop quantum gravity. II. Searching for semi-classical states - Lanery, Suzanne; Thiemann, Thomas (Journal of Mathematical Physics, 2017)
In the first paper of this series, an extension of the Ashtekar-Lewandowski state space of loop quantum gravity was set up with the help of a projective formalism introduced by Kijowski. The motivation for this work was to achieve a more balanced treatment of the position and momentum variables (also known as holonomies and fluxes). While this is the first step toward the construction of states semi-classical with respect to a full set of observables, one uncovers a deeper issue, which we analyse in the present article in the case of real-valued holonomies. Specifically, we show that, in this case, there does not exist any state on the holonomy-flux algebra in which the variances of the holonomy and flux observables would all be finite, let alone small. It is important to note that this obstruction cannot be bypassed by further enlarging the quantum state space, for it arises from the structure of the algebra itself. Away out would be to suitably restrict the algebra of observables: we take the first step in this direction in a companion paper. Published by AIP Publishing.

Projective loop quantum gravity. I. State space - Lanery, Suzanne; Thiemann, Thomas (Journal of Mathematical Physics, 2016)
Instead of formulating the state space of a quantum field theory over one big Hilbert space, it has been proposed by Kijowski to describe quantum states as projective families of density matrices over a collection of smaller, simpler Hilbert spaces. Beside the physical motivations for this approach, it could help designing a quantum state space holding the states we need. In a latter work by Okolow, the description of a theory of Abelian connections within this framework was developed, an important insight being to use building blocks labeled by combinations of edges and surfaces. The present work generalizes this construction to an arbitrary gauge group G (in particular, G is neither assumed to be Abelian nor compact). This involves refining the definition of the label set, as well as deriving explicit formulas to relate the Hilbert spaces attached to different labels. If the gauge group happens to be compact, we also have at our disposal the well-established Ashtekar-Lewandowski Hilbert space, which is defined as an inductive limit using building blocks labeled by edges only. We then show that the quantum state space presented here can be thought as a natural extension of the space of density matrices over this Hilbert space. In addition, it is manifest from the classical counterparts of both formalisms that the projective approach allows for a more balanced treatment of the holonomy and flux variables, so it might pave the way for the development of more satisfactory coherent states. Published by AIP Publishing.

Projective State Spaces for Theories of Connections - Lanéry, Suzanne; Thiemann, Thomas (Friedrich-Alexander-Universität Erlangen-Nürnberg, 2015)

Proliferation of anomalous symmetries in colloidal monolayers subjected to quasiperiodic light fields - Mikhael, Jules; Schmiedeberg, Michael; Rausch, Sebastian; Roth, Johannes; Stark, Holger; Bechinger, Clemens (Proceedings of the National Academy of Sciences of the United States of America, 2010)
Quasicrystals provide a fascinating class of materials with intriguing properties. Despite a strong potential for numerous technical applications, the conditions under which quasicrystals form are still poorly understood. Currently, it is not clear why most quasicrystals hold 5- or 10-fold symmetry but no single example with 7- or 9-fold symmetry has ever been observed. Here we report on geometrical constraints which impede the formation of quasicrystals with certain symmetries in a colloidal model system. Experimentally, colloidal quasicrystals are created by subjecting micron-sized particles to two-dimensional quasiperiodic potential landscapes created by n = 5 or seven laser beams. Our results clearly demonstrate that quasicrystalline order is much easier established for n 5 compared to n = 7. With increasing laser intensity we observe that the colloids first adopt quasiperiodic order at local areas which then laterally grow until an extended quasicrystalline layer forms. As nucleation sites where quasiperiodicity originates, we identify highly symmetric motifs in the laser pattern. We find that their density strongly varies with n and surprisingly is smallest exactly for those quasicrystalline symmetries which have never been observed in atomic systems. Since such high-symmetry motifs also exist in atomic quasicrystals where they act as preferential adsorption sites, this suggests that it is indeed the deficiency of such motifs which accounts for the absence of materials with e. g., 7-fold symmetry.
Keywords: 7-fold symmetry; growth mechanism; light patterns

Properties of odd nuclei and the impact of time-odd mean fields: A systematic Skyrme-Hartree-Fock analysis - Pototzky, K. J.; Erler, J.; Reinhard, Paul-Gerhard; Nesterenko, V. O. (European Physical Journal A, 2010)
We present a systematic analysis of the description of odd nuclei by the Skyrme-Hartree-Fock approach augmented with pairing in BCS approximation and blocking of the odd nucleon. Current and spin densities in the Skyrme functional produce time-odd mean fields (TOMF) for odd nuclei. Their effect on basic properties (binding energies, odd-even staggering, separation energies and spectra) is investigated for the three Skyrme parameterizations SkI3, SLy6, and SV-bas. About 1300 spherical and axially deformed odd nuclei with 16 ≤ Z ≤ 92 are considered. The calculations demonstrate that the TOMF effect is generally small, although not fully negligible. The influence of the Skyrme parameterization and the consistency of the calculations are much more important. With a proper choice of the parameterization, a good description of binding energies and their differences is obtained, comparable to that for even nuclei. The description of low-energy excitation spectra of odd nuclei is of varying quality depending on the nucleus. © 2010 SIF, Springer-Verlag Berlin Heidelberg.
CRIS-Team Scopus Importer:2023-07-24

Prospects of probing dark matter condensates with gravitational waves - Banerjee, Shreya; Bera, Sayantani; Mota, David F. (Journal of Cosmology and Astroparticle Physics, 2023)
The Lambda-Cold Dark Matter model explains cosmological observations most accurately till date. However, it is still plagued with various shortcomings at galactic scales. Models of dark matter such as superfluid dark matter, Bose-Einstein Condensate(BEC) dark matter and fuzzy dark matter have been proposed to overcome some of these drawbacks. In this work, we probe these models using the current constraint on the gravitational wave (GW) propagation speed coming from the binary neutron star GW170817 detection by LIGO-Virgo detector network and use it to study the allowed parameter space for these three models for Advanced LIGO+Virgo, LISA, IPTA and SKA detection frequencies. The speed of GW has been shown to depend upon the refractive index of the medium, which in turn, depends on the dark matter model parameters through the density profile of the galactic halo. We constrain the parameter space for these models using the bounds coming from GW speed measurement and the Milky Way radius bound. Our findings suggest that with Advanced LIGO-Virgo detector sensitivity, the three models considered here remain unconstrained. A meaningful constraint can only be obtained for detection frequencies ≤ 10-9 Hz, which falls in the detection range of radio telescopes such as IPTA and SKA. Considering this best possible case, we find that out of the three condensate models, the fuzzy dark matter model is the most feasible scenario to be falsified/validated in near future.
CRIS-Team Scopus Importer:2023-04-07
Keywords: dark matter theory; gravitational waves / theory

Protein induced lipid demixing in homogeneous membranes - Stumpf, Henning; Nowakowski, Piotr; Eggeling, Christian; Maciolek, Anna; Smith, Ana-Suncana (Physical Review Research, 2021)
Specific lipid environments are necessary for the establishment of protein signaling platforms in membranes, yet their origin has been highly debated. We present a continuum, exactly solvable model of protein induced local demixing of lipid membranes. The coupling between a local composition and a local thickness of the membrane induces lipid domains around inclusions with hydrophobic mismatch, even for temperatures above the miscibility critical point of the membrane. The model qualitatively explains the experimentally observed formation of lipid domains induced by anchoring of reconstituted actin in flat supported lipid bilayers.
CRIS-Team WoS Importer:2021-11-12

Pseudomagnetic fields for sound at the nanoscale - Brendel, Christian; Peano, Vittorio; Painter, Oskar J.; Marquardt, Florian (Proceedings of the National Academy of Sciences of the United States of America, 2017)
There is a growing effort in creating chiral transport of sound waves. However, most approaches so far have been confined to the macroscopic scale. Here, we propose an approach suitable to the nanoscale that is based on pseudomagnetic fields. These pseudomagnetic fields for sound waves are the analogue of what electrons experience in strained graphene. In our proposal, they are created by simple geometrical modifications of an existing and experimentally proven phononic crystal design, the snowflake crystal. This platform is robust, scalable, and well-suited for a variety of excitation and readout mechanisms, among them optomechanical approaches.
CRIS-Team Scopus Importer:2023-03-27
Keywords: Nanomechanics; Optomechanics; Phononic crystal; Pseudomagnetic field; Topological physics

Pseudoparticle vertex solver for quantum impurity models - Kim, Aaram J.; Li, Jiajun; Eckstein, Martin; Werner, Philipp (Physical Review B, 2022)
We present a quantum impurity solver based on a pseudoparticle framework, which combines diagrammatic resummations for a three-point vertex with diagrammatic Monte Carlo sampling of a four-point vertex. This recently proposed approach (A. J. Kim, arXiv:2112.15549) is generalized here to fermionic impurity problems and we discuss the technical details of the implementation, including the time-stepping approach, the Monte Carlo updates, and the routines for checking the two-particle irreducibility of the four-point vertex. We also explain how the vertex information can be efficiently stored using a Dubiner basis representation. The convergence properties of the algorithm are demonstrated with applications to exactly solvable impurity models and dynamical mean field theory simulations of the single-orbital Hubbard model. It is furthermore shown that the algorithm can handle a two-orbital problem with off-diagonal hybridizations, which would cause a severe sign problem in standard hybridization-expansion Monte Carlo simulations. Since the vertex-based algorithm successfully handles sign-oscillating integrals in equilibrium and samples only connected diagrams, it may be a promising approach for real-time simulations.
CRIS-Team Scopus Importer:2022-09-02

Pulling tethers from adhered vesicles - Smith, Ana-Suncana; Sackmann, Erich; Seifert, Udo (Physical Review Letters, 2004)
The competition between adhesion and tether formation in bound vesicles is investigated. A theoretical model is developed in which tethers are induced by the application of a pulling force to the top of a strongly adhered vesicle. A critical onset force is identified where the tether spontaneously appears as part of a first order shape transition. Further growth of the tether initiates a detachment process that culminates in a continuous unbinding of the vesicle at a finite detachment force. Both critical forces, as well as all shape parameters, are calculated as a function of the reduced volume and the strength of adhesive potential.
Keywords: BENDING STIFFNESS; BLOOD-CELLS; FORCE; GIANT; ADHESION; SHAPES

Purely entropic self-assembly of the bicontinuous Ia3d gyroid phase in equilibrium hard-pear systems - Schoenhoefer, Philipp W. A.; Ellison, Laurence J.; Marechal, Mattheus; Cleaver, Douglas J.; Schroder-Turk, Gerd E. (Interface Focus, 2017)
We investigate a model of hard pear-shaped particles which forms the bicontinuous Ia[Formula: see text]d structure by entropic self-assembly, extending the previous observations of Barmes et al. (2003 Phys. Rev. E 68, 021708. (doi:10.1103/PhysRevE.68.021708)) and Ellison et al. (2006 Phys. Rev. Lett. 97, 237801. (doi:10.1103/PhysRevLett.97.237801)). We specifically provide the complete phase diagram of this system, with global density and particle shape as the two variable parameters, incorporating the gyroid phase as well as disordered isotropic, smectic and nematic phases. The phase diagram is obtained by two methods, one being a compression-decompression study and the other being a continuous change of the particle shape parameter at constant density. Additionally, we probe the mechanism by which interdigitating sheets of pears in these systems create surfaces with negative Gauss curvature, which is needed to form the gyroid minimal surface. This is achieved by the use of Voronoi tessellation, whereby both the shape and volume of Voronoi cells can be assessed in regard to the local Gauss curvature of the gyroid minimal surface. Through this, we show that the mechanisms prevalent in this entropy-driven system differ from those found in systems which form gyroid structures in nature (lipid bilayers) and from synthesized materials (di-block copolymers) and where the formation of the gyroid is enthalpically driven. We further argue that the gyroid phase formed in these systems is a realization of a modulated splay-bend phase in which the conventional nematic has been predicted to be destabilized at the mesoscale due to molecular-scale coupling of polar and orientational degrees of freedom.
EAM Import::2019-03-08

Quantitative Structural Organization of Bulk Apical Membrane Traffic in Pollen Tubes - Grebnev, Gleb; Cvitkovic, Mislav; Fritz, Carolin; Cai, Giampiero; Smith, Ana-Suncana; Kost, Benedikt (Plant Physiology, 2020)
Pollen tube tip growth depends on balancing secretion of cell wall material with endocytic recycling of excess material incorporated into the plasma membrane (PM). The classical model of tip growth, which predicts bulk secretion, occurs apically, and is compensated by subapical endocytosis, has been challenged in recent years. Many signaling proteins and lipids with important functions in the regulation of membrane traffic underlying tip growth associate with distinct regions of the pollen tube PM, and understanding the mechanisms responsible for the targeting of these regulatory factors to specific PM domains requires quantitative information concerning the sites of bulk secretion and endocytosis. Here, we quantitatively characterized the spatial organization of membrane traffic during tip growth by analyzing steady-state distributions and dynamics of FM4-64-labeled lipids and YFP-tagged transmembrane (TM) proteins in tobacco (Nicotiana tabacum) pollen tubes growing normally or treated with Brefeldin A to block secretion. We established that (1) secretion delivers TM proteins and recycled membrane lipids to the same apical PM domain, and (2) FM4-64-labeled lipids, but not the analyzed TM proteins, undergo endocytic recycling within a clearly defined subapical region. We mathematically modeled the steady-state PM distributions of all analyzed markers to better understand differences between them and to support the experimental data. Finally, we mapped subapical F-actin fringe and trans-Golgi network positioning relative to sites of bulk secretion and endocytosis to further characterize functions of these structures in apical membrane traffic. Our results support and further define the classical model of apical membrane traffic at the tip of elongating pollen tubes.
CRIS-Team Scopus Importer:2020-08-14

QUANTIZATION OF DIFFEOMORPHISM INVARIANT THEORIES OF CONNECTIONS WITH LOCAL DEGREES OF FREEDOM - Ashtekar, Abhay; Lewandowski, Jerzy; Marolf, Donald; Mourao, José Manuel; Thiemann, Thomas (Journal of Mathematical Physics, 1995)
Quantization of diffeomorphism invariant theories of connections is studied and the quantum diffeomorphism constraint is solved. The space of solutions is equipped with an inner product that is shown to satisfy the physical reality conditions. This provides, in particular, a quantization of the Husain-Kuchar model. The main results also pave the way to quantization of other diffeomorphism invariant theories such as general relativity. In the Riemannian case (i.e., signature ++++), the approach appears to contain all the necessary ingredients already. In the Lorentzian case, it will have to be combined in an appropriate fashion with a coherent state transform to incorporate complex connections. (C) 1995 American Institute of Physics.

Quantum algorithms with local particle-number conservation: Noise effects and error correction - Streif, Michael; Leib, Martin; Wudarski, Filip; Rieffel, Eleanor; Wang, Zhihui (Physical Review A, 2021)
Quantum circuits with local particle-number conservation restrict the quantum computation to a subspace of the Hilbert space of the qubit register. In a noiseless or fault-tolerant quantum computation, such quantities are preserved. In the presence of noise, however, the evolution's symmetry could be broken and nonvalid states could be sampled at the end of the computation. On the other hand, the restriction to a subspace in the ideal case suggests the possibility of more resource-efficient error-mitigation techniques for circuits preserving symmetries that are not possible for general circuits. Here, we analyze the probability of staying in such symmetry-preserved subspaces under noise, providing an exact formula for local depolarizing noise. We apply our findings to benchmark, under depolarizing noise, the symmetry robustness of the XY quantum alternating operator Ansatz, which has local particle-number conserving symmetries and is a special case of the quantum alternating operator Ansatz. We also analyze the influence of the choice of encoding the problem on the symmetry robustness of the algorithm and discuss a simple adaption of the bit-flip code to correct for symmetry-breaking errors with reduced resources.
CRIS-Team Scopus Importer:2021-04-30

Quantum approximate optimization of non-planar graph problems on a planar superconducting processor - Harrigan, Matthew P.; Sung, Kevin J.; Neeley, Matthew; Satzinger, Kevin J.; Arute, Frank; Arya, Kunal; Atalaya, Juan; Bardin, Joseph C.; Barends, Rami; Boixo, Sergio; Broughton, Michael; Buckley, Bob B.; et al. (Nature Physics, 2021)
Faster algorithms for combinatorial optimization could prove transformative for diverse areas such as logistics, finance and machine learning. Accordingly, the possibility of quantum enhanced optimization has driven much interest in quantum technologies. Here we demonstrate the application of the Google Sycamore superconducting qubit quantum processor to combinatorial optimization problems with the quantum approximate optimization algorithm (QAOA). Like past QAOA experiments, we study performance for problems defined on the planar connectivity graph native to our hardware; however, we also apply the QAOA to the Sherrington–Kirkpatrick model and MaxCut, non-native problems that require extensive compilation to implement. For hardware-native problems, which are classically efficient to solve on average, we obtain an approximation ratio that is independent of problem size and observe that performance increases with circuit depth. For problems requiring compilation, performance decreases with problem size. Circuits involving several thousand gates still present an advantage over random guessing but not over some efficient classical algorithms. Our results suggest that it will be challenging to scale near-term implementations of the QAOA for problems on non-native graphs. As these graphs are closer to real-world instances, we suggest more emphasis should be placed on such problems when using the QAOA to benchmark quantum processors.
CRIS-Team Scopus Importer:2021-03-05

Quantum Boltzmann equation for strongly correlated electrons - Picano, Antonio; Li, Jiajun; Eckstein, Martin (Physical Review B, 2021)
Collective orders and photoinduced phase transitions in quantum matter can evolve on timescales which are orders of magnitude slower than the femtosecond processes related to electronic motion in the solid. Quantum Boltzmann equations can potentially resolve this separation of timescales, but are often constructed by assuming the existence of quasiparticles. Here we derive a quantum Boltzmann equation which only assumes a separation of timescales (taken into account through the gradient approximation for convolutions in time), but is based on a nonperturbative scattering integral, and makes no assumption on the spectral function such as the quasiparticle approximation. In particular, a scattering integral corresponding to nonequilibrium dynamical mean-field theory is evaluated in terms of an Anderson impurity model in a nonequilibrium steady state with prescribed distribution functions. This opens the possibility to investigate dynamical processes in correlated solids with quantum impurity solvers designed for the study of nonequilibrium steady states.
CRIS-Team Scopus Importer:2021-08-20

Quantum-coherent phase oscillations in synchronization - Weiß, Talitha; Walter, Stefan; Marquardt, Florian (Physical Review A, 2017)
Recently, several studies have investigated synchronization in quantum-mechanical limit-cycle oscillators. However, the quantum nature of these systems remained partially hidden, since the dynamics of the oscillator's phase was overdamped and therefore incoherent. We show that there exist regimes of underdamped and even quantum-coherent phase motion, opening up new possibilities to study quantum synchronization dynamics. To this end, we investigate the Van der Pol oscillator (a paradigm for a self-oscillating system) synchronized to an external drive. We derive an effective quantum model which fully describes the regime of underdamped phase motion and additionally allows us to identify the quality of quantum coherence. Finally, we identify quantum limit cycles of the phase itself.
CRIS-Team Scopus Importer:2023-03-27

Quantum cosmological backreactions. I. Cosmological space adiabatic perturbation theory - Schander, Susanne; Thiemann, Thomas (Physical Review D, 2022)
Quantum cosmology, including quantum cosmological inhomogeneities, is a promising framework for describing the very early universe in which all degrees of freedom are being considered as dynamical and quantum. However, many previous discussions on this subject consider the quantum cosmological perturbations as test fields on a curved spacetime with effective quantum modifications and thus neglect certain interactions of the subsystems, namely the backreaction of the inhomogeneous quantum fields on the cosmological background. In a series of four papers, of which this is the first, we aim at improving on the treatment of quantum effects that arise due to backreactions between matter and geometry as well as between the cosmological perturbations and the homogeneous degrees of freedom. We employ the technique of space adiabatic perturbation theory in the form developed by Panati, Spohn, and Teufel which relies on the familiar Born-Oppenheimer approximation. We extend the scheme to quantum field theory in the cosmological perturbative setting and show that this leads to presently neglected correction terms in the quantum Friedmann equations. In this first article, we provide a detailed introduction to the iterative scheme and examine the generic challenges encountered in its application to perturbative quantum cosmology. Our results will allow for a direct comparison to cosmological observations.
CRIS-Team WoS Importer:2022-06-24

Quantum cosmological backreactions. II. Purely homogeneous quantum cosmology - Neuser, Jonas; Schander, Susanne; Thiemann, Thomas (Physical Review D, 2022)
In this second paper in a series of four, we continue with the program of incorporating backreaction among the homogeneous and between the homogeneous and inhomogeneous degrees of freedom in quantum cosmological perturbation theory. The purpose of the present paper is to illustrate the formalism of space adiabatic perturbation theory for two simple quantum mechanical models, and to prove that backreaction indeed leads to additional correction terms in effective Hamiltonians that one would otherwise neglect in a crude Born-Oppenheimer approximation. The first model consists of a harmonic oscillator coupled to an anharmonic oscillator. The second model describes the coupling between a scalar matter field and gravity restricted to the purely homogeneous and isotropic sector. These results have potential phenomenological consequences for quantum cosmological models.
CRIS-Team WoS Importer:2022-06-24

Quantum cosmological backreactions. IV. Constrained quantum cosmological perturbation theory - Schander, Susanne; Thiemann, Thomas (Physical Review D, 2022)
This is the final paper in a series of four in which we incorporate backreaction among the homogeneous and between the homogeneous and inhomogeneous degrees of freedom in quantum cosmological perturbation theory using space adiabatic methods. Here, we consider the gauge-invariant scalar (Mukhanov-Sasaki) and tensor (primordial gravitational wave) perturbations of general relativity coupled to a scalar field which arise from a careful constraint analysis of this system up to second order in the perturbations. The simultaneous quantization of the homogeneous and inhomogeneous degrees of freedom requires the use of a suitable constructive perturbation scheme for which we employ space adiabatic perturbation theory, a rigorous extension of the standard Born-Oppenheimer theory. We are confronted with several challenges arising for theories with an infinite number of degrees of freedom. We are able to compute the backreaction effects up to second order in the adiabatic parameter and find modifications as compared to earlier derivations of the effective quantum dynamics of the homogeneous sector.
CRIS-Team WoS Importer:2022-06-24

Quantum criticality and excitations of a long-range anisotropic XY chain in a transverse field - Adelhardt, Patrick; Koziol, Jan; Schellenberger, Andreas; Schmidt, Kai Phillip (Physical Review B, 2020)
The critical breakdown of a one-dimensional quantum magnet with long-range interactions is studied by investigating the high-field polarized phase of the anisotropic XY model in a transverse field for the ferromagnetic and antiferromagnetic cases. While for the limiting case of the isotropic long-range XY model we can extract the elementary one-quasiparticle dispersion analytically and calculate two-quasiparticle excitation energies quantitatively in a numerical fashion, for the long-range Ising limit as well as in the intermediate regime we use perturbative continuous unitary transformations on white graphs in combination with classical Monte Carlo simulations for the graph embedding to extract high-order series expansions in the thermodynamic limit. This enables us to determine the quantum-critical breakdown of the high-field polarized phase by analyzing the gap closing including associated critical exponents and multiplicative logarithmic corrections. In addition, for the ferromagnetic isotropic XY model we determined the critical exponents z and v analytically by a bosonic quantum-field theory.
CRIS-Team WoS Importer:2020-11-27

Quantum criticality of the transverse-field Ising model with long-range interactions on triangular-lattice cylinders - Koziol, Jan; Fey, Sebastian; Kapfer, Sebastian; Schmidt, Kai Phillip (Physical Review B, 2019)
To gain a better understanding of the interplay between frustrated long-range interactions and zero-temperature quantum fluctuations, we investigate the ground-state phase diagram of the transverse-field Ising model with algebraically decaying long-range Ising interactions on quasi-one-dimensional infinite-cylinder triangular lattices. Technically, we apply various approaches including low- and high-field series expansions. For the classical long-range Ising model, we investigate cylinders with an arbitrary even circumference. We show the occurrence of gapped stripe-ordered phases emerging out of the infinitely degenerate nearest-neighbor Ising ground-state space on the two-dimensional triangular lattice. Further, while cylinders with circumferences 6, 10, 14, etc., are always in the same stripe phase for any decay exponent of the long-range Ising interaction, the family of cylinders with circumferences 4, 8, 12, etc., displays a phase transition between two different types of stripe structures. For the full long-range transverse-field Ising model, we concentrate on cylinders with circumference four and six. The ground-state phase diagram consists of several quantum phases in both cases including an x-polarized phase, stripe-ordered phases, and clock-ordered phases which emerge from an order-by-disorder scenario already present in the nearest-neighbor model. In addition, the generic presence of a potential intermediate gapless phase with algebraic correlations and associated Kosterlitz-Thouless transitions is discussed for both cylinders.
CRIS-Team WoS Importer:2019-10-22

Quantum Criticality of Two-Dimensional Quantum Magnets with Long-Range Interactions - Fey, Sebastian; Kapfer, Sebastian; Schmidt, Kai Phillip (Physical Review Letters, 2019)
We study the critical breakdown of two-dimensional quantum magnets in the presence of algebraically decaying long-range interactions by investigating the transverse-field Ising model on the square and triangular lattice. This is achieved technically by combining perturbative continuous unitary transformations with classical Monte Carlo simulations to extract high-order series for the one-particle excitations in the high-field quantum paramagnet. We find that the unfrustrated systems change from mean-field to nearest-neighbor universality with continuously varying critical exponents. In the frustrated case on the square lattice the system remains in the universality class of the nearest-neighbor model independent of the long-range nature of the interaction, while we argue that the quantum criticality for the triangular lattice is terminated by a first-order phase transition line.
CRIS-Team Scopus Importer:2019-05-14

Quantum critical phase transition between two topologically ordered phases in the Ising tonic code bilayer - Wiedmann, Raymond; Lenke, Lea; Walther, Matthias; Mühlhauser, Matthias; Schmidt, Kai Phillip (Physical Review B, 2020)
We demonstrate that two toric code layers on the square lattice coupled by an Ising interaction display two distinct phases with intrinsic topological order. The second-order quantum phase transition between the weakly coupled Z(2) x Z(2) and the strongly coupled Z(2) topological order can be described by the condensation of bosonic quasiparticles from both sides and belongs to the three-dimensional Ising* universality class. This can be shown by an exact duality transformation to the transverse-field Ising model on the square lattice, which builds on the existence of an extensive number of local Z(2) conserved parities. These conserved quantities correspond to the product of two adjacent star operators on different layers. Notably, we show that the low-energy effective model derived about the limit of large Ising coupling is given by an effective single-layer toric code in terms of the conserved quantities of the Ising toric code bilayer. The two topological phases are further characterized by the topological entanglement entropy which serves as a nonlocal order parameter.
CRIS-Team WoS Importer:2021-01-08

Quantum-critical properties of the long-range transverse-field Ising model from quantum Monte Carlo simulations - Koziol, Jan; Langheld, Anja; Kapfer, Sebastian; Schmidt, Kai Phillip (Physical Review B, 2021)
The quantum-critical properties of the transverse-field Ising model with algebraically decaying interactions are investigated by means of stochastic series expansion quantum Monte Carlo, on both the one-dimensional linear chain and the two-dimensional square lattice. We extract the critical exponents nu and beta as a function of the decay exponent of the long-range interactions. For ferromagnetic Ising interactions, we resolve the limiting regimes known from field theory, ranging from the nearest-neighbor Ising to the long-range Gaussian universality classes, as well as the intermediate regime with continuously varying critical exponents. In the long-range Gaussian regime, we treat the effect of dangerous irrelevant variables on finite-size scaling forms. For antiferromagnetic and therefore competing Ising interactions, the stochastic series expansion algorithm displays growing autocorrelation times leading to a reduced performance. Nevertheless, our results are consistent with the nearest-neighbor Ising universality for all investigated interaction ranges both on the linear chain and the square lattice.
CRIS-Team WoS Importer:2021-07-09

Quantum disorder and local modes of the fully-frustrated transverse field Ising model on a diamond chain - Coester, K.; Malitz, W.; Fey, S.; Schmidt, Kai Phillip (Physical Review B, 2013)
We investigate the transverse field Ising model on a diamond chain using series expansions about the high-field limit and exact diagonalizations. For the unfrustrated case we accurately determine the quantum critical point and its expected 2d Ising universality separating the polarized and the Z(2) symmetry broken phase. In contrast, we find strong evidence for a disorder by disorder scenario for the fully-frustrated transverse field Ising model, i.e., except for the pure Ising model, having an extensive number of ground states, the system is always in a quantum disordered polarized phase. The low-energy excitations in this polarized phase are understood in terms of exact local modes of the model. Furthermore, an effective low-energy description for an infinitesimal transverse field allows us to pinpoint the quantum disordered nature of the ground state via mapping to an effective transverse field Ising chain and to determine the induced gap to the elementary effective domain wall excitation very accurately.

Quantum dynamical screening of the local magnetic moment in Fe-based superconductors - Toschi, Alessandro; Arita, Ryotaro; Hansmann, Philipp; Sangiovanni, Giorgio; Held, Karsten (Physical Review B, 2012)
We have calculated the local magnetic susceptibility of one of the prototypical Fe-based superconductors (LaFeAsO) by means of the local density approximation + dynamical mean field theory as a function of both (imaginary) time and real frequencies with and without vertex corrections. Vertex corrections are essential for obtaining the correct omega dependence, in particular, a pronounced low-energy peak at omega similar to 0.2 eV, which constitutes the hallmark of the dynamical screening of a large instantaneous magnetic moment on the Fe atoms. In experiments, however, except for the case of x-ray absorption spectroscopy, the magnetic moment or the susceptibility represent typically the average over long time scales. In this respect, the frequency range of typical neutron experiments would be too limited to directly estimate the magnitude of the short-time moment.
CRIS-Team WoS Importer:2020-02-19

Quantum geometry - Giesel, Kristina (Loop Quantum Gravity - The First 30 Years, 2017)
CRIS-Team Scopus Importer:2022-05-09

Quantum gravity phenomenology at the dawn of the multi-messenger era—A review - Addazi, A.; Alvarez-Muniz, J.; Alves Batista, R.; Amelino-Camelia, G.; Antonelli, V.; Arzano, M.; Asorey, M.; Atteia, J. L.; Bahamonde, S.; Bajardi, F.; Ballesteros, A.; Baret, B.; et al. (Progress in Particle and Nuclear Physics, 2022)
The exploration of the universe has recently entered a new era thanks to the multi-messenger paradigm, characterized by a continuous increase in the quantity and quality of experimental data that is obtained by the detection of the various cosmic messengers (photons, neutrinos, cosmic rays and gravitational waves) from numerous origins. They give us information about their sources in the universe and the properties of the intergalactic medium. Moreover, multi-messenger astronomy opens up the possibility to search for phenomenological signatures of quantum gravity. On the one hand, the most energetic events allow us to test our physical theories at energy regimes which are not directly accessible in accelerators; on the other hand, tiny effects in the propagation of very high energy particles could be amplified by cosmological distances. After decades of merely theoretical investigations, the possibility of obtaining phenomenological indications of Planck-scale effects is a revolutionary step in the quest for a quantum theory of gravity, but it requires cooperation between different communities of physicists (both theoretical and experimental). This review, prepared within the COST Action CA18108 “Quantum gravity phenomenology in the multi-messenger approach”, is aimed at promoting this cooperation by giving a state-of-the art account of the interdisciplinary expertise that is needed in the effective search of quantum gravity footprints in the production, propagation and detection of cosmic messengers.
CRIS-Team Scopus Importer:2022-04-01
Keywords: Cosmic neutrinos; Gamma-ray astronomy; Gravitational waves; Lorentz invariance violation and deformation; Ultra-high-energy cosmic rays

Quantum Imprints on CMBR - Banerjee, Shreya (Universe, 2023)
Quantum cosmology aims to develop a quantum theory of the universe, attempting to answer open questions of physical cosmology, mainly related to the early epochs of the universe. Such a theory aims to unite relativity theory and quantum theory. Here, the whole universe is treated as a quantum mechanical system and is described by a wave function rather than by a classical spacetime. In this review, I shall describe the mathematical structure and primary formulations that form the backbone of quantum cosmology. We know that over a period of time, several approaches were developed to form a quantum theory of gravity. However, in order to decide which approach is the best, we need testable predictions, effects that can be observed in cosmic microwave background radiation (CMBR). I shall discuss the methodologies for generating quantum gravitational corrections to inflationary background leading to testable predictions. Another aspect of finding quantum imprints on CMBR results through the application of resolution of the ‘quantum measurement problem’ to early universe physics. In this article, I shall also discuss two such promising models explaining the classicalization of inflationary perturbation and are capable of leaving distinct observational imprints on the observables.
CRIS-Team Scopus Importer:2023-10-06
Keywords: cosmic microwave background; inflation; quantum cosmology; Wheeler–DeWitt

Quantum Interference and Decoherence in Single-Molecule Junctions: How Vibrations Induce Electrical Current - Härtle, Rainer; Butzin, M.; Rubio-Pons, O.; Thoss, Michael (Physical Review Letters, 2011)
Quantum interference and decoherence in single-molecule junctions is analyzed employing a nonequilibrium Green's function approach. Electrons tunneling through quasidegenerate states of a molecular junction exhibit interference effects. We show that electronic-vibrational coupling, inherent to any molecular junction, strongly quenches such interference effects. This decoherence mechanism may cause significantly larger electrical currents and is particularly pronounced if the junction is vibrationally highly excited, e.g., due to inelastic processes in the resonant transport regime. © 2011 American Physical Society.
UnivIS-Import:2015-03-09:Pub.2011.nat.dphy.ITP.ppem.quantu

Quantum Mechanics in the Polymer Particle Representation - Reichert, Thorsten; Giesel, Kristina (Friedrich-Alexander-Universität Erlangen-Nürnberg, 2013)

Quantum Natural Policy Gradients: Towards Sample-Efficient Reinforcement Learning - Meyer, Nico; Scherer, Daniel D.; Plinge, Axel; Mutschler, Christopher; Hartmann, Michael (Proceedings - 2023 IEEE International Conference on Quantum Computing and Engineering, QCE 2023, 2023)
Reinforcement learning is a growing field in AI with a lot of potential. Intelligent behavior is learned automatically through trial and error in interaction with the environment. However, this learning process is often costly. Using variational quantum circuits as function approximators potentially can reduce this cost. In order to implement this, we propose the quantum natural policy gradient (QNPG) algorithm - a second-order gradient-based routine that takes advantage of an efficient approximation of the quantum Fisher information matrix. We experimentally demonstrate that QNPG outperforms first-order based training on different Contextual Bandits environments regarding convergence speed and stability and moreover reduces the sample complexity. Furthermore, we provide evidence for the practical feasibility of our approach by training on a 12-qubit hardware device.
CRIS-Team Scopus Importer:2023-12-29
Keywords: contextual bandits; natural gradient; policy gradient; reinforcement learning; variational quantum computing

Quantum nondemolition measurement of mechanical motion quanta - Dellantonio, Luca; Kyriienko, Oleksandr; Marquardt, Florian; Sorensen, Anders S. (Nature Communications, 2018)
The fields of optomechanics and electromechanics have facilitated numerous advances in the areas of precision measurement and sensing, ultimately driving the studies of mechanical systems into the quantum regime. To date, however, the quantization of the mechanical motion and the associated quantum jumps between phonon states remains elusive. For optomechanical systems, the coupling to the environment was shown to make the detection of the mechanical mode occupation difficult, typically requiring the single-photon strong-coupling regime. Here, we propose and analyse an electromechanical setup, which allows us to overcome this limitation and resolve the energy levels of a mechanical oscillator. We found that the heating of the membrane, caused by the interaction with the environment and unwanted couplings, can be suppressed for carefully designed electromechanical systems. The results suggest that phonon number measurement is within reach for modern electromechanical setups.
CRIS-Team Scopus Importer:2023-03-27

Quantum phase transitions in the K-layer Ising toric code - Schamriss, Lukas; Lenke, Lea; Mühlhauser, Matthias; Schmidt, Kai Phillip (Physical Review B, 2022)
We investigate the quantum phase diagram of the K-layer Ising toric code corresponding to K layers of twodimensional toric codes coupled by Ising interactions. While for small Ising interactions the system displays 1K2 topological order originating from the toric codes in each layer, the system shows 12 topological order in the high-Ising limit. The latter is demonstrated for general K by deriving an effective low-energy model in Kth-order degenerate perturbation theory, which is given as an effective anisotropic single-layer toric code in terms of collective pseudospins 1/2 referring to the two ground states of isolated Ising chain segments. For the specific cases K = 3 and K = 4 we apply high-order series expansions to determine the gap series in the low- and high-Ising limit. Extrapolation of the elementary energy gaps gives convincing evidence that the ground-state phase diagram consists of a single quantum critical point and our findings suggest a quantum phase transition in the 3D Ising* universality class for both K separating both types of topological order, which is consistent with former findings for the bilayer Ising toric code.
CRIS-Team WoS Importer:2022-07-01

Quantum phase transitions out of a Z(2) x Z(2) topological phase - Jahromi, Saeed S.; Masoudi, S. Farhad; Kargarian, Mehdi; Schmidt, Kai Phillip (Physical Review B, 2013)
We investigate the low-energy spectral properties and robustness of the topological phase of color code, which is a quantum spin model for the aim of fault-tolerant quantum computation, in the presence of a uniform magnetic field or Ising interactions, using high-order series expansion and exact diagonalization. In a uniform magnetic field, we find first-order phase transitions in all field directions. In contrast, our results for the Ising interactions unveil that for strong enough Ising couplings, the Z(2) x Z(2) topological phase of color code breaks down to symmetry broken phases by first-or second-order phase transitions.

Quantum phase transitions to topological Haldane phases in spin-one chains studied by linked-cluster expansions - Adelhardt, Patrick; Gritsch, Julian; Hille, M.; Reiß, David; Schmidt, Kai Phillip (Physical Review B, 2017)
We use linked-cluster expansions to analyze the quantum phase transitions between symmetry-unbroken trivial and topological Haldane phases in two different spin-one chains. The first model is the spin-one Heisenberg chain in the presence of a single-ion anisotropy, while the second one is the dimerized spin-one Heisenberg chain. For both models, we determine the ground-state energy and the one-particle gap inside the nontopological phase as a high-order series using perturbative continuous unitary transformations. Extrapolations of the gap series are applied to locate the quantum critical point and to extract the associated critical exponent. We find that this approach works unsatisfactorily for the anisotropic chain, since the quality of the extrapolation appears insufficient due to the large correlation length exponent. In contrast, extrapolation schemes display very good convergence for the gap closing in the case of the dimerized spin-one Heisenberg chain.

Quantum Policy Gradient Algorithm with Optimized Action Decoding - Meyer, Nico; Scherer, Daniel D.; Plinge, Axel; Mutschler, Christopher; Hartmann, Michael (Proceedings of Machine Learning Research, 2023)
Quantum machine learning implemented by variational quantum circuits (VQCs) is considered a promising concept for the noisy intermediate-scale quantum computing era. Focusing on applications in quantum reinforcement learning, we propose an action decoding procedure for a quantum policy gradient approach. We introduce a quality measure that enables us to optimize the classical post-processing required for action selection, inspired by local and global quantum measurements. The resulting algorithm demonstrates a significant performance improvement in several benchmark environments. With this technique, we successfully execute a full training routine on a 5-qubit hardware device. Our method introduces only negligible classical overhead and has the potential to improve VQC-based algorithms beyond the field of quantum reinforcement learning.
CRIS-Team Scopus Importer:2023-10-27

Quantum robustness and phase transitions of the 3D toric code in a field - Reiss, David A.; Schmidt, Kai Phillip (SciPost Physics, 2019)
We study the robustness of 3D intrinsic topogical order under external perturbations by investigating the paradigmatic microscopic model, the 3D toric code in an external magnetic field. Exact dualities as well as variational calculations reveal a ground-state phase diagram with first and second-order quantum phase transitions. The variational approach can be applied without further approximations only for certain field directions. In the general field case, an approximative scheme based on an expansion of the variational energy in orders of the variational parameters is developed. For the breakdown of the 3D intrinsic topological order, it is found that the (im-) mobility of the quasiparticle excitations is crucial in contrast to their fractional statistics.
CRIS-Team WoS Importer:2019-07-19

Quantum robustness of fracton phases - Mühlhauser, Matthias; Walther, Matthias; Reiss, D. A.; Schmidt, Kai Phillip (Physical Review B, 2020)
The quantum robustness of fracton phases is investigated by studying the influence of quantum fluctuations on the X-Cube model and Haah's code, which realize a type-I and type-II fracton phase, respectively. To this end, a finite uniform magnetic field is applied to induce quantum fluctuations in the fracton phase, resulting in zero-temperature phase transitions between fracton phases and polarized phases. Using high-order series expansions and a variational approach, all phase transitions are classified as strongly first order, which turns out to be a consequence of the (partial) immobility of fracton excitations. Indeed, single fractons as well as few-fracton composites can hardly lower their excitation energy by delocalization due to the intriguing properties of fracton phases, as demonstrated in this work explicitly in terms of fracton quasiparticles.
CRIS-Team WoS Importer:2020-03-06

Quantum Simplicity Constraints and Area Spectrum - Zilker, Thomas; Thiemann, Thomas (2010)

Quantum Simulation Meets Nonequilibrium Dynamical Mean-Field Theory: Exploring the Periodically Driven, Strongly Correlated Fermi-Hubbard Model - Sandholzer, Kilian; Murakami, Yuta; Görg, Frederik; Minguzzi, Joaquín; Messer, Michael; Desbuquois, Rémi; Eckstein, Martin; Werner, Philipp; Esslinger, Tilman (Physical Review Letters, 2019)
We perform an ab initio comparison between nonequilibrium dynamical mean-field theory and optical lattice experiments by studying the time evolution of double occupations in the periodically driven Fermi-Hubbard model. For off-resonant driving, the range of validity of a description in terms of an effective static Hamiltonian is determined and its breakdown due to energy absorption close to resonance is demonstrated. For near-resonant driving, we investigate the response to a change in driving amplitude and discover an asymmetric excitation spectrum with respect to the detuning. In general, we find good agreement between experiment and theory, which cross validates the experimental and numerical approaches in a strongly correlated nonequilibrium system.
CRIS-Team Scopus Importer:2019-11-22

Quantum spacetime on a quantum simulator - Li, Keren; Li, Youning; Han, Muxin; Lu, Sirui; Zhou, Jie; Ruan, Dong; Long, Guilu; Wan, Yidun; Lu, Dawei; Zeng, Bei; Laflamme, Raymond (Communications Physics, 2019)
Quantum simulation has shown its irreplaceable role in many fields, where it is difficult for classical computers to do much. On a four-qubit Nuclear Magnetic Resonance (NMR) quantum simulator, we experimentally simulate the spin-network states by simulating quantum spacetime tetrahedra. The fidelities of our experimentally prepared quantum tetrahedra are all above 95%. We then use the quantum tetradedra prepared by the Nuclear Magnetic Resonance to simulate a spinfoam vertex amplitude, which displays the local dynamics of quantum spacetime. By measuring the geometric properties on the corresponding quantum tetrahedra and simulating their interaction, our experiment serves as a basic module that represents the Feynman diagram vertex in the spinfoam formulation of Loop Quantum Gravity(LQG). This is an initial attempt to study LQG by quantum information processing.
CRIS-Team WoS Importer:2019-10-18

Quantum spin dynamics (QSD) - Thiemann, Thomas (Classical and Quantum Gravity, 1998)
This paper is the first in a series of seven papers with the title 'quantum spin dynamics (QSD)'.

Quantum spin dynamics (QSD): III. Quantum constraint algebra and physical scalar product in quantum general relativity - Thiemann, Thomas (Classical and Quantum Gravity, 1998)
(vii) Equipped with this inner product, the construction of physical observables is straightforward.

Quantum spin dynamics (QSD): II. The kernel of the Wheeler-DeWitt constraint operator - Thiemann, Thomas (Classical and Quantum Gravity, 1998)
Finally, we comment on the status of the Wick rotation transform in the light of the present results and give an intuitive description of the action of the Hamiltonian constraint.

Quantum spin dynamics (QSD): IV. 2+1 Euclidean quantum gravity as a model to test 3+1 Lorentzian quantum gravity - Thiemann, Thomas (Classical and Quantum Gravity, 1998)
The space of solutions to all constraints turns out to be much larger than that obtained by traditional approaches, however, it is fully included. Thus, by a suitable restriction of the solution space, we can recover all former results which gives confidence in the new quantization methods. The meaning of the remaining 'spurious solutions' is discussed.

Quantum spin dynamics (QSD): VII. Symplectic structures and continuum lattice formulations of gauge field theories - Thiemann, Thomas (Classical and Quantum Gravity, 2001)
The framework developed here is the classical cornerstone on which the semiclassical analysis in a new series of papers called `gauge theory coherent states' is based.

Quantum spin dynamics (QSD): VI. Quantum Poincare algebra and a quantum positivity of energy theorem for canonical quantum gravity - Thiemann, Thomas (Classical and Quantum Gravity, 1998)
The quantum symmetry algebra corresponding to the generators of the little group faithfully represents the classical algebra.

Quantum spin dynamics (QSD): V. Quantum gravity as the natural regulator of the Hamiltonian constraint of matter quantum field theories - Thiemann, Thomas (Classical and Quantum Gravity, 1998)
Of course, to show that the entire theory is finite requires more: one would need to know what the physical observables are, apart from the Hamiltonian constraint, and whether they are also finite. However, with the results given in this paper this question can now be answered, at least in principle.

Quantum spin dynamics: VIII. The master constraint - Thiemann, Thomas (Classical and Quantum Gravity, 2006)
Recently the master constraint programme (MCP) for loop quantum gravity (LQG) was launched which replaces the infinite number of Hamiltonian constraints by a single master constraint. The MCP is designed to overcome the complications associated with the non-Lie-algebra structure of the Dirac algebra of Hamiltonian constraints and was successfully tested in various field theory models. For the case of 3+1 gravity itself, so far only a positive quadratic form for the master constraint operator was derived. In this paper, we close this gap and prove that the quadratic form is closable and thus stems from a unique self-adjoint master constraint operator. The proof rests on a simple feature of the general pattern according to which Hamiltonian constraints in LQG are constructed and thus extends to arbitrary matter coupling and holds for any metric signature. With this result the existence of a physical Hilbert space for LQG is established by standard spectral analysis.

Quantum supremacy using a programmable superconducting processor - Arute, Frank; Arya, Kunal; Babbush, Ryan; Bacon, Dave; Bardin, Joseph C.; Barends, Rami; Biswas, Rupak; Boixo, Sergio; Brandao, Fernando G.S.L.; Buell, David A.; Burkett, Brian; Chen, Yu; et al. (Nature, 2019)
The promise of quantum computers is that certain computational tasks might be executed exponentially faster on a quantum processor than on a classical processor1. A fundamental challenge is to build a high-fidelity processor capable of running quantum algorithms in an exponentially large computational space. Here we report the use of a processor with programmable superconducting qubits2–7 to create quantum states on 53 qubits, corresponding to a computational state-space of dimension 253 (about 1016). Measurements from repeated experiments sample the resulting probability distribution, which we verify using classical simulations. Our Sycamore processor takes about 200 seconds to sample one instance of a quantum circuit a million times—our benchmarks currently indicate that the equivalent task for a state-of-the-art classical supercomputer would take approximately 10,000 years. This dramatic increase in speed compared to all known classical algorithms is an experimental realization of quantum supremacy8–14 for this specific computational task, heralding a much-anticipated computing paradigm.
CRIS-Team Scopus Importer:2019-11-05

Quantum surface holonomies for loop quantum gravity and their application to black hole horizons - Sahlmann, Hanno; Zilker, Thomas (Physical Review D - Particles, Fields, Gravitation and Cosmology, 2020)
In this work we define a new type of flux operators on the Hilbert space of loop quantum gravity. We use them to solve an equation of the form F(A) = c Sigma in loop quantum gravity. This equation, which relates the curvature of a connection A with its canonical conjugate Sigma = *E, plays an important role for spherically symmetric isolated horizons, and, more generally, for maximally symmetric geometries and for the Kodama state. If the equation holds, the new flux operators can be interpreted as a quantization of surface holonomies from higher gauge theory. Also, they represent a kind of quantum deformation of SU(2). We investigate their properties and discuss how they can be used to define states that satisfy the isolated horizon boundary condition in the quantum theory.
CRIS-Team WoS Importer:2020-07-24

Quantum tetrahedron and loop quantum gravity: The monochromatic four-vertex - Eder, Konstantin; Sahlmann, Hanno (2015)

Quantum theory of charged black hole horizons - Eder, Konstantin; Sahlmann, Hanno (Friedrich-Alexander-Universität Erlangen-Nürnberg, 2017)
This thesis is devoted to the study of the quantum theory of charged black holes in the context of loop quantum gravity, extending the model of the quantum black hole as considered so far in the literature. We therefore consider Maxwell theory coupled to gravity de ned on a spacetime manifold with internal boundary described by an isolated horizon, construct the Hamiltonian formulation of the classical system, quantize the corresponding symplectic phase space and nally go over to the computation of the black hole entropy. We consider the spherically symmetric case in the U(1) framework as well as the distorted case following the SU(2) approach. The resulting picture depends signi cantly on the choices made for the quantization and the de nition of the state counting. We show that there is a choice such that the Bekenstein-Hawking relation holds. At the end, we use the theory in order to address the question whether there is a correspondence between the highly damped quasinormal modes and the area spectra of quantum charged black holes in the framework of loop quantum gravity

Quantum theory of charged isolated horizons - Sahlmann, Hanno; Eder, Konstantin (Physical Review D, 2018)
We describe the quantum theory of isolated horizons with electromagnetic or non-Abelian gauge charges in a setting in which both the gauge and gravitational field are quantized. We consider the distorted case, and its spherically symmetric limit. We show that the gravitational horizon d.o.f. give rise to the Bekenstein-Hawking relation, with lower-order terms giving some corrections for small black holes. We also demonstrate that one can include matter d.o.f. in the state counting. We show that one can expect (potentially divergent) contributions proportional to the area, as well as logarithmic corrections proportional to the horizon charge. This is qualitatively similar to results on matter contributions obtained with other methods in the literature.