“The Turn to the Mind, Inside and Out” - Sinding, Michael (2010)
<p class="Bibl_Angaben_rez_Text" style="margin: 0mm 0mm 6.35mm 20mm; border-style: hidden; border-color: rgb(0, 0, 0); float: none; line-height: normal; padding: 0mm; break-after: auto; font-weight: bold; font-family: Arial, sans-serif; font-size: 11pt; vertical-align: baseline; color: rgb(0, 0, 0);"> »Imagining Minds: Cognitive Approaches to Narrative, Embodied Simulation, Metaphor and Complex Tropes.« Vienna, May 21 to 24, 2008.  <p class="Absatz_Haupttext" style="margin: 0mm 0mm 4.23mm; border-style: hidden; border-color: rgb(0, 0, 0); float: none; line-height: normal; padding: 0mm; break-after: auto; font-family: Arial, sans-serif; font-size: 11pt; vertical-align: baseline; color: rgb(0, 0, 0);"> Michael Kimmel and Thomas Eder brought together established experts and younger researchers at the University of Vienna in May 2008 to reflect on and contribute to the »cognitive turn«, a major development in recent cultural theory and criticism. This turn, as the conference program said, grounds literary reception in general human psychology and everyday knowledge without losing sight of the specificity of literary aesthetics. It addresses central topics of literary structure and response in new ways. I will briefly contextualize the conference’s themes, review the papers and comment on them, then discuss them in light of some recent debates in the Journal of Literary Theory.

Thickness Dependent Properties in Oxide Heterostructures Driven by Structurally Induced Metal-Oxygen Hybridization Variations - Liao, Zhaoliang; Gauquelin, Nicolas; Green, Robert J.; Macke, Sebastian; Gonnissen, Julie; Thomas, Sean; Zhong, Zhicheng; Li, Lin; Si, Liang; Van Aert, Sandra; Hansmann, Philipp; Held, Karsten; et al. (Advanced Functional Materials, 2017)
Thickness-driven electronic phase transitions are broadly observed in different types of functional perovskite heterostructures. However, uncertainty remains whether these effects are solely due to spatial confinement, broken symmetry, or rather to a change of structure with varying film thickness. Here, this study presents direct evidence for the relaxation of oxygen-2p and Mn-3d orbital (p-d) hybridization coupled to the layer-dependent octahedral tilts within a La2/3Sr1/3MnO3 film driven by interfacial octahedral coupling. An enhanced Curie temperature is achieved by reducing the octahedral tilting via interface structure engineering. Atomically resolved lattice, electronic, and magnetic structures together with X-ray absorption spectroscopy demonstrate the central role of thickness-dependent p-d hybridization in the widely observed dimensionality effects present in correlated oxide heterostructures.
CRIS-Team WoS Importer:2020-02-19

Thin-film flow influenced by thermal noise - Grün, Günther; Rauscher, Markus; Mecke, Klaus (Journal of Statistical Physics, 2006)

Thin liquid polymer films rupture via defects - Mecke, Klaus; et al. (Langmuir, 1998)
The standard rupture scenario of thin polymer films on solid substrates is investigated in detail. The geometry of the dewetting structure is analyzed by means of the Minkowski functionals in two dimensions. Furthermore, the temporal behavior and the impact of ambient conditions are considered. In contrast to what has been frequently asserted, we show that spinodal dewetting does very probably not play any significant role in the rupture process. On the contrary, it is demonstrated that rupture is dominated by nucleation from defects, which are intrinsic to the polymer due to its peculiar morphology. It is concluded that polymer films on solid surfaces are either stable (in the case of complete wetting) or unstable, but never metastable.

Time-dependent Hartree-Fock approach to nuclear pasta at finite temperature - Schuetrumpf, B.; Klatt, Michael; Iida, K.; Maruhn, J. A.; Mecke, Klaus; Reinhard, Paul-Gerhard (Journal of Physics: Conference Series, 2013)
We present simulations of neutron-rich matter at subnuclear densities, like supernova matter, with the time-dependent Hartree-Fock approximation at temperatures of several MeV. 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. This matter evolves into spherical, rod-like, and slab-like shapes and mixtures thereof. The simulations employ a full Skyrme interaction in a periodic three-dimensional grid. By an improved morphological analysis based on Minkowski functionals, all eight pasta shapes can be uniquely identified by the sign of only two valuations, namely the Euler characteristic and the integral mean curvature.
CRIS-Team Scopus Importer:2023-07-24

Time-dependent Hartree-Fock approach to nuclear "pasta" at finite temperature - Schuetrumpf, Bastian; Klatt, Michael; Iida, K.; Maruhn, J. A.; Mecke, Klaus; Reinhard, Paul-Gerhard (Physical Review C - Nuclear Physics, 2013)
We present simulations of neutron-rich matter at subnuclear densities, like supernova matter, with the time-dependent Hartree-Fock approximation at temperatures of several MeV. The initial state consists of alpha particles randomly distributed in space that have aMaxwell-Boltzmann distribution in momentum space. Adding a neutron background initialized with Fermi distributed plane waves the calculations reflect a reasonable approximation of astrophysical matter. This matter evolves into spherical, rod-like, and slab-like shapes and mixtures thereof. The simulations employ a full Skyrme interaction in a periodic three-dimensional grid. By an improved morphological analysis based on Minkowski functionals, all eight pasta shapes can be uniquely identified by the sign of 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.

Time-dependent mass of cosmological perturbations in the hybrid and dressed metric approaches to loop quantum cosmology - Elizaga de Navascués, Beatriz; Martin de Blas, Daniel; Mena Marugan, Guillermo (Physical Review D - Particles, Fields, Gravitation and Cosmology, 2018)
Loop quantum cosmology has recently been applied in order to extend the analysis of primordial perturbations to the planck era and discuss the possible effects of quantum geometry on the cosmic microwave background. Two approaches to quantum cosmology with admissible ultraviolet behaviors leading to predictions that are compatible with observations are the so-called hybrid and dressed metric approaches. The evolution of the tensor and scalar perturbations are somewhat different. When backreaction is neglected, the discrepancy appears only in the time-dependent mass term of the corresponding field equations. We explain the origin of this difference, arising from the distinct quantization procedures. Besides, given the privileged role that the big bounce plays in the quantum cosmology loop, eg as a natural instant of time to set initial conditions for the perturbations, so we analyze the positivity of the time-dependent mass when this bounce occurs. While the mass of the tensor perturbations is positive in the hybrid approach, the kinetic contribution to the energy density of the inflation dominates over its potential, as well as to a large area of ​​the situation, while this mass is always nonpositive in the dressed metric approach. Similar results are demonstrated for the scalar perturbations in a sector of background that includes the kinetically dominated ones; namely, the mass then is positive for the hybrid approach, it is becoming negative in the dressed metric case. More precisely, this last statement is strictly valid when the potential is quadratic for values ​​of the inflation.<br

Time-resolved photoemission and resonant inelastic x-ray scattering study of a site-selective Mott insulator - Werner, Philipp; Petocchi, Francesco; Eckstein, Martin (Physical Review B, 2023)
Inspired by the physics of rare-earth nickelates, we study the photoemission (PES) and resonant inelastic x-ray scattering (RIXS) spectra of a correlated electron system with two types of insulating sublattices. Sublattice A is characterized by a hybridization gap and a low-spin state, while sublattice B features a Mott gap and a local magnetic moment. We show how the coupling of these two qualitatively different insulating states affects the dynamics of photoinduced charge carriers and how the nonequilibrium states manifest themselves in the PES and RIXS signals. In particular, we find that charge carriers created on the B sublattice migrate to the A sublattice, where they contribute to the creation of in-gap states in the PES signal and to characteristic peaks in the nonequilibrium RIXS spectrum. While the contributions from the two sublattices cannot be easily distinguished in the local photoemission spectrum, the weights of the RIXS signals in the two-dimensional ωin-ωout space provide information on the local state evolution on both sublattices.
CRIS-Team Scopus Importer:2023-02-17

Time-resolved photoemission spectroscopy on correlated electrons: Insights from dynamical mean-field theory - Eckstein, Martin (Journal of Electron Spectroscopy and Related Phenomena, 2021)
Time- and angular-resolved photoemission spectroscopy (trARPES) can directly probe the electronic structure of quantum materials out of equilibrium. This can shed light on the interaction of the electrons with spin, lattice, and orbital degrees of freedom, and help to unravel pathways towards novel out-of-equilibrium phases. Dynamical mean-field theory (DMFT) and its extensions provide a versatile toolbox to interpret such experiments through a theoretical simulation of the underlying microscopic processes. The approach can be applied both to Mott insulators and correlated metals, and it is formulated in terms of non-equilibrium Green's functions, which directly relate to the photoemission spectrum. This article reviews the theoretical description of trARPES within DMFT and related diagrammatic non-equilibrium Green's function techniques. Several applications are discussed, including the photo-induced melting of excitonic order, femtosecond relaxation processes in Mott insulators, and the manipulation of the electronic structure of Mott and charge transfer insulators using photodoping and strong THz fields.
CRIS-Team WoS Importer:2021-11-05

Time-reversal symmetry breaking Abelian chiral spin liquid in Mott phases of three-component fermions on the triangular lattice - Boos, Carolin; Ganahl, C. J.; Lajko, M.; Nataf, P.; Lauchli, A. M.; Penc, Karlo; Schmidt, Kai Phillip; Mila, F. (Physical Review Research, 2020)
We provide numerical evidence in favor of spontaneous chiral symmetry breaking and the concomitant appearance of an Abelian chiral spin liquid for three-component fermions on the triangular lattice described by an SU(3) symmetric Hubbard model with hopping amplitude -t (t > 0) and on-site interaction U. This chiral phase is stabilized in the Mott phase with one particle per site in the presence of a uniform pi flux per plaquette, and in the Mott phase with two particles per site without any flux. Our approach relies on effective spin models derived in the strong-coupling limit in powers of t/U for general SU(N) and arbitrary uniform charge flux per plaquette, which are subsequently studied using exact diagonalizations and variational Monte Carlo simulations for N = 3, as well as exact diagonalizations of the SU(3) Hubbard model on small clusters. Up to third order in t/U, and for the time-reversal symmetric cases (flux 0 or pi), the low-energy description is given by the J-K model with Heisenberg coupling J and real ring exchange K. The phase diagram in the full J-K parameter range contains, apart from three already known, magnetically long-range ordered phases, two previously unreported phases: (i) a lattice nematic phase breaking the lattice rotation symmetry and (ii) a spontaneous time-reversal and parity symmetry breaking Abelian chiral spin liquid. For the Hubbard model, an investigation that includes higher-order itinerancy effects supports the presence of a phase transition inside the insulating region, occurring at (t/U)(c) approximate to 0.07 [(U/t)(c) approximate to 13] between the three-sublattice magnetically ordered phase at small t/U and this Abelian chiral spin liquid.
CRIS-Team WoS Importer:2021-01-22

TMM-Fast, a transfer matrix computation package for multilayer thin-film optimization: tutorial - Luce, Alexander; Mahdavi, Ali; Marquardt, Florian; Wankerl, Heribert (Journal of the Optical Society of America A-Optics Image Science and Vision, 2022)
Achieving the desired optical response from a multilayer thin-film structure over a broad range of wavelengths and angles of incidence can be challenging. An advanced thin-film structure can consist of multiple materials with different thicknesses and numerous layers. Design and optimization of complex thin-film structures with multiple variables is a computationally heavy problem that is still under active research. To enable fast and easy experimentation with new optimization techniques,we propose the Python packageTransferMatrixMethod - Fast (TMM-Fast), which enables parallelized computation of reflection and transmission of light at different angles of incidence and wavelengths through the multilayer thin film.By decreasing computational time, generating datasets for machine learning becomes feasible, and evolutionary optimization can be used effectively. Additionally, the subpackageTMM-Torch allows us to directly compute analytical gradients for local optimization by usingPyTorch Autograd functionality. Finally, an OpenAI Gym environment is presented, which allows the user to train new reinforcement learning agents on the problem of finding multilayer thin-film configurations.
CRIS-Team Scopus Importer:2022-08-19

Tomographic Analysis of Jammed Ellipsoid Packings - Schröter, Matthias; Schaller, Fabian; Neudecker, Max; Saadatfar, Mohammad; Delaney, Gary; Mecke, Klaus; Schröder-Turk, Gerd (American Institute of Physics, 2013)
Disordered packings of ellipsoidal particles are an important model for disordered granular matter. Here we report a way to determine the average contact number of ellipsoid packings from tomographic analysis. Tomographic images of jammed ellipsoid packings prepared by vertical shaking of loose configurations are recorded and the positions and orientations of the ellipsoids are reconstructed. The average contact number can be extracted from a contact number scaling (CNS) function. The size of the particles, that may vary due to production inaccuracies, can also be determined by this method.
Keywords: Granular matter; Ellipsoids; Tomography; Contact numbers

Topological edge states for disordered bosonic systems - Peano Cavasola, Vittorio; Schulz-Baldes, Hermann (Journal of Mathematical Physics, 2018)
Quadratic bosonic Hamiltonians over a one-particle Hilbert space can be described by a Bogoliubov-de Gennes (BdG) Hamiltonian on a particle-hole Hilbert space. In general, the BdG Hamiltonian is not self-adjoint, but only J-self-adjoint on the particle-hole space viewed as a Krein space. Nevertheless, its energy bands can have non-trivial topological invariants like Chern numbers or winding numbers. By a thorough analysis for tight-binding models, it is proved that these invariants lead to bosonic edge modes which are robust to a large class of possibly disordered perturbations. Furthermore, general scenarios are presented for these edge states to be dynamically unstable even though the bulk modes are stable. Published by AIP Publishing.

Topological estimation of percolation thresholds - Neher, Richard A.; Mecke, Klaus; Wagner, Herbert (Journal of Statistical Mechanics-Theory and Experiment, 2008)
Global physical properties of random media change qualitatively at a percolation threshold, where isolated clusters merge to form one infinite connected component. The precise knowledge of percolation thresholds is thus of paramount importance. For two-dimensional lattice graphs, we use the universal scaling form of the cluster size distributions to derive a relation between the mean Euler characteristic of the critical percolation patterns and the threshold density pc. From this relation, we deduce a simple rule to estimate pc, which is remarkably accurate. We present some evidence that similar relations might hold for continuum percolation and percolation in higher dimensions.
Keywords: topology and combinatorics; classical phase transitions (theory); percolation problems (theory)

Topological Phase Transitions in the Golden String-Net Model - Schulz, Marc Daniel; Dusuel, Sebastien; Schmidt, Kai Phillip; Vidal, Julien (Physical Review Letters, 2013)
We examine the zero-temperature phase diagram of the two-dimensional Levin-Wen string-net model with Fibonacci anyons in the presence of competing interactions. Combining high-order series expansions around three exactly solvable points and exact diagonalizations, we find that the non-Abelian doubled Fibonacci topological phase is separated from two nontopological phases by different second-order quantum critical points, the positions of which are computed accurately. These trivial phases are separated by a first-order transition occurring at a fourth exactly solvable point where the ground-state manifold is infinitely many degenerate. The evaluation of critical exponents suggests unusual universality classes. DOI: 10.1103/PhysRevLett.110.147203

Topological phonon transport in an optomechanical system - Ren, Hengjiang; Shah, Tirth; Pfeifer, Hannes; Brendel, Christian; Peano, Vittorio; Marquardt, Florian; Painter, Oskar (2021 Conference on Lasers and Electro-Optics, CLEO 2021 - Proceedings, 2021)
We report the observation of topological phonon transport within a multiscale optome-chanical crystal structure consisting of an array of over 800 cavity-optomechanical ele-ments. Using sensitive, spatially resolved optical read-out we detect thermal phonons in a 0.325-0.34 GHz band traveling along a topological edge channel, with substantial reduc-tion in backscattering. This represents an important step from the pioneering macroscopic mechanical systems work towards topological phononic systems at the nanoscale.
CRIS-Team Scopus Importer:2021-12-24

Topological phonon transport in an optomechanical system - Ren, Hengjiang; Shah, Tirth; Pfeifer, Hannes; Brendel, Christian; Peano, Vittorio; Marquardt, Florian; Painter, Oskar (Nature Communications, 2022)
Light is a powerful tool for controlling mechanical motion, as shown by numerous applications in the field of cavity optomechanics. Recently, small scale optomechanical circuits, connecting a few optical and mechanical modes, have been demonstrated in an ongoing push towards multi-mode on-chip optomechanical systems. An ambitious goal driving this trend is to produce topologically protected phonon transport. Once realized, this will unlock the full toolbox of optomechanics for investigations of topological phononics. Here, we report the realization of topological phonon transport in an optomechanical device. Our experiment is based on an innovative multiscale optomechanical crystal design and allows for site-resolved measurements in an array of more than 800 cavities. The sensitivity inherent in our optomechanical read-out allowed us to detect thermal fluctuations traveling along topological edge channels. This represents a major step forward in an ongoing effort to downscale mechanical topological systems.
CRIS-Team Scopus Importer:2022-09-16

Toward black hole entropy in chiral loop quantum supergravity - Eder, Konstantin; Sahlmann, Hanno (Physical Review D - Particles, Fields, Gravitation and Cosmology, 2022)
Recently, many geometric aspects of N-extended anti–de Sitter supergravity in chiral variables have been encountered and clarified. In particular, if the theory is supposed to be invariant under supersymmetry transformations also on boundaries, the boundary term has to be the action of an OSp(N|2)C super Chern-Simons theory, and particular boundary conditions must be met. Based on this, we propose a way to calculate an entropy S for surfaces, presumably including black hole horizons, in the supersymmetric version of loop quantum gravity for the minimal case N=1. It proceeds in analogy to the nonsupersymmetric theory, by calculating dimensions of quantum state spaces of the super Chern-Simons theory with punctures, for a fixed quantum (super) area of the surface. We find S=a_H/4 for large areas and determine the subleading correction. Because of the noncompactness of OSp(1|2)C and the corresponding difficulties with the Chern-Simons quantum theory, we use analytic continuation from the Verlinde formula for a compact real form, UOSp(1|2), in analogy to work by Noui et al. This also entails studying some properties of OSp(1|2)_C representations that we have not found elsewhere in the literature.<br

Toward explaining black hole entropy quantization in loop quantum gravity - Sahlmann, Hanno (Physical Review D - Particles, Fields, Gravitation and Cosmology, 2007)
In a remarkable numerical analysis of the spectrum of states for a spherically symmetric black hole in loop quantum gravity, Corichi, Diaz-Polo and Fernandez-Borja found that the entropy of the black hole horizon increases in what resembles discrete steps as a function of area. In the present article we reformulate the combinatorial problem of counting horizon states in terms of paths through a certain space. This formulation sheds some light on the origins of this steplike behavior of the entropy. In particular, using a few extra assumptions we arrive at a formula that reproduces the observed step length to a few tenths of a percent accuracy. However, in our reformulation the periodicity ultimately arises as a property of some complicated process, the properties of which, in turn, depend on the properties of the area spectrum in loop quantum gravity in a rather opaque way. Thus, in some sense, a deep explanation of the observed periodicity is still lacking. © 2007 The American Physical Society.

Towards a First-Principles Determination of Effective Coulomb Interactions in Correlated Electron Materials: Role of Intershell Interactions - Seth, Priyanka; Hansmann, Philipp; Van Roekeghem, Ambroise; Vaugier, Loig; Biermann, Silke (Physical Review Letters, 2017)
The determination of the effective Coulomb interactions to be used in low-energy Hamiltonians for materials with strong electronic correlations remains one of the bottlenecks for parameter-free electronic structure calculations. We propose and benchmark a scheme for determining the effective local Coulomb interactions for charge-transfer oxides and related compounds. Intershell interactions between electrons in the correlated shell and ligand orbitals are taken into account in an effective manner, leading to a reduction of the effective local interactions on the correlated shell. Our scheme resolves inconsistencies in the determination of effective interactions as obtained by standard methods for a wide range of materials, and allows for a conceptual understanding of the relation of cluster model and dynamical mean field-based electronic structure calculations.
CRIS-Team WoS Importer:2020-02-19

Towards a reduced phase space quantization in loop quantum cosmology with an inflationary potential. - Giesel, Kristina; Li, Bao-Fei; Singh, Parampreet (Physical Review D - Particles, Fields, Gravitation and Cosmology, 2020)
We explore a reduced phase space quantization of loop quantum cosmology (LQC) for a spatially flat Friedmann-Lemaître-Robertson-Walker (FLRW) universe filled with reference fields and an inflaton field in a Starobinsky inflationary potential. We consider three separate cases in which the reference fields are taken to be the Gaussian dust, the Brown-Kuchař dust, and massless Klein-Gordon scalar reference fields respectively. This is a “two-fluid” model in which reference fields act as global clocks providing a physical time in an inflationary spacetime, and allow bypassing various technical hurdles in conventional quantum cosmological models. The reduced phase space is obtained in terms of the Dirac observables of the gravitational as well as the inflaton degrees of freedom. The physical Hamiltonians of the two dust models take the same form but turn out to be quite different from that of the Klein-Gordon reference field which reflects an aspect of the multiple choice problem of time. Loop quantization is implemented using the so-called ¯μ scheme and the Schrödinger equations involving the physical Hamiltonian operators generating the evolution in the physical time in the dust and massless Klein-Gordon models are obtained. These turn out to be quantum difference equations with the same nonsingular structure as for other models in LQC. We study some phenomenological implications of the quantization using the effective dynamics resulting from the reduced phase space quantization including the resolution of the big bang singularity via a quantum bounce, and effects of the different reference fields on e-foldings in both the preinflationary and the slow-roll inflationary phases. We find that different clocks, even when starting with a small but same energy density, can leave tiny but different imprints on the inflationary dynamics. In addition, for Brown-Kuchař dust, the choice of a negative energy density can result in a cyclic evolution before the onset of inflation constraining certain values for ideal clocks.<br

Towards Gaussian states for loop quantum gravity - Sahlmann, Hanno; Seeger, Robert (Physical Review D, 2020)
An important challenge in loop quantum gravity is to find semiclassical states-states that are as close to classical as quantum theory allows. This is difficult because the states in the Hilbert space used in loop quantum gravity are excitations over a vacuum in which geometry is highly degenerate. Additionally, fluctuations are distributed very unevenly between configuration and momentum variables. Coherent states that have been proposed to balance the uncertainties more evenly can, up to now, only do this for finitely many degrees of freedom. Our work is motivated by the desire to obtain Gaussian states that encompass all degrees of freedom. We reformulate the U(1) holonomy-flux algebra in any dimension as a Weyl algebra. We then define and investigate a new class of states on this algebra which behave as quasifree states on the momentum variables. Using a general result on representations of the holonomy-flux algebra, we define analogous representations also in the case of non-Abelian compact structure groups. For the case of SU(2), we study an explicit example of such a representation and the consequences for quantum geometry. This kind of state, with Gaussian fluctuations in the spatial geometry, seems well suited to investigate problems related to structure formation in cosmology.
CRIS-Team WoS Importer:2020-05-29

Towards Gaussian States for the Holonomy-Flux Weyl Algebra - Seeger, Robert; Sahlmann, Hanno (Friedrich-Alexander-Universität Erlangen-Nürnberg, 2018)
The objective of this Master’s thesis is to consider the well-known framework of Weyl algebras and quasifree states in order to find out if it is possible to apply the general ideas, coming from algebraic quantum theory, to the theory of loop quantum gravity. Starting from the U(1) toy-model of the canonical commutation relation of the holonomy-flux algebra, underlying loop quantum gravity, we construct a Weyl C*-algebra generated by so-called Weyl elements that arise from combining holonomies and exponentiated electric fluxes, which are the canonically conjugated variables of the theory. Quasifree states are a certain notion of Gaussian states, directly emerging from Weyl algebras. Because it seems to be impossible to establish such states on the algebra we found, we develop a different notion states that is only Gaussian in one of the variables and hence is referred to as almost-quasifree states. For such a state, which is Gaussian in the fluxes, we find a representation on a Hilbert space that combines the Hilbert space of loop quantum gravity with the Fock space of a scalar field. For the canonical commutation relation of the actual theory, which involves SU(2) Yang-Mills holonomies and the corresponding fluxes, we try to generalize our results. It is possible to define Weyl-like elements for holonomies along a single path and a set of exponentiated fluxes. We work toward a notion of elements that take care of more distinct edges or even graphs. It is, however, not clear if these also generate a C*-algebra. Without an underlying Weyl algebra we successfully generalize the almost-quasifree representation, found for the toy-model, and analyze its properties by re-deriving the area operator of loop quantum gravity in this new representation.<br

Towards Hartle-Hawking states for connection variables - Dhandhukiya, Satya; Sahlmann, Hanno (Physical Review D, 2017)
The Hartle-Hawking state is a proposal for a preferred initial state for quantum gravity, based on a path integral over all compact Euclidean four-geometries which have a given three-geometry as a boundary. The wave function constructed this way satisfies the (Lorentzian) Hamiltonian constraint of general relativity in ADM variables in a formal sense. In this article, we address the question of whether this construction is dependent on the canonical variables used. We give a precise derivation of the properties of the Hartle-Hawking state in terms of formal manipulations of the path integral expressions. Then we mimic the construction in terms of Ashtekar-Barbero variables, and observe that the resulting wave function does not satisfy the Lorentzian Hamiltonian constraint even in a formal sense. We also investigate this issue for the relativistic particle, with a similar conclusion. We finally suggest a modification of the proposal that does satisfy the constraint at least in a formal sense and start to consider its implications in quantum cosmology. We find that for certain variables, and in the saddle point approximation, the state is very similar to the Ashtekar-Lewandowski state of loop quantum gravity. In the process, we have calculated the on-shell action for several cosmological models in connection variables.

Towards Loop Quantum Supergravity (LQSG) - Bodendorfer, Norbert; Thurn, Andreas; Thiemann, Thomas (Physics Letters B, 2012)
Should nature be supersymmetric, then it will be described by Quantum Supergravity at least in some energy regimes. The currently most advanced description of Quantum Supergravity and beyond is Superstring Theory/M-Theory in 10/11 dimensions. String Theory is a top-to-bottom approach to Quantum Supergravity in that it postulates a new object, the string, from which classical Supergravity emerges as a low energy limit. On the other hand, one may try more traditional bottom-to-top routes and apply the techniques of Quantum Field Theory. Loop Quantum Gravity (LQG) is a manifestly background independent and non-perturbative approach to the quantisation of classical General Relativity, however, so far mostly without supersymmetry. The main obstacle to the extension of the techniques of LQG to the quantisation of higher dimensional Supergravity is that LQG rests on a specific connection formulation of General Relativity which exists only in D + 1 = 4 dimensions. In this Letter we introduce a new connection formulation of General Relativity which exists in all space-time dimensions. We show that all LQG techniques developed in D + 1 = 4 can be transferred to the new variables in all dimensions and describe how they can be generalised to the new types of fields that appear in Supergravity theories as compared to standard matter, specifically Rarita-Schwinger and p-form gauge fields. (C) 2012 Elsevier B.V. All rights reserved.
Keywords: Loop quantum gravity; Supergravity; Higher dimensional gravity

Towards loop quantum supergravity (LQSG): II. p-form sector - Bodendorfer, Norbert; Thiemann, Thomas; Thurn, Andreas (Classical and Quantum Gravity, 2013)
In our companion paper, we focused on the quantization of the Rarita-Schwinger sector of supergravity theories in various dimensions by using an extension of loop quantum gravity to all spacetime dimensions. In this paper, we extend this analysis by considering the quantization of additional bosonic fields necessary to obtain a complete SUSY multiplet next to graviton and gravitino in various dimensions. As a generic example, we study concretely the quantization of the 3-index photon of minimal 11d SUGRA, but our methods easily extend to more general p-form fields. Due to the presence of a Chern-Simons term for the 3-index photon, which is due to local SUSY, the theory is self-interacting and its quantization far from straightforward. Nevertheless, we show that a reduced phase space quantization with respect to the 3-index photon Gauss constraint is possible. Specifically, the Weyl algebra of observables, which deviates from the usual CCR Weyl algebras by an interesting twist contribution proportional to the level of the Chern-Simons theory, admits a background independent state of the Narnhofer-Thirring type.

Towards loop quantum supergravity (LQSG): I. Rarita-Schwinger sector - Bodendorfer, Norbert; Thiemann, Thomas; Thurn, Andreas (Classical and Quantum Gravity, 2013)
In our companion papers, we managed to derive a connection formulation of Lorentzian general relativity in D + 1 dimensions with compact gauge group SO(D + 1) such that the connection is Poisson-commuting, which implies that loop quantum gravity quantization methods apply. We also provided the coupling to standard matter. In this paper, we extend our methods to derive a connection formulation of a large class of Lorentzian signature supergravity theories, in particular 11D SUGRA and 4D, N = 8 SUGRA, which was in fact the motivation to consider higher dimensions. Starting from a Hamiltonian formulation in the time gauge which yields a Spin(D) theory, a major challenge is to extend the internal gauge group to Spin(D + 1) in the presence of the Rarita-Schwinger field. This is non-trivial because SUSY typically requires the Rarita-Schwinger field to be a Majorana fermion for the Lorentzian Clifford algebra and Majorana representations of the Clifford algebra are not available in the same spacetime dimension for both Lorentzian and Euclidean signatures. We resolve the arising tension and provide a background-independent representation of the non-trivial Dirac antibracket *-algebra for the Majorana field which significantly differs from the analogous construction for Dirac fields already available in the literature.

Towards the fundamental spectrum of the quantum Yang-Mills theory - Liegener, Klaus; Thiemann, Thomas (Physical Review D - Particles, Fields, Gravitation and Cosmology, 2016)
In this work we focus on the quantum Einstein-Yang-Mills sector quantized by the methods of loop quantum gravity. We point out the improved UV behavior of the coupled system as compared to pure quantum Yang-Mills theory on a fixed, classical background spacetime as was considered in a seminal work by Kogut and Susskind. Furthermore, we develop a calculational scheme by which the fundamental spectrum of the quantum Yang-Mills Hamiltonian can be computed in principle and by which one can make contact with the Wilsonian renormalization group, possibly purely within the Hamiltonian framework. Finally, we comment on the relationship of the fundamental spectrum to that of pure Yang-Mills theory on a (flat) classical spacetime.

Towards the QFT on curved spacetime limit of QGR: I. A general scheme - Sahlmann, Hanno; Thiemann, Thomas (Classical and Quantum Gravity, 2006)
In this paper and the companion paper (Sahlmann and Thiemann 2006 Towards the QFT on curved spacetime limit of QGR: II. A concrete implementation Class. Quantum Grav. 23 909), we address the question of how one might obtain the semiclassical limit of ordinary matter quantum fields (QFT) propagating on curved spacetimes (CST) from full-fledged quantum general relativity (QGR), starting from first principles. We stress that we do not claim to have a satisfactory answer to this question, rather our intention is to ignite a discussion by displaying the problems that have to be solved when carrying out such a programme. In the first paper of this series of two, we propose a general scheme of logical steps that one has to take in order to arrive at such a limit. We discuss the technical and conceptual problems that arise in doing so and how they can be solved in principle. As to be expected, completely new issues arise due to the fact that QGR is a background-independent theory. For instance, fundamentally the notion of a photon involves not only the Maxwell quantum field but also the metric operator - in a sense, there is no photon vacuum state but a 'photon vacuum operator'! Such problems have, to the best of our knowledge, not been discussed in the literature before, we are facing squarely one aspect of the deep conceptual difference between a background-dependent and a background-free theory. While in this first paper we focus on conceptual and abstract aspects, for instance the definition of (fundamental) n-particle states (e.g. photons), in the second paper we perform detailed calculations including, among other things, coherent state expectation values and propagation on random lattices. These calculations serve as an illustration of how far one can get with present mathematical techniques. Although they result in detailed predictions for the size of first quantum corrections such as the γ-ray burst effect, these predictions should not be taken too seriously because (a) the calculations are carried out at the kinematical level only and (b) while we can classify the amount of freedom in our constructions, the analysis of the physical significance of possible choices has just begun. © 2006 IOP Publishing Ltd.

Towards the QFT on curved spacetime limit of QGR: II. A concrete implementation - Sahlmann, Hanno; Thiemann, Thomas (Classical and Quantum Gravity, 2006)
The present paper is the companion of Sahlmann and Thiemann (2006 Towards the QFT on curved spacetime limit of QGR: I. A general scheme Class. Quantum Grav. 23 867) in which we proposed a scheme that tries to derive the quantum field theory (QFT) on curved spacetimes (CST) limit from background-independent quantum general relativity (QGR). The constructions of the companion paper make heavy use of the notion of semiclassical states for QGR. In the present paper, we employ the complexifier coherent states for QGR recently proposed by Thiemann and Winkler as semiclassical states, and thus fill the general formulae obtained in the companion paper with life. We demonstrate how one can, under some simplifying assumptions, explicitly compute expectation values of the operators relevant for the gravity-matter Hamiltonians of the companion paper in the complexifier coherent states. These expectation values give rise to effective matter Hamiltonians on the background on which the gravitational coherent state is peaked and thus induce approximate notions of n-particle states and matter propagation on fluctuating spacetimes. We display the details for the scalar and the electromagnetic field. The effective theories exhibit two types of corrections as compared to the ordinary QFT on CST. The first is due to the quantum fluctuations of the gravitational field and the second arises from the fact that background independence forces both geometry and matter to propagate on a spacetime of the form , where γ is a (random) graph. Finally, we obtain explicit numerical predictions for non-standard dispersion relations for the scalar and the electromagnetic field. They should, however, not be taken too seriously, due to the many ambiguities in our scheme, the analysis of the physical significance of which has only begun. We show, however, that one can classify these ambiguities at least in broad terms. © 2006 IOP Publishing Ltd.

Tracking down the origin of superbubbles and supergiant shells in the Magellanic Clouds with Minkowski tensor analysis - Collischon, Caroline; Sasaki, Manami; Mecke, Klaus; Points, Sean D.; Klatt, Michael A. (Astronomy & Astrophysics, 2021)
Aims. We develop an automatic bubble-recognition routine based on Minkowski functionals (MF) and tensors (MT) to detect bubble-like interstellar structures in optical emission line images. Methods. Minkowski functionals and MT are powerful mathematical tools for parameterizing the shapes of bodies. Using the papaya2-library, we created maps of the desired MF or MT of structures at a given window size. We used maps of the irreducible MT ψ2, which is sensitive to elongation, to find filamentary regions in Hα, [S II], and [O III] images of the Magellanic Cloud Emission Line Survey. Using the phase of ψ2, we were able to draw lines perpendicular to each filament and thus obtain line-density maps. This allowed us to find the center of a bubble-like structure and to detect structures at different window sizes. Results. The detected bubbles in all bands are spatially correlated to the distribution of massive stars, showing that we indeed detect interstellar bubbles without large spatial bias. Eighteen out of 59 supernova remnants in the Large Magellanic Cloud (LMC) and 13 out of 20 superbubbles are detected in at least one wavelength. The lack of detection is mostly due to surrounding emission that disturbs the detection, a too small size, or the lack of a (circular) counterpart in our emission line images. In line-density maps at larger scales, maxima can be found in regions with high star formation in the past, often inside supergiant shells (SGS). In SGS LMC 2, there is a maximum west of the shell where a collision of large gas clouds is thought to have occurred. In the Small Magellanic Cloud (SMC), bubble detection is impaired by the more complex projected structure of the galaxy. Line maps at large scales show large filaments in the SMC in a north-south direction, especially in the [S II] image. The origin of these filaments is unknown.
CRIS-Team Scopus Importer:2021-09-17
Keywords: ISM: structure; Magellanic Clouds; Methods: data analysis; Methods: statistical

Tracking the Footprints of Spin Fluctuations: A MultiMethod, MultiMessenger Study of the Two-Dimensional Hubbard Model - Schaefer, Thomas; Wentzell, Nils; Simkovic, Fedor; He, Yuan-Yao; Hille, Cornelia; Klett, Marcel; Eckhardt, Christian J.; Arzhang, Behnam; Harkov, Viktor; Le Regent, Francois-Marie; Kirsch, Alfred; Wang, Yan; et al. (Physical Review X, 2021)
The Hubbard model represents the fundamental model for interacting quantum systems and electronic correlations. Using the two-dimensional half-filled Hubbard model at weak coupling as a testing ground, we perform a comparative study of a comprehensive set of state-of-the-art quantum many-body methods. Upon cooling into its insulating antiferromagnetic ground state, the model hosts a rich sequence of distinct physical regimes with crossovers between a high-temperature incoherent regime, an intermediate-temperature metallic regime, and a low-temperature insulating regime with a pseudogap created by antiferromagnetic fluctuations. We assess the ability of each method to properly address these physical regimes and crossovers through the computation of several observables probing both quasiparticle properties and magnetic correlations, with two numerically exact methods (diagrammatic and determinantal quantumMonte Carlo methods) serving as a benchmark. By combining computational results and analytical insights, we elucidate the nature and role of spin fluctuations in each of these regimes. Based on this analysis, we explain how quasiparticles can coexist with increasingly long-range antiferromagnetic correlations and why dynamical mean-field theory is found to provide a remarkably accurate approximation of local quantities in the metallic regime. We also critically discuss whether imaginary-time methods are able to capture the non-Fermi-liquid singularities of this fully nested system.
CRIS-Team WoS Importer:2021-04-02

Training the quantum approximate optimization algorithm without access to a quantum processing unit - Streif, Michael; Leib, Martin (Quantum Science and Technology, 2020)
In this paper, we eliminate the classical outer learning loop of the quantum approximate optimization algorithm (QAOA) and present a strategy to find good parameters for QAOA based on topological arguments of the problem graph and tensor network techniques. Starting from the observation of the concentration of control parameters of QAOA, we find a way to classically infer parameters which scales polynomially in the number of qubits and exponentially with the depth of the circuit. Using this strategy, the quantum processing unit (QPU) is only needed to sample from the final state of QAOA. This method paves the way for a variation-free version of QAOA and makes QAOA more practical for applications on NISQ devices. We investigate the performance of the proposed approach for the initial assumptions and its resilience with respect to situations where they are not fulfilled. Moreover, we investigate the applicability of our method beyond the scope of QAOA, in improving schedules for quantum annealing.
CRIS-Team WoS Importer:2020-06-05

Trajectories of colloidal particles in laser fields with eight-, ten-, or twelve-fold symmetry and phasonic drift - Sandbrink, Matthias; Schmiedeberg, Michael (Aperiodic Crystal, 2013)

Transient nuclear inversion by x-ray free electron laser in a tapered x-ray waveguide - Chen, Yu-Hsueh; Lin, Po-Han; Wang, Guan-Ying; Pálffy, Adriana; Liao, Wen-Te (Physical Review Research, 2022)
The enhancement of x-ray-matter interaction by guiding and focusing radiation from x-ray free electron lasers is investigated theoretically. We show that elliptical waveguides using a cladding material with a high atomic number, such as platinum, can maintain an x-ray intensity of up to three orders of magnitude larger than in free space. This feature can be used to place a nuclear sample in the waveguide focal area and drive nuclear Mossbauer transitions up to transient nuclear population inversion. The latter is a long-standing goal related to gamma-ray lasers or nuclear state population control for energy storage. We show that inverted nuclei numbers of up to approximately 2 x 10(5) are achievable in the realistic region of longitudinal x-ray-free-electron-laser coherence time <= 10 fs. Our results anticipate the important role of tapered x-ray waveguides and strategically embedded samples in the field of x-ray quantum optics.
CRIS-Team WoS Importer:2022-08-05

Tree tensor-network real-time multiorbital impurity solver: Spin-orbit coupling and correlation functions in Sr2RuO4 - Cao, X.; Lu, Y.; Hansmann, Philipp; Haverkort, M. W. (Physical Review B, 2021)
We present a tree tensor-network impurity solver suited for general multiorbital systems. The network is constructed to efficiently capture the entanglement structure and symmetry of an impurity problem. The solver works directly on the real-time/frequency axis and generates spectral functions with energy-independent resolution of the order of one percent of the correlated bandwidth. Combined with an optimized representation of the impurity bath, it efficiently solves self-consistent dynamical mean-field equations and calculates various dynamical correlation functions for systems with off-diagonal Green's functions. For the archetypal correlated Hund's metal Sr2RuO4, we show that both the low-energy quasiparticle spectra related to the van Hove singularity and the high-energy atomic multiplet excitations can be faithfully resolved. In particular, we show that while the spin-orbit coupling has only minor effects on the orbital-diagonal one-particle spectral functions, it has a more profound impact on the low-energy spin and orbital response functions.
CRIS-Team Scopus Importer:2021-09-24

Tunable bimodal explorations of space from memory-driven deterministic dynamics - Hubert, Maxime; Perrard, Stephane; Labousse, Matthieu; Vandewalle, Nicolas; Couder, Yves (Physical Review E, 2019)
We present a wave-memory-driven system that exhibits intermittent switching between two propulsion modes in free space. The model is based on a pointlike particle emitting periodically cylindrical standing waves. Submitted to a force related to the local wave-field gradient, the particle is propelled, while the wave field stores positional information on the particle trajectory. For long memory, the linear motion is unstable and we observe erratic switches between two propulsive modes: linear motion and diffusive motion. We show that the bimodal propulsion and the stochastic aspect of the dynamics at long time are generated by a Shil'nikov chaos. The memory of the system controls the fraction of time spent in each phase. The resulting bimodal dynamics shows analogies with intermittent search strategies usually observed in living systems of much higher complexity.
CRIS-Team WoS Importer:2019-09-17

Tuning cellular uptake of nanoparticles via ligand density: Contribution of configurational entropy - Zhang, Yudie; Li, Long; Wang, Jizeng (Physical Review E, 2021)
The bioactivity of nanoparticles (NPs) crucially depends on their ability to cross biological membranes. A fundamental understanding of cell-NP interaction is hence essential to improve the performance of the NP-based biomedical applications. Although extensive studies of cellular uptake have converged upon the idea that the uptake process is mainly regulated by the elastic deformation of the cell membrane or NP, recent experimental observations indicate the ligand density as another critical factor in modulating NP uptake into cells. In this study, we propose a theoretical model of the wrapping of an elastic vesicle NP by a finite lipid membrane to depict the relevant energetic and morphological evolutions during the wrapping process driven by forming receptor-ligand bonds. In this model, the deformations of the membrane and the vesicle NP are assumed to follow the continuum Canham-Helfrich framework, whereas the change of configurational entropy of receptors is described from statistical thermodynamics. Results show that the ligand density strongly affects the binding energy and configurational entropy of free receptors, thereby altering the morphology of the vesicle-membrane system in the steady wrapping state. For the wrapping process by the finite lipid membrane, we also find that there exists optimal ligand density for the maximum wrapping degree. These predictions are consistent with relevant experimental observations reported in the literature. We have further observed that there are transitions of various wrapping phases (no wrapping, partial wrapping, and full wrapping) in terms of ligand density, membrane tension, and molecular binding energy. In particular, the ligand and receptor shortage regimes for the small and high ligand density are, respectively, identified. These results may provide guidelines for the rational design of nanocarriers for drug delivery.
CRIS-Team WoS Importer:2021-12-03

Tuning Elasticity of Open-Cell Solid Foams and Bone Scaffolds via Randomized Vertex Connectivity - Nachtrab, Susan; Kapfer, Sebastian; Rietzel, Dominik; Drummer, Dietmar; Madadi, Mahyar; Arns, Christoph; Kraynik, Andrew M.; Schröder-Turk, Gerd; Mecke, Klaus (Advanced Engineering Materials, 2012)
Tuning mechanical properties of and fluid flow through open-cell solid structures is a challenge for material science, in particular for the design of porous structures used as artificial bone scaffolds in tissue engineering. We present a method to tune the effective elastic properties of custom-designed open-cell solid foams and bone scaffold geometries by almost an order of magnitude while approximately preserving the pore space geometry and hence fluid transport properties. This strong response is achieved by a change of topology and node coordination of a network-like geometry underlying the scaffold design. Each node of a four-coordinated network is disconnected with probability p into two two-coordinated nodes, yielding network geometries that change continuously from foam- or network-like cellular structures to entangled fiber bundles. We demonstrate that increasing p leads to a strong, approximately exponential decay of mechanical stiffness while leaving the pore space geometry largely unchanged. This result is obtained by both voxel-based finite element methods and compression experiments on laser sintered models. The physical effects of randomizing network topology suggest a new design paradigm for solid foams, with adjustable mechanical properties. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
UnivIS-Import:2015-03-09:Pub.2012.tech.FT.FT-KT.tuning

Tuning the work function in transition metal oxides and their heterostructures - Zhong, Zhicheng; Hansmann, Philipp (Physical Review B, 2016)
The development of novel functional materials in experimental labs combined with computer-based compound simulation brings the vision of materials design on a microscopic scale continuously closer to reality. For many applications interface and surface phenomena rather than bulk properties are key. One of the most fundamental qualities of a material-vacuum interface is the energy required to transfer an electron across this boundary, i. e., the work function. It is a crucial parameter for numerous applications, including organic electronics, field electron emitters, and thermionic energy converters. Being generally very resistant to degradation at high temperatures, transition metal oxides present a promising materials class for such devices. We have performed a systematic study for perovskite oxides that provides reference values and, equally important, reports on materials trends and the tunability of work functions. Our results identify and classify dependencies of the work function on several parameters including specific surface termination, surface reconstructions, oxygen vacancies, and heterostructuring.
CRIS-Team WoS Importer:2020-02-19

Tuning topological surface magnetism by bulk alloying - Klier, Nicolas; Sharma, S.; Rost, Fabian; Pankratov, Oleg; Shallcross, Sam (Physical Review B, 2019)
Deploying an analytical atomistic model of the bulk band structure of the IV-VI crystalline topological insulators, we connect the spin structure of the surface state to the crystal field and spin-orbit coupling parameters of the bulk material. While the Dirac-Weyl-type topological surface state is often assumed to be universal, we show that the physics of the surface state is strikingly nonuniversal, carrying a profound imprint of the bulk physics. To see this explicitly we calculate the Ruderman-Kittel-Kasuya-Yosida interaction, which may be viewed as a probe of this surface state spin structure, finding that its qualitative form depends on the values the bulk spin-orbit and crystal field parameters take. This opens the way to tune the spin interaction on the surface of a IV-VI topological insulator by, for instance, varying the composition of the IV-VI ternary compounds.
CRIS-Team WoS Importer:2021-10-18

Tunneling-induced fractal transmission in valley Hall waveguides - Shah, Tirth; Marquardt, Florian; Peano, Vittorio (Physical Review B, 2023)
The valley Hall effect provides a popular route to engineer robust waveguides for bosonic excitations such as photons and phonons. The almost complete absence of backscattering in many experiments has its theoretical underpinning in a smooth-envelope approximation that neglects large momentum transfer and is accurate only for small bulk band gaps and/or smooth domain walls. For larger bulk band gaps and hard domain walls, backscattering is expected to become significant. Here, we show that in this experimentally relevant regime, the reflection of a wave at a sharp corner becomes highly sensitive to the orientation of the outgoing waveguide relative to the underlying lattice. Enhanced backscattering can be understood as being triggered by resonant tunneling transitions in quasimomentum space. Tracking the resonant tunneling energies as a function of the waveguide orientation reveals a self-repeating fractal pattern that is also imprinted in the density of states and the backscattering rate at a sharp corner.
CRIS-Team Scopus Importer:2023-03-03

Tunneling in the Brillouin zone: Theory of backscattering in valley Hall edge channels - Shah, Tirth; Marquardt, Florian; Peano, Vittorio (Physical Review B, 2021)
A large set of recent experiments has been exploring topological transport in bosonic systems, e.g., of photons or phonons. In the vast majority, time-reversal symmetry is preserved, and band structures are engineered by a suitable choice of geometry to produce topologically nontrivial band gaps in the vicinity of high-symmetry points. However, this leaves open the possibility of large-quasimomentum backscattering, destroying the topological protection. Up to now, it has been unclear what precisely the conditions are where this effect can be sufficiently suppressed. In the present paper, we introduce a comprehensive semiclassical theory of tunneling transitions in momentum space, describing backscattering for one of the most important system classes, based on the valley Hall effect. We predict that even for a smooth domain wall, effective scattering centers develop at locations determined by both the local slope of the wall and the energy. Moreover, our theory provides a quantitative analysis of the exponential suppression of the overall reflection amplitude with increasing domainwall smoothness.
CRIS-Team WoS Importer:2022-01-14

Turning a Nickelate Fermi Surface into a Cupratelike One through Heterostructuring - Hansmann, Philipp; Yang, Xiaoping; Toschi, Alessandro; Khaliullin, G.; Andersen, O. K.; Held, Karsten (Physical Review Letters, 2009)
Using the local density approximation and its combination with dynamical mean-field theory, we show that electronic correlations induce a single-sheet, cupratelike Fermi surface for hole-doped 1/1 LaNiO(3)/LaAlO(3) heterostructures, even though both e(g) orbitals contribute to it. The Ni 3d(3z)(2)-1 orbital plays the role of the axial Cu 4s-like orbital in the cuprates. These two results indicate that "orbital engineering" by means of heterostructuring should be possible. As we also find strong antiferromagnetic correlations, the low-energy electronic and spin excitations in nickelate heterostructures resemble those of high-temperature cuprate superconductors.
CRIS-Team WoS Importer:2020-02-19

Two computational models for simulating the tumbling motion of elongated particles in fluids - Bartuschat, Dominik; Fischermeier, Ellen; Gustavsson, Katarina; Rüde, Ulrich (Computers & Fluids, 2016)
Suspensions with fiber-like particles in the low Reynolds number regime are modeled by two different approaches that both use a Lagrangian representation of individual particles. The first method is the well established formulation based on Stokes flow that is formulated as integral equations. It uses a slender body approximation for the fibers to represent the interaction between them directly without explicitly computing the flow field. The second is a new technique using the 3D lattice Boltzmann method on parallel supercomputers. Here the flow computation is coupled to a computational model of the dynamics of rigid bodies using fluid-structure interaction techniques. Both methods can be applied to simulate fibers in fluid flow. They are carefully validated and compared against each other, exposing systematically their strengths and weaknesses regarding their accuracy, the computational cost, and possible model extensions. (C) 2015 Elsevier Ltd. All rights reserved.
UnivIS-Import:2015-04-14:Pub.2015.tech.IMMD.lsinfs.twocom_7
Keywords: Fluid–particle interaction; Tumbling fibers; Slender body formulation; Lattice Boltzmann method

Two dimensionality of magnetic excitations on the trellis lattice: (La,Sr,Ca)(14)Cu24O41 and SrCu2O3 - Schmidt, Kai Phillip; et al. (Physical Review B, 2007)
We explore the properties of magnetic excitations on the trellis lattice which is relevant for the so-called telephone-number compounds A(14)Cu(24)O(41) with A=(La,Sr,Ca) and the system SrCu2O3. The trellis lattice consists of two-leg ladders which are coupled in a strongly frustrated fashion. We use the effective model obtained for a single two-leg spin ladder to calculate the two-dimensional one-triplon dispersion and the corresponding one-triplon contribution to the dynamical structure factor. Special attention is laid on signatures of the frustrating interladder magnetic exchange. A detailed suggestion is made for an experimental detection of this exchange in inelastic neutron-scattering experiments.

Two-dimensional quasicrystals of decagonal order in one-component monolayer films - Comment - Schmiedeberg, Michael; Stark, Holger (Physical Review Letters, 2008)
A Comment on the Letter by A. Patrykiejew and S. Sokolowski, Phys. Rev. Lett. 99, 156101 (2007). The authors of the Letter offer a Reply.

Two intertwined facets of adherent membranes: membrane roughness and correlations between ligand-receptors bonds - Reister, Ellen; Bihr, Timo; Seifert, Udo; Smith, Ana-Suncana (New Journal of Physics, 2011)
We study equilibrium fluctuations of adherent membranes by means of Langevin simulations in the case when the interaction of the membrane with the substrate is twofold: a non-specific homogeneous harmonic potential is placed at large distances, whereas discrete ligand-receptor interactions occur at short distances from the flat substrate. We analyze the correlations between neighboring ligand-receptor bonds in a regime of relatively strong membrane fluctuations. By comparison with the random distribution of bonds, we find that the correlations between the bonds are always positive, suggesting spontaneous formation of domains. The equilibrium roughness of the membrane is then determined by fluctuations in the number density of bonds within the domains. Furthermore, we show that the excess number of bonds arising due to correlations and the instantaneous roughness of the membrane both follow master curves that depend only on the instantaneous bond density and not on the intrinsic binding strength of the ligand-receptor pair. The master curves show identical trends, further corroborating the link between membrane roughness and bond correlations.
Keywords: SUPPORTED MEMBRANES; GIANT VESICLES; ADHESION; DIFFUSION; DYNAMICS; MOLECULES; SURFACE; MODELS; CELLS

Ultrafast control of spin-orbital separation probed with time-resolved resonant inelastic x-ray scattering - Müller, Aaron; Grandi, Francesco; Eckstein, Martin (Physical Review B, 2022)
Quasi-one-dimensional systems exhibit many-body effects elusive in higher dimensions. A prime example is spin-orbital separation, which has been measured by resonant inelastic x-ray scattering (RIXS) in Sr2CuO3. Here, we theoretically analyze the time-resolved RIXS spectrum of Sr2CuO3 under the action of a time-dependent electric field. We show that the external field can reversibly modify the parameters in the effective t-J model used to describe spinon and orbiton dynamics in the material. For strong driving amplitudes, we find that the spectrum changes qualitatively as a result of reversing the relative spinon to orbiton velocity. The analysis shows that in general, the spin-orbital dynamics in Mott insulators in combination with time-resolved RIXS should provide a suitable platform to explore the reversible control of many-body physics in the solid with strong laser fields.
CRIS-Team Scopus Importer:2022-10-07

Ultrafast electric field controlled spin correlations in the Hubbard model - Dasari, Nagamalleswara Rao; Eckstein, Martin (Physical Review B, 2019)
Highly intense electric field pulses can move the electronic momentum occupation in correlated metals over large portions of the Brillouin zone, leading to phenomena such as dynamic Bloch oscillations. Using the nonequilibrium fluctuation-exchange approximation for the two-dimensional Hubbard model, we study how such nonthermal electron distributions drive collective spin and charge fluctuations. Suitable pulses can induce a highly anisotropic modification of the occupied momenta, and the corresponding spin dynamics results in a transient change from antiferromagnetic to anisotropic ferromagnetic correlations. To good approximation this behavior is understood in terms of an instantaneous response of the spin correlations to the single-particle properties.
CRIS-Team WoS Importer:2019-10-18

Ultrafast Mott transition driven by nonlinear electron-phonon interaction - Grandi, Francesco; Li, Jiajun; Eckstein, Martin (Physical Review B, 2021)
Nonlinear phononics holds the promise for controlling properties of quantum materials on the ultrashort timescale. Using nonequilibrium dynamical mean-field theory, we solve a model for the description of organic solids, where correlated electrons couple nonlinearly to a quantum phonon mode. Unlike previous works, we exactly diagonalize the local phonon mode within the noncrossing approximation to include the full phononic fluctuations. By exciting the local phonon in a broad range of frequencies near resonance with an ultrashort pulse, we show it is possible to induce a Mott insulator-to-metal phase transition. Conventional semiclassical and mean-field calculations, where the electron-phonon interaction decouples, underestimate the onset of the quasiparticle peak. This fact, together with the nonthermal character of the photoinduced metal, suggests a leading role of the phononic fluctuations and of the dynamic nature of the state in the vibrationally induced quasiparticle coherence.
CRIS-Team Scopus Importer:2021-02-12

Ultrafast Spin Dynamics in Photodoped Spin-Orbit Mott Insulator Sr2IrO4 - Afanasiev, D.; Gatilova, A.; Groenendijk, D. J.; Ivanov, B. A.; Gibert, M.; Gariglio, S.; Mentink, J.; Li, Jiajun; Dasari, Nagamalleswara Rao; Eckstein, Martin; Rasing, Th.; Caviglia, A. D.; et al. (Physical Review X, 2019)
Ultrafast photodoping of the Mott insulators, possessing strong correlation between electronic and magnetic degrees of freedom, holds promise for launching an ultrafast dynamics of spins which cannot be described in terms of conventional models of ultrafast magnetism. Here we study the ultrafast laser-induced dynamics of the magnetic order in a novel spin-orbit Mott insulator Sr2IrO4 featuring an uncompensated pattern of antiferromagnetic spin ordering. Using the transient magneto-optical Kerr effect sensitive to the net magnetization, we reveal that photodoping by femtosecond laser pulses with photon energy above the Mott gap launches melting of the antiferromagnetic order seen as ultrafast demagnetization with a characteristic time of 300 fs followed by a sub-10-ps recovery. Nonequilibrium dynamical mean-field theory calculations based on the single-band Hubbard model confirm that ultrafast demagnetization is primarily governed by the laser-induced generation of electron-hole pairs, although the precise simulated time dependencies are rather different from the experimentally observed ones. To describe the experimental results, here we suggest a phenomenological model which is based on Onsager's formalism and accounts for the photogenerated electron-hole pairs using the concepts of holons and doublons.
CRIS-Team WoS Importer:2019-06-04

Unboundedness of triad-like operators in loop quantum gravity - Brunnemann, Johannes; Thiemann, Thomas (Classical and Quantum Gravity, 2006)
In this paper we deliver the proofs for the claims, made in a companion paper, concerning the avoidance of cosmological curvature singularities in full loop quantum gravity (LQG).

Uncertainty principle for experimental measurements: Fast versus slow probes - Hansmann, Philipp; Ayral, Thomas; Tejeda, A.; Biermann, Silke (Scientific Reports, 2016)
The result of a physical measurement depends on the time scale of the experimental probe. In solid-state systems, this simple quantum mechanical principle has far-reaching consequences: the interplay of several degrees of freedom close to charge, spin or orbital instabilities combined with the disparity of the time scales associated to their fluctuations can lead to seemingly contradictory experimental findings. A particularly striking example is provided by systems of adatoms adsorbed on semiconductor surfaces where different experiments-angle-resolved photoemission, scanning tunneling microscopy and core-level spectroscopy-suggest different ordering phenomena. Using most recent first principles many-body techniques, we resolve this puzzle by invoking the time scales of fluctuations when approaching the different instabilities. These findings suggest a re-interpretation of ordering phenomena and their fluctuations in a wide class of solid-state systems ranging from organic materials to high-temperature superconducting cuprates.
CRIS-Team WoS Importer:2020-02-19

Unconventional magnetization plateaus in a Shastry-Sutherland spin tube - Manmana, S. R.; Picon, J. -D.; Schmidt, Kai Phillip; Mila, F. (EPL - Europhysics Letters, 2011)
Using density matrix renormalization group (DMRG) and perturbative continuous unitary transformations (PCUTs), we study the magnetization process in a magnetic field for all coupling strengths of a quasi-1D version of the 2D Shastry-Sutherland lattice, a frustrated spin tube made of two orthogonal dimer chains. At small inter-dimer coupling, plateaus in the magnetization appear at 1/6, 1/4, 1/3, 3/8, and 1/2. As in 2D, they correspond to a Wigner crystal of triplons. However, close to the boundary of the product singlet phase, plateaus of a new type appear at 1/5 and 3/4. They are stabilized by the localization of bound states of triplons. Their magnetization profile differs significantly from that of single-triplon plateaus and leads to specific NMR signatures. We address the possibility to stabilize such plateaus in further geometries by analyzing small finite clusters using exact diagonalizations and the PCUTs. Copyright (C) EPLA, 2011

Unified model for singlet fission within a non-conjugated covalent pentacene dimer - Basel, Bettina; Zirzlmeier, Johannes; Hetzer, Constantin; Phelan, Brian T.; Krzyaniak, Matthew D.; Seelam, Rajagopala Reddy; Brana Coto, Pedro; Horwitz, Noah E.; Young, Ryan M.; White, Fraser J.; Hampel, Frank; Clark, Timothy; et al. (Nature Communications, 2017)
When molecular dimers, crystalline films or molecular aggregates absorb a photon to produce a singlet exciton, spin-allowed singlet fission may produce two triplet excitons that can be used to generate two electron-hole pairs, leading to a predicted similar to 50% enhancement in maximum solar cell performance. The singlet fission mechanism is still not well understood. Here we report on the use of time-resolved optical and electron paramagnetic resonance spectroscopy to probe singlet fission in a pentacene dimer linked by a non-conjugated spacer. We observe the key intermediates in the singlet fission process, including the formation and decay of a quintet state that precedes formation of the pentacene triplet excitons. Using these combined data, we develop a single kinetic model that describes the data over seven temporal orders of magnitude both at room and cryogenic temperatures.

Uniform discretizations for spherically symmetric gravity coupled to a scalar field: A proposal for the vacuum state - Strobel, Eckhard; Thiemann, Thomas (Friedrich-Alexander-Universität Erlangen-Nürnberg, 2012)

Unifying magnons and triplons in stripe-ordered cuprate superconductors - Schmidt, Kai Phillip; et al. (Physical Review Letters, 2004)
Based on a two-dimensional model of coupled two-leg spin ladders, we derive a unified picture of recent neutron scattering data of stripe-ordered La15/8Ba1/8CuO4, namely, of the low-energy magnons around the superstructure satellites and of the triplon excitations at higher energies. The resonance peak at the antiferromagnetic wave vector Q(AF) in the stripe-ordered phase corresponds to a saddle point in the dispersion of the magnetic excitations. Quantitative agreement with the neutron data is obtained for J=130-160 meV and J(cyc)/J=0.2-0.25.

Unimodular lattice triangulations as small-world and scale-free random graphs - Mecke, Klaus; Krüger, Benedikt; Schmidt, Ella (New Journal of Physics, 2015)
Real-world networks, e.g., the social relations or world-wide-web graphs, exhibit both small-world and scale-free behaviour. We interpret lattice triangulations as planar graphs by identifying triangulation vertices with graph nodes and one-dimensional simplices with edges. Since these triangulations are ergodic with respect to a certain Pachner flip, applying different Monte Carlo simulations enables us to calculate average properties of random triangulations, as well as canonical ensemble averages, using an energy functional that is approximately the variance of the degree distribution. All considered triangulations have clustering coefficients comparable with real-world graphs; for the canonical ensemble there are inverse temperatures with small shortest path length independent of system size. Tuning the inverse temperature to a quasi-critical value leads to an indication of scale-free behaviour for degrees k >= 5. Using triangulations as a random graph model can improve the understanding of real-world networks, especially if the actual distance of the embedded nodes becomes important.
Keywords: unimodular lattice triangulations; networks; maximal planar graphs

Unique Crystal Structure of Ca2RuO4 in the Current Stabilized Semimetallic State - Bertinshaw, J.; Gurung, N.; Jorba, P.; Liu, H.; Schmid, M.; Mantadakis, D. T.; Daghofer, Maria; Krautloher, Maximilian Josef; Jain, A.; Ryu, G. H.; Fabelo, O.; Hansmann, Philipp; et al. (Physical Review Letters, 2019)
The electric-current stabilized semimetallic state in the quasi-two-dimensional Mott insulator Ca2RuO4 exhibits an exceptionally strong diamagnetism. Through a comprehensive study using neutron and x-ray diffraction, we show that this nonequilibrium phase assumes a crystal structure distinct from those of equilibrium metallic phases realized in the ruthenates by chemical doping, high pressure, and epitaxial strain, which in turn leads to a distinct electronic band structure. Dynamical mean field theory calculations based on the crystallographically refined atomic coordinates and realistic Coulomb repulsion parameters indicate a semimetallic state with partially gapped Fermi surface. Our neutron diffraction data show that the nonequilibrium behavior is homogeneous, with antiferromagnetic long-range order completely suppressed. These results provide a new basis for theoretical work on the origin of the unusual nonequilibrium diamagnetism in Ca2RuO4.
CRIS-Team Scopus Importer:2020-02-19

Unique fermionic vacuum in de Sitter spacetime from hybrid quantum cosmology - Navascues, Beatriz Elizaga; Mena Marugan, Guillermo A.; Prado, Santiago (Physical Review D - Particles, Fields, Gravitation and Cosmology, 2020)
In this work we show how the criterion of asymptotic Hamiltonian diagonalization originated in hybrid quantum cosmology serves to pick out a unique vacuum for the Dirac field in de Sitter, in the context of quantum field theory in curved spacetimes. This criterion is based on the dynamical definition of annihilation and creationlike variables for the fermionic field, which obey the linearized dynamics of a Hamiltonian that has been diagonalized in a way that is adapted to its local spatial structure. This leads to fermionic variables that possess a precise asymptotic expansion in the ultraviolet limit of large wave numbers. We explicitly show that, when the cosmological background is fixed as a de Sitter solution, this expansion uniquely selects the choice of fermionic annihilation and creationlike variables for all spatial scales, and thus picks out a unique privileged Fock representation and vacuum state for the Dirac field in de Sitter. The explicit form of the basis of solutions to the Dirac equation associated with this vacuum is then computed.
CRIS-Team WoS Importer:2020-07-10

Uniqueness Criteria for the Fock Quantization of Dirac Fields and Applications in Hybrid Loop Quantum Cosmology - Cortez, Jeronimo; Elizaga de Navascués, Beatriz; Mena Marugan, Guillermo A.; Prado, Santiago; Velhinho, Jose M. (Universe, 2020)
In generic curved spacetimes, the unavailability of a natural choice of vacuum state introduces a serious ambiguity in the Fock quantization of fields. In this review, we study the case of fermions described by a Dirac field in non-stationary spacetimes, and present recent results obtained by us and our collaborators about well-motivated criteria capable to ensure the uniqueness in the selection of a vacuum up to unitary transformations, at least in certain situations of interest in cosmology. These criteria are based on two reasonable requirements. First, the invariance of the vacuum under the symmetries of the Dirac equations in the considered spacetime. These symmetries include the spatial isometries. Second, the unitary implementability of the Heisenberg dynamics of the annihilation and creation operators when the curved spacetime is treated as a fixed background. This last requirement not only permits the uniqueness of the Fock quantization but, remarkably, it also allows us to determine an essentially unique splitting between the phase space variables assigned to the background and the fermionic annihilation and creation variables. We first consider Dirac fields in 2 + 1 dimensions and then discuss the more relevant case of 3 + 1 dimensions, particularizing the analysis to cosmological spacetimes with spatial sections of spherical or toroidal topology. We use this analysis to investigate the combined, hybrid quantization of the Dirac field and a flat homogeneous and isotropic background cosmology when the latter is treated as a quantum entity, and the former as a perturbation. Specifically, we focus our study on a background quantization along the lines of loop quantum cosmology. Among the Fock quantizations for the fermionic perturbations admissible according to our criteria, we discuss the possibility of further restricting the choice of a vacuum by the requisite of a finite fermionic backreaction and, moreover, by the diagonalization of the fermionic contribution to the total Hamiltonian in the asymptotic limit of large wave numbers of the Dirac modes. Finally, we argue in support of the uniqueness of the vacuum state selected by the extension of this diagonalization condition beyond the commented asymptotic region, in particular proving that it picks out the standard Poincare and Bunch-Davies vacua for fixed flat and de Sitter background spacetimes, respectively.
CRIS-Team WoS Importer:2021-01-15

Uniqueness of diffeomorphism invariant states on holonomy-flux algebras - Lewandowski, Jerzy; Okolow, Andrzej; Sahlmann, Hanno; Thiemann, Thomas (Communications in Mathematical Physics, 2006)
Loop quantum gravity is an approach to quantum gravity that starts from the Hamiltonian formulation in terms of a connection and its canonical conjugate. Quantization proceeds in the spirit of Dirac: First one defines an algebra of basic kinematical observables and represents it through operators on a suitable Hilbert space. In a second step, one implements the constraints. The main result of the paper concerns the representation theory of the kinematical algebra: We show that there is only one cyclic representation invariant under spatial diffeomorphisms. While this result is particularly important for loop quantum gravity, we are rather general: The precise definition of the abstract*-algebra of the basic kinematical observables we give could be used for any theory in which the configuration variable is a connection with a compact structure group. The variables are constructed from the holonomy map and from the fluxes of the momentum conjugate to the connection. The uniqueness result is relevant for any such theory invariant under spatial diffeomorphisms or being a part of a diffeomorphism invariant theory.

Uniqueness of the Representation in Homogeneous Isotropic LQC - Engle, Jonathan; Hanusch, Maximilian; Thiemann, Thomas (Communications in Mathematical Physics, 2017)
We show that the standard representation of homogeneous isotropic loop quantum cosmology (LQC) is the GNS-representation that corresponds to the unique state on the reduced quantum holonomy-flux *-algebra that is invariant under residual diffeomorphisms-both when the standard algebra is used as well as when one uses the extended algebra proposed by Fleischhack. More precisely, we find that in both situations the GNS-Hilbert spaces coincide, and that in the Fleischhack case the additional algebra elements are just mapped to zero operators. In order for the residual diffeomorphisms to have a well-defined action on the quantum algebra, we have let them act on the fiducial cell as well as on the dynamical variables, thereby recovering covariance. Consistency with Ashtekar and Campilgia in the Bianchi case is also shown.

Universal Definition of the Nonconformal Trace Anomaly - Ferrero, Renata; Franchino-Viñas, Sebastián A.; Fröb, Markus B.; Lima, William C.C. (Physical Review Letters, 2024)
We show that there exists a generalized, universal notion of the trace anomaly for theories which are not conformally invariant at the classical level. The definition is suitable for any regularization scheme and clearly states to what extent the classical equations of motion should be used, thus resolving existing controversies surrounding previous proposals. Additionally, we exhibit the link between our definition of the anomaly and the functional Jacobian arising from a Weyl transformation.
CRIS-Team Scopus Importer:2024-02-23

Universal hidden order in amorphous cellular geometries - Klatt, Michael A.; Lovric, Jakov; Chen, Duyu; Kapfer, Sebastian; Schaller, Fabian M.; Schoenhoefer, Philipp W. A.; Gardiner, Bruce S.; Smith, Ana-Suncana; Schroeder-Turk, Gerd E.; Torquato, Salvatore (Nature Communications, 2019)
Partitioning space into cells with certain extreme geometrical properties is a central problem in many fields of science and technology. Here we investigate the Quantizer problem, defined as the optimisation of the moment of inertia of Voronoi cells, i.e., similarly-sized 'sphere-like' polyhedra that tile space are preferred. We employ Lloyd's centroidal Voronoi diagram algorithm to solve this problem and find that it converges to disordered states associated with deep local minima. These states are universal in the sense that their structure factors are characterised by a complete independence of a wide class of initial conditions they evolved from. They moreover exhibit an anomalous suppression of long-wavelength density fluctuations and quickly become effectively hyperuniform. Our findings warrant the search for novel amorphous hyperuniform phases and cellular materials with unique physical properties.
CRIS-Team WoS Importer:2019-03-06

Universal jamming phase diagram in the hard-sphere limit - Schmiedeberg, Michael; Haxton, Tom; Liu, Andrea (Physical Review E, 2011)
We present a new formulation of the jamming phase diagram for a class of glass-forming fluids consisting of spheres interacting via finite-ranged repulsions at temperature T, packing fraction phi or pressure p, and applied shear stress Sigma. We argue that the natural choice of axes for the phase diagram are the dimensionless quantities T/p sigma(3), p sigma(3)/epsilon, and Sigma/p, where T is the temperature, p is the pressure, Sigma is the stress, sigma is the sphere diameter, epsilon is the interaction energy scale, and m is the sphere mass. We demonstrate that the phase diagram is universal at low p sigma(3)/epsilon; at low pressure, observables such as the relaxation time are insensitive to details of the interaction potential and collapse onto the values for hard spheres, provided the observables are nondimensionalized by the pressure. We determine the shape of the jamming surface in the jamming phase diagram, organize previous results in relation to the jamming phase diagram, and discuss the significance of various limits.

Unraveling the Mott-Peierls intrigue in vanadium dioxide - Grandi, Francesco; Amaricci, Adriano; Fabrizio, M. (Physical Review Research, 2020)
Vanadium dioxide is one of the most studied strongly correlated materials. Nonetheless, the intertwining between electronic correlation and lattice effects has precluded a comprehensive description of the rutile metal to monoclinic insulator transition, in turn triggering a longstanding "the chicken or the egg" debate about which comes first, the Mott localization or the Peierls distortion. Here, we suggest that this problem is in fact ill posed: The electronic correlations and the lattice vibrations conspire to stabilize the monoclinic insulator, and so they must be both considered to not miss relevant pieces of the VO2 physics. Specifically, we design a minimal model for VO2 that includes all the important physical ingredients: the electronic correlations, the multiorbital character, and the two components of the antiferrodistortive mode that condense in the monoclinic insulator. We solve this model by dynamical mean-field theory within the adiabatic Born-Oppenheimer approximation. Consistently with the first-order character of the metal-insulator transition, the Born-Oppenheimer potential has a rich landscape, with minima corresponding to the undistorted phase and to the four equivalent distorted ones, and which translates into an equally rich thermodynamics that we uncover by the Monte Carlo method. Remarkably, we find that a distorted metal phase intrudes between the low-temperature distorted insulator and high-temperature undistorted metal, which sheds new light on the debated experimental evidence of a monoclinic metallic phase.
CRIS-Team WoS Importer:2021-01-22

Utilizing Minkowski functionals for image analysis: a marching square algorithm - Mecke, Klaus; Jacobs, Karin; et al. (Journal of Statistical Mechanics-Theory and Experiment, 2008)
Comparing noisy experimental image data with statistical models requires a quantitative analysis of grey-scale images beyond mean values and two-point correlations. A real-space image analysis technique is introduced for digitized grey-scale images, based on Minkowski functionals of thresholded patterns. A novel feature of this marching square algorithm is the use of weighted side lengths for pixels, so that boundary lengths are captured accurately. As examples to illustrate the technique we study surface topologies emerging during the dewetting process of thin films and analyse spinodal decomposition as well as turbulent patterns in chemical reaction-diffusion systems. The grey-scale value corresponds to the height of the film or to the concentration of chemicals, respectively. Comparison with analytic calculations in stochastic geometry models reveals a remarkable agreement of the examples with a Gaussian random field. Thus, a statistical test for non-Gaussian features in experimental data becomes possible with this image analysis technique-even for small image sizes. Implementations of the software used for the analysis are offered for download.
Keywords: patterns; liquid films (experiment); pattern formation (theory); pattern formation (experiment)

Variational Hamiltonian simulation for translational invariant systems via classical pre-processing - Mansuroglu, Refik; Eckstein, Timo; Nützel, Ludwig; Wilkinson, Samuel; Hartmann, Michael (Quantum Science and Technology, 2023)
The simulation of time evolution of large quantum systems is a classically challenging and in general intractable task, making it a promising application for quantum computation. A Trotter-Suzuki approximation yields an implementation thereof, where a higher approximation accuracy can be traded for an increased gate count. In this work, we introduce a variational algorithm which uses solutions of classical optimizations to predict efficient quantum circuits for time evolution of translationally invariant quantum systems. Our strategy can improve upon the Trotter-Suzuki accuracy by several orders of magnitude. It translates into a reduction in gate count and hence gain in overall fidelity at the same algorithmic accuracy. This is important in noisy intermediate scale quantum-applications where the fidelity of the output state decays exponentially with the number of gates. The performance advantage of our classical assisted strategy can be extended to open boundaries with translational symmetry in the bulk. We can extrapolate our method to beyond classically simulatable system sizes, maintaining its total fidelity advantage over a Trotter-Suzuki approximation making it an interesting candidate for beyond classical time evolution.
CRIS-Team Scopus Importer:2023-02-17
Keywords: Hamiltonian simulation; noisy intermediate scale quantum; time evolution; translational invariance; Trotter-Suzuki decomposition; variational algorithms

Variational quantum eigensolver ansatz for the J1-J2 -model - Feulner, Verena; Hartmann, Michael (Physical Review B, 2022)
The ground state properties of the two-dimensional J1-J2-model are very challenging to analyze via classical numerical methods due to the high level of frustration. This makes the model a promising candidate for which quantum computers could be helpful to possibly explore regimes that classical computers cannot reach. The J1-J2-model is a quantum spin model composed of Heisenberg interactions along the rectangular lattice edges and along diagonal edges between next-nearest-neighbor spins. We propose an ansatz for the variational quantum eigensolver to approximate the ground state of an antiferromagnetic J1-J2 Hamiltonian for different lattice sizes and different ratios of J1 and J2. Moreover, we demonstrate that this ansatz can work without the need for gates along the diagonal next-nearest-neighbor interactions. This simplification is of great importance for solid-state-based hardware with qubits on a rectangular grid, where it eliminates the need for swap gates. In addition, we provide an extrapolation for the number of gates and parameters needed for larger lattice sizes, showing that these are expected to grow linearly in the qubit number up to lattice sizes which eventually can no longer be treated with classical computers.
CRIS-Team Scopus Importer:2022-11-04

Variations on a QM/MM approach to cluster dynamics - Dinh, P. M.; Berger, D.; Faber, Bernd; Reinhard, Paul-Gerhard; Suraud, E. (Journal of Physics: Conference Series, 2012)
We discuss two extensions of a hierarchical QM/MM approach developed to describe clusters embedded into a polarizable environment. The first extension consists in considering the presence of a supporting metal. More specifically we consider the structure and dynamics of Na clusters on a surface built from Ar layers grown on a metal support. We study the effect of the Ar substrate and of the metal support. We find a faint binding on the surface and the effect of the dielectric response of the metal (DRM) turns out to be negligible. Deposit of small Na clusters is crucially influenced by the mechanical hardness of the metal support and the number of Ar layers, while the DRM makes little effect. The second extension consists in using the QM/MM approach to study color centers at an MgO surface. The particular efficiency of the QM/MM approach allows to consider large system's sizes and thus test the impact of the (necessarily) finite (surface) sample. We find that our results compare favorably with ab-initio approaches as well as with available experimental results.
CRIS-Team Scopus Importer:2023-07-24

Variations on a theme by Skyrme: A systematic study of adjustments of model parameters - Klüpfel, P.; Reinhard, Paul-Gerhard; Bürvenich, T. J.; Maruhn, J. A. (Physical Review C - Nuclear Physics, 2009)
We present a survey of the phenomenological adjustment of the parameters of the Skyrme-Hartree-Fock (SHF) model for a self-consistent description of nuclear structure and low-energy excitations. A large sample of reliable input data from nuclear bulk properties (energy, radii, surface thickness) is selected guided by the criterion that ground-state correlations should remain small. Least-squares fitting techniques are used to determine the SHF parameters that accommodate best the given input data. The question of the predictive value of the adjustment is scrutinized by performing systematic variations with respect to chosen nuclear matter properties (incompressibility, effective mass, symmetry energy, and sum-rule enhancement factor). We find that the ground-state data, although representing a large sample, leave a broad range of choices, i.e., a broad range of nuclear matter properties. Information from giant resonances is added to pin down more precisely the open features. We then apply the set of newly adjusted parametrizations to several more detailed observables such as neutron skin, isotope shifts, and super-heavy elements. The techniques of least-squares fitting provide safe estimates for the uncertainties of such extrapolations. The systematic variation of forces allows to disentangle the various influences on a given observable and to estimate the predictive value of the SHF model. The results depend very much on the observable under consideration. © 2009 The American Physical Society.
CRIS-Team Scopus Importer:2023-07-24

Verification of Surface Tension in the Parallel Free Surface Lattice Boltzmann Method in waLBerla - Donath, Stefan; Mecke, Klaus; Rabha, Swapna; Buwa, Vivek; Rüde, Ulrich (Computers & Fluids, 2011)
The free surface lattice Boltzmann method in the parallel software framework waLBerla solves a wide range of two-phase flow scenarios efficiently by neglecting the gas flow and only taking the gas pressure into account. To obtain a good parallel efficiency, an extensive algorithm computes the curvature needed to calculate the surface tension pressure by using only a restricted set of neighboring data, reducing the communication effort. This, however, is assumed to be the main limitation of the accuracy of the surface tension computation. Nevertheless, this work shows that this method is able to simulate typical two-phase applications accurately enough. It is validated by comparing the evolution of capillary waves to analytical solution, and checking the agreement of a rising bubble's velocity with experiments. The results show that the method differs less than 10% from theory in the micro-scale, and lies well within the confidence interval of experimental measurements. © 2010 Elsevier Ltd.
UnivIS-Import:2015-04-14:Pub.2011.tech.IMMD.lsinfs.verifi_4
Keywords: Bubbly flows; Lattice Boltzmann method; Massively parallel; Surface tension

Vertex-Based Diagrammatic Treatment of Light-Matter-Coupled Systems - Kim, Aaram J.; Lenk, Katharina; Li, Jiajun; Werner, Philipp; Eckstein, Martin (Physical Review Letters, 2023)
We propose a diagrammatic Monte Carlo approach for quantum impurity models, which can be regarded as a generalization of the strong-coupling expansion for fermionic impurity models. The algorithm is based on a self-consistently computed three-point vertex and a stochastically sampled four-point vertex, and it allows one to obtain numerically exact results in a wide parameter regime. The performance of the algorithm is demonstrated with applications to a spin-boson model representing an emitter in a waveguide. As a function of the coupling strength, the spin exhibits a delocalization-localization crossover at low temperatures, signaling a qualitative change in the real-time relaxation. In certain parameter regimes, the response functions of the emitter coupled to the electromagnetic continuum can be described by an effective Rabi model with appropriately defined parameters. We also discuss the spatial distribution of the photon density around the emitter.
CRIS-Team Scopus Importer:2023-02-10

Vertex corrections in the dynamic structure factor in spin ladders - Exius, I.; Schmidt, Kai Phillip; Lake, Bella; Tennant, D. A.; Uhrig, G. S. (Physical Review B, 2010)
We combine the results of perturbative continuous unitary transformations with a mean-field calculation to determine the evolution of the single-mode, i.e., one-triplon, contribution to the dynamic structure factor of a two-leg S=1/2 ladder on increasing temperature from zero to a finite value. The temperature dependence is induced by two effects: (i) no triplon can be excited on a rung where a thermally activated triplon is present and (ii) conditional excitation processes take place if a thermally activated triplon is present. Both effects diminish the one-triplon spectral weight upon heating. It is shown that the second effect is the dominant vertex correction in the calculation of the dynamic structure factor. The matrix elements describing the conditional triplon excitation in the two-leg Heisenberg ladder with additional four-spin ring exchange are calculated perturbatively up to order 9. The calculated results are compared to those of an inelastic neutron scattering experiment on the cuprate-ladder compound La(4)Sr(10)Cu(24)O(41) showing convincing agreement for established values of the exchange constants.

Vesicles as a model for controlled (de-) adhesion of cells: a thermodynamic approach - Smith, Ana-Suncana; Seifert, Udo (Soft Matter, 2007)
We review the specific adhesion between ligand-containing vesicles and receptor-functionalized substrates as an established model system used to study the cell recognition process and its control mechanisms. In order to provide better understanding of the underlying physics and to allow for quantitative exploitation of this system, we develop a simple theoretical framework that accounts for the equilibrium state of adhesion and successfully merges the macroscopic and microscopic aspects of the problem. Several mechanisms that are used to control adhesion or induce de-adhesion are studied on the same level of theory. Specifically, the repelling properties of adhesive molecules, the role of repelling molecules, the action of antagonists for a specific binder as well as the influence of an externally applied force are addressed independently within the same formalism.
Keywords: FLUID MEMBRANES; UNBINDING TRANSITIONS; SUPPORTED MEMBRANES; BOUND VESICLES; SURFACE; LIGAND; FORCE; BILAYER; MOLECULES; DYNAMICS

Vibrational instabilities in resonant electron transport through single-molecule junctions - Härtle, Rainer; Thoss, Michael (Physical Review B, 2011)
We analyze various limits of vibrationally coupled resonant electron transport in single-molecule junctions. Based on a master equation approach, we discuss analytic and numerical results for junctions under a high bias voltage or weak electronic-vibrational coupling. It is shown that in these limits, the vibrational excitation of the molecular bridge increases indefinitely, i.e., the junction exhibits a vibrational instability. Moreover, our analysis provides analytic results for the vibrational distribution function and reveals that these vibrational instabilities are related to electron-hole pair creation processes. © 2011 American Physical Society.
UnivIS-Import:2015-03-09:Pub.2011.nat.dphy.ITP.ppem.vibrat

Vibrationally coupled electron transport in single-molecule junctions: The importance of electron-hole pair creation processes - Haertle, R.; Peskin, U.; Thoss, Michael (physica status solidi (b), 2013)
Vibrationally coupled electron transport through single-molecule junctions is considered. Reviewing our recent theoretical work, we show that electron-hole pair creation processes represent a key to understand the vibrational excitation characteristic of a single-molecule contact. Moreover, these processes can lead to a number of interesting transport phenomena such as, for example, negative differential resistance, rectification, mode-selective vibrational excitation, and a pronounced temperature dependence of the electrical current. Thus, electron-hole pair creation processes are crucial to elucidate the basic mechanisms of vibrationally coupled electron transport through a single-molecule contact, despite the fact that these processes do not directly contribute to the electrical current that is flowing through the junction. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
UnivIS-Import:2015-03-09:Pub.2013.nat.dphy.ITP.ppem.vibrat_4
Keywords: Electron-hole pair creation; Electronic-vibrational coupling; Molecular electronics; Nonequilibrium Greens-function theory; Vibrationally coupled electron transport

Vibrationally dependent electron-electron interactions in resonant electron transport through single-molecule junctions - Erpenbeck, André; Haertle, R.; Bockstedte, Michel Georg; Thoss, Michael (Physical Review B, 2016)
We investigate the role of electronic-vibrational coupling in resonant electron transport through single-molecule junctions, taking into account that the corresponding coupling strengths may depend on the charge and excitation state of the molecular bridge. Within an effective-model Hamiltonian approach for a molecule with multiple electronic states, this requires to extend the commonly used model and include vibrationally dependent electron-electron interaction. We use Born-Markov master equation methods and consider selected models to exemplify the effect of the additional interaction on the transport characteristics of a single-molecule junction. In particular, we show that it has a significant influence on local cooling and heating mechanisms, it may result in negative differential resistance, and it may cause pronounced asymmetries in the conductance map of a single-molecule junction.
UnivIS-Import:2017-12-18:Pub.2016.nat.dphy.ITP.ppem.vibrat

Vibrationally induced decoherence in single-molecule junctions - Härtle, Rainer; Thoss, Michael; et al. (Physical Review B, 2013)
We investigate the interplay of quantum interference effects and electronic-vibrational coupling in electron transport through single-molecule junctions, employing a nonequilibrium Green's function approach. Our findings show that inelastic processes lead, in general, to a quenching of quantum interference effects. This quenching is more pronounced for increasing bias voltages and levels of vibrational excitation. As a result of this vibrationally induced decoherence, vibrational signatures in the transport characteristics of a molecular contact may strongly deviate from a simple Franck-Condon picture. This includes signatures in both the resonant and the nonresonant transport regimes. Moreover, it is shown that local cooling by electron-hole pair creation processes can influence the transport characteristics profoundly, giving rise to a significant temperature dependence of the electrical current. © 2013 American Physical Society.
UnivIS-Import:2015-03-09:Pub.2013.nat.dphy.ITP.ppem.vibrat

Water in an electric field does not dance alone: The relation between equilibrium structure, time dependent viscosity and molecular motions - Baer, Andreas; Milicevic, Zoran; Smith, David M.; Smith, Ana-Suncana (Journal of Molecular Liquids, 2019)
Dynamic structuring of water is a key player in a large class of processes underlying biochemical and technological developments today, the latter often involving electric fields. However, the anisotropic coupling between the water structure and the field has not been understood on a molecular level so far. Here we perform extensive molecular dynamics simulations to explore the influence of an externally imposed electric field on liquid water under ambient conditions. Using self-developed analysis tools and rigorous statistical analysis, we unambiguously show that water hydration shells break into subcompartments, which were hitherto not observed due to radial averaging. The shape of subcompartments is sensitive to the field magnitude and affects excitations of the hydrogen bond network including the femtosecond stretching and the sub-picosecond restructuring of hydrogen bonds. Furthermore, by analysing the reorientational dynamics of water molecules, we ascertain the existence of cooperative excitations of small water clusters. Enabled by the interplay between hydrogen bonding, and the coupling of water dipoles to the field, these coordinated motions, occurring on the picosecond time scale, are associated with fluctuations between torque-free states of water dipoles. We show that unlike the coupling between translation and reorientation of water molecules, which takes place on even longer time scales, these coordinated motions are the key for understanding the emergent anisotropy of diffusion and viscosity of water. Particular effort is invested to provide an analysis that allows for future experimental validation. (C) 2019 Published by Elsevier B.V.
CRIS-Team WoS Importer:2019-05-09

Wave packet simulations for the insulator-metal transition in dense hydrogen - Jakob, B.; Reinhard, Paul-Gerhard; Toepffer, Christian; Zwicknagel, Günter (Journal of Physics A: Mathematical and Theoretical, 2009)
Dense hydrogen is studied in the framework of wave packet simulations. In this semi-quantal method the electrons are represented by wave packets which are suitably parameterized, e.g. Gaussians. The time evolution of the system and the equilibrium properties are obtained with the help of a variational principle and by Monte Carlo sampling, respectively. A transition from a molecular to a metallic state is observed. The wave packets become delocalized and the electrical conductivity increases sharply. The phase diagram is calculated in a wide range of the pressure-density-temperature space. In the transition from the molecular to the metallic state the density increases in agreement with recent reverberating shock wave experiments. © 2009 IOP Publishing Ltd.
CRIS-Team Scopus Importer:2023-07-24

Wave propagation on a random lattice - Sahlmann, Hanno (Physical Review D - Particles, Fields, Gravitation and Cosmology, 2010)
Motivated by phenomenological questions in quantum gravity, we consider the propagation of a scalar field on a random lattice. We describe a procedure to calculate the dispersion relation for the field by taking a limit of a periodic lattice. We use this to calculate the lowest order coefficients of the dispersion relation for a specific one-dimensional model. © 2010 The American Physical Society.

Weak Poisson structures on infinite dimensional manifolds and hamiltonian actions - Neeb, Karl Hermann; Thiemann, Thomas; Sahlmann, Hanno (Springer Proceedings in Mathematics & Statistics, 2015)
We introduce a notion of a weak Poisson structure on a manifold M modeled on a locally convex space. This is done by specifying a Poisson bracket on a subalgebra A⊆C∞(M)" id="MathJax-Element-1-Frame" role="presentation" style="position: relative;" tabindex="0">A⊆C∞(M) which has to satisfy a non-degeneracy condition (the differentials of elements of A" id="MathJax-Element-2-Frame" role="presentation" style="position: relative;" tabindex="0">A separate tangent vectors) and we postulate the existence of smooth Hamiltonian vector fields. Motivated by applications to Hamiltonian actions, we focus on affine Poisson spaces which include in particular the linear and affine Poisson structures on duals of locally convex Lie algebras. As an interesting byproduct of our approach, we can associate to an invariant symmetric bilinear form κ on a Lie algebra g" id="MathJax-Element-3-Frame" role="presentation" style="position: relative;" tabindex="0">g and a κ-skew-symmetric derivation D a weak affine Poisson structure on g" id="MathJax-Element-4-Frame" role="presentation" style="position: relative;" tabindex="0">g itself. This leads naturally to a concept of a Hamiltonian G-action on a weak Poisson manifold with a g" id="MathJax-Element-5-Frame" role="presentation" style="position: relative;" tabindex="0">g -valued momentum map and hence to a generalization of quasi-hamiltonian group actions.

What about U on surfaces? Extended Hubbard models for adatom systems from first principles - Hansmann, Philipp; Vaugier, Loig; Jiang, Hong; Biermann, Silke (Journal of Physics: Condensed Matter, 2013)
Electronic correlations together with dimensional constraints lead to some of the most fascinating properties known in condensed matter physics. As possible candidates where these conditions are realized, semiconductor (111) surfaces and adatom systems on surfaces have been under investigation for quite some time. However, state-of-the-art theoretical studies on these materials that include many-body effects beyond the band picture are rare. First principles estimates of inter-electronic Coulomb interactions for the correlated states are missing entirely, and usually these interactions are treated as adjustable parameters. In this work, we report on calculations of the interaction parameters for the group IV surface-adatom systems in the alpha-phase series of Si(111):C, Si, Sn, Pb. For all systems investigated, the inter-electronic Coulomb interactions are indeed large compared to the kinetic energies of the states in question. Moreover, our study reveals that intersite interactions cannot be disregarded. We explicitly construct an extended Hubbard model for the series of group IV surface-adatom systems on silicon, which can be used for further many-body calculations.
CRIS-Team WoS Importer:2020-02-19

What does the Penrose operator measure in loop quantum gravity? - Wichert, Josef; Sahlmann, Hanno (2016)

What Phasons Look Like: Particle Trajectories in a Quasicrystalline Potential - Kromer, Justus; Schmiedeberg, Michael; Roth, Johannes; Stark, Holger (Physical Review Letters, 2012)
Among the distinctive features of quasicrystals-structures with long-range order but without periodicity-are phasons. Phasons are hydrodynamic modes that, like phonons, do not cost free energy in the long-wavelength limit. For light-induced colloidal quasicrystals, we analyze the collective rearrangements of the colloids that occur when the phasonic displacement of the light field is changed. The colloidal model system is employed to study the link between the continuous description of phasonic modes in quasicrystals and collective phasonic flips of atoms. We introduce characteristic areas of reduced phononic and phasonic displacements and use them to predict individual colloidal trajectories. In principle, our method can be employed with all quasicrystalline systems in order to derive collective rearrangements of particles from the continuous description of phasons.

When do measures on the space of connections support the triad operators of loop quantum gravity? - Sahlmann, Hanno (Journal of Mathematical Physics, 2011)
In this work we investigate the question under what conditions Hilbert spaces that are induced by measures on the space of generalized connections carry a representation of certain non-Abelian analogues of the electric flux. We give the problem a precise mathematical formulation and start its investigation. For the technically simple case of U(1) as gauge group, we establish a number of "no-go theorems" asserting that for certain classes of measures, the flux operators can not be represented on the corresponding Hilbert spaces. The flux-observables we consider, play an important role in loop quantum gravity since they can be defined without recurse to a background geometry and they might also be of interest in the general context of quantization of non-Abelian gauge theories. © 2011 American Institute of Physics.

When do microswimmers exit the Stokes regime - Pickl, Kristina; Köstler, Harald; Pande, Jayant; Smith, Ana-Suncana; Rüde, Ulrich (2016)
We compare fully-resolved, 3D lattice Boltzmann (LB) simulations of a three sphere microswimmer to analytical calculations, and show thereby that (i) LB simulations reproduce the motion very well in the Stokes regime, and (ii) the swimmer exits this regime at Reynolds numbers Re ∼ 10−2 , significantly lower than previously realised. Below this Re value Stokesian theory describes the motion accurately, but fails above it due to inertial effects. In the latter case, the swimmer’s relaxation matches that of an underdamped harmonic oscillator, and this specifies its effective hydrodynamic radius in a narrow Re range, as we show by calculating the radius analytically. The method can be used to determine the limit of the Stokes regime and the effective radius for a general mechanical microswimmer.

X-ray synchrotron study of liquid-vapor interfaces at short length scales: Effect of long-range forces and bending energies - Mecke, Klaus; et al. (Physical Review Letters, 2003)
We have investigated the small-scale structure of the liquid-vapor interface using synchrotron x-ray scattering for liquids with different molecular structures and interactions. The effective momentum-dependent surface energy first decreases from its macroscopic value due to the effect of long-range forces, and then increases with increasing wave vector. The results are analyzed using a recent density functional theory. The large wave-vector increase is attributed to a bending energy for which local and nonlocal contributions are equally important.

An average stochastic approach to two-body dissipation in finite fermion systems - Lacombe, L.; Suraud, E.; Reinhard, Paul-Gerhard; Dinh, P. M. (Annals of Physics, 2019)
We present average stochastic time-dependent Hartree–Fock (ASTDHF) as a theory for efficient handling of incoherent dynamical correlations in finite fermion systems at fully quantum-mechanical level. The basic time evolution is given by time-dependent mean-field theory. The two-body collisions beyond mean-field propagation are evaluated stochastically in an incoherent ensemble of mean-field states as was done in the previously developed STDHF. The new, and time saving, feature in ASTDHF is that it employs one common mean field throughout the whole ensemble. A detailed presentation of ASTDHF is given and extensive comparison with STDHF and other approaches to dissipation are discussed. ASTDHF is found to be a very efficient approach for well bound systems where mean-field fluctuations remain small.
CRIS-Team Scopus Importer:2019-05-28
Keywords: Dissipation dynamics; Mean-field theory; Stochastic approach; Two-body collisions

Asymptotic Growth of the Local Ground-State Entropy of the Ideal Fermi Gas in a Constant Magnetic Field - Leschke, Hajo; Sobolev, Alexander; Spitzer, Wolfgang (Communications in Mathematical Physics, 2020)
We consider the ideal Fermi gas of indistinguishable particles without spin but with electric charge, confined to a Euclidean plane R2 perpendicular to an external constant magnetic field of strength B > 0. We assume this (infinite) quantum gas to be in thermal equilibrium at zero temperature, that is, in its ground state with chemical potential mu = B (in suitable physical units). For this (pure) state we define its local entropy S(Lambda) associated with a bounded (sub)region Lambda subset of R-2 as the von Neumann entropy of the (mixed) local substate obtained by reducing the infinite-area ground state to this region Lambda of finite area vertical bar Lambda vertical bar. In this setting we prove that the leading asymptotic growth of S(L Lambda), as the dimensionless scaling parameter L > 0 tends to infinity, has the form L root B vertical bar partial derivative Lambda vertical bar up to a precisely given (positive multiplicative) coefficient which is independent of Lambda and dependent on B and mu only through the integer part of (mu/B-1)/2. Here we have assumed the boundary curve partial derivative Lambda of Lambda to be sufficiently smooth which, in particular, ensures that its arc length vertical bar partial derivative Lambda vertical bar is well-defined. This result is in agreement with a so-called area-law scaling (for two spatial dimensions). It contrasts the zero-field case B = 0, where an additional logarithmic factor ln( L) is known to be present. We also have a similar result, with a slightly more explicit coefficient, for the simpler situation where the underlying single-particle Hamiltonian, known as the Landau Hamiltonian, is restricted from its natural Hilbert space L-2(R-2) to the eigenspace of a single but arbitrary Landau level. Both results extend to the whole one-parameter family of quantum Renyi entropies. As opposed to the case B = 0, the corresponding asymptotic coefficients depend on the Renyi index in a non-trivial way.
CRIS-Team WoS Importer:2021-01-15

Beyond the charge radius: The information content of the fourth radial moment - Reinhard, Paul-Gerhard; Nazarewicz, W.; Ruiz, R. F. Garcia (Physical Review C - Nuclear Physics, 2020)
Measurements of atomic transitions in different isotopes offer key information on the nuclear charge radius. The anticipated high-precision experimental techniques, augmented by atomic calculations, will soon enable extraction of the higher order radial moments of the charge density distribution. To assess the value of such measurements for nuclear structure research, we study the information content of the fourth radial moment < r(4)> by means of nuclear density functional theory and a multiple correlation analysis. We show that < r(4)> can be directly related to the surface thickness of nuclear density, a fundamental property of the atomic nucleus that is difficult to obtain for radioactive systems. Precise knowledge of these radial moments is essential to establish reliable constraints on the existence of new forces from precision isotope shift measurements.
CRIS-Team WoS Importer:2020-02-18

First-order phase boundaries of the massive (1+1)-dimensional Nambu-Jona-Lasinio model with isospin - Thies, Michael (Physical Review D, 2020)
The massive two-dimensional Nambu-Jona-Lasinio model with isospin (isoNJL) is reconsidered in the large N-c limit. We continue the exploration of its phase diagram by constructing missing first-order phase boundaries. At zero temperature, a phase boundary in the plane of baryon and isospin chemical potentials separates the vacuum from a crystal phase. We derive it from the baryon spectrum of the isoNJL model which, in turn, is obtained via a numerical Hartree-Fock (HF) calculation. At finite temperature, a first-order phase boundary sheet is found using a thermal HF calculation. It interpolates smoothly between the zero temperature phase boundary and the perturbative sheet. The calculations remain tractable owing to the assumption that the charged pion condensate vanishes. In that case, most of the calculations can be done with methods developed in the past for solving the massive one-flavor NJL model.
CRIS-Team WoS Importer:2020-04-28

Laser spectroscopy of neutron-rich tin isotopes: A discontinuity in charge radii across the N=82 shell closure - Gorges, C.; Rodríguez, L. V.; Balabanski, D. L.; Bissell, M. L.; Blaum, K.; Cheal, B.; Garcia Ruiz, R. F.; Georgiev, G.; Gins, W.; Heylen, H.; Kanellakopoulos, A.; Kaufmann, S.; et al. (Physical Review Letters, 2019)
The change in mean-square nuclear charge radii δ2© along the even-A tin isotopic chain Sn108-134 has been investigated by means of collinear laser spectroscopy at ISOLDE/CERN using the atomic transitions 5p2 S10→5p6 sP11 and 5p2 P30→5p6s P31. With the determination of the charge radius of Sn134 and corrected values for some of the neutron-rich isotopes, the evolution of the charge radii across the N=82 shell closure is established. A clear kink at the doubly magic Sn132 is revealed, similar to what has been observed at N=82 in other isotopic chains with larger proton numbers, and at the N=126 shell closure in doubly magic Pb208. While most standard nuclear density functional calculations struggle with a consistent explanation of these discontinuities, we demonstrate that a recently developed Fayans energy density functional provides a coherent description of the kinks at both doubly magic nuclei, Sn132 and Pb208, without sacrificing the overall performance. A multiple correlation analysis leads to the conclusion that both kinks are related to pairing and surface effects.
CRIS-Team Scopus Importer:2019-05-28

Measurement and microscopic description of odd–even staggering of charge radii of exotic copper isotopes - De Groote, R. P.; Billowes, J.; Binnersley, C. L.; Bissell, M. L.; Cocolios, T. E.; Goodacre, T. Day; Farooq-Smith, G. J.; Fedorov, D. V.; Flanagan, K. T.; Franchoo, S.; Ruiz, R. F. Garcia; Gins, W.; et al. (Nature Physics, 2020)
Nuclear charge radii globally scale with atomic mass number A as A1∕3, and isotopes with an odd number of neutrons are usually slightly smaller in size than their even-neutron neighbours. This odd–even staggering, ubiquitous throughout the nuclear landscape1, varies with the number of protons and neutrons, and poses a substantial challenge for nuclear theory2–4. Here, we report measurements of the charge radii of short-lived copper isotopes up to the very exotic 78Cu (with proton number Z = 29 and neutron number N = 49), produced at only 20 ions s–1, using the collinear resonance ionization spectroscopy method at the Isotope Mass Separator On-Line Device facility (ISOLDE) at CERN. We observe an unexpected reduction in the odd–even staggering for isotopes approaching the N = 50 shell gap. To describe the data, we applied models based on nuclear density functional theory5,6 and A-body valence-space in-medium similarity renormalization group theory7,8. Through these comparisons, we demonstrate a relation between the global behaviour of charge radii and the saturation density of nuclear matter, and show that the local charge radii variations, which reflect the many-body polarization effects, naturally emerge from A-body calculations fitted to properties of A ≤ 4 nuclei.
CRIS-Team Scopus Importer:2020-05-02

Phase structure of the (1+1)-dimensional Nambu-Jona-Lasinio model with isospin - Thies, Michael (Physical Review D, 2020)
The phase diagram of the two-dimensional Nambu-Jona-Lasinio model with isospin is explored in the large N-c limit with semiclassical methods. We consider finite temperature and include chemical potentials for all conserved charges. In the chiral limit, a full analytical solution is presented, expressed in terms of known results for the single-flavor Gross-Neveu and Nambu-Jona-Lasinio models. A novel crystalline structure appears and is shown explicitly to be thermodynamically more stable than the homogeneous phase at zero temperature. If we include a bare fermion mass, the problem reduces again to solved problems in one-flavor models provided that either the fermionic or the isospin chemical potentials vanish. In the general case, a stability analysis is used to construct the perturbative phase boundary between homogeneous and inhomogeneous phases. This is sufficient to get a good overview of the complete phase diagram. Missing nonperturbative phase boundaries requiring a full numerical Hartree-Fock calculation will be presented in future work.
CRIS-Team WoS Importer:2020-01-31

Progress towards a realistic theoretical description of C60 photoelectron-momentum imaging experiments using time-dependent density-functional theory - Wopperer, P.; Gao, C. Z.; Barillot, T.; Cauchy, C.; Marciniak, A.; Despre, Victor; Loriot, Vincent; Celep, G.; Bordas, Christian; Lepine, F.; Dinh, P. M.; Suraud, E.; et al. (Physical Review A - Atomic, Molecular, and Optical Physics, 2015)
We have studied theoretical photoelectron-momentum distributions of C60 using time-dependent density functional theory (TDDFT) in real time and including a self-interaction correction. Our calculations furthermore account for a proper orientation averaging allowing a direct comparison with experimental results. To illustrate the capabilities of this direct (microscopic and time-dependent) approach, two very different photo-excitation conditions are considered: excitation with a high-frequency XUV light at 20 eV and with a low-frequency IR femtosecond pulse at 1.55 eV. The interaction with the XUV light leads to one-photon transitions and a linear ionization regime. In that situation, the spectrum of occupied single-electron states in C60 is directly mapped to the photoelectron spectrum. On the contrary, the IR pulse leads to multiphoton ionization in which only the two least-bound states contribute to the process. In both dynamical regimes (mono- and multiphoton), calculated and experimental angle-resolved photoelectron spectra compare reasonably well. The observed discrepancies can be understood by the theoretical underestimation of higher-order many-body interaction processes such as electron-electron scattering and by the fact that experiments are performed at finite temperature. These results pave the way to a multiscale description of the C60 ionization mechanisms that is required to render justice to the variety of processes observed experimentally for fullerene molecules.
Created from Fastlane, Scopus look-up
Keywords: 31.15.ee; 33.60.+q; 36.20.Kd; 36.40.Cg

Proton superfluidity and charge radii in proton-rich calcium isotopes - Miller, A. J.; Minamisono, K.; Klose, A.; Garand, D.; Kujawa, C.; Lantis, J. D.; Liu, Y.; Maaß, B.; Mantica, P. F.; Nazarewicz, W.; Nörtershäuser, W.; Pineda, S. V.; et al. (Nature Physics, 2019)
One of the most important global properties of the atomic nucleus is its size. Experimentally determined nuclear charge radii carry unique information on the nuclear force and complex dynamics of protons and neutrons moving inside the nucleus. The intricate behaviour of charge radii along the chain of Ca isotopes, including the unexpectedly large charge radius of neutron-rich 52 Ca, poses a daunting challenge for nuclear theory 1 . Here we present the measurements of the charge radii of proton-rich isotopes 36,37,38 Ca, whose properties are impacted by the interplay between nuclear superfluidity and weak binding. Calculations carried out within nuclear density functional theory show that the combination of a novel interaction 2 and a state-of-the-art theoretical method can successfully explain the behaviour of charge radii from the lightest to the heaviest Ca isotopes. Through this model, we show how the new data on 36,37,38 Ca elucidate the nature of nucleonic pairing in weakly bound proton-rich isotopes.
CRIS-Team Scopus Importer:2019-05-14

Statistical aspects of nuclear mass models - Kejzlar, V.; Neufcourt, L.; Nazarewicz, W.; Reinhard, Paul-Gerhard (Journal of Physics G: Nuclear and Particle Physics, 2020)
We study the information content of nuclear masses from the perspective of global models of nuclear binding energies. To this end, we employ a number of statistical methods and diagnostic tools, including Bayesian calibration, Bayesian model averaging, chi-square correlation analysis, principal component analysis and empirical coverage probability. Using a Bayesian framework, we investigate the structure of the four-parameter liquid drop model by considering discrepant mass domains for calibration. We then use the chi-square correlation framework to analyze the 14-parameter Skyrme energy density functional calibrated using homogeneous and heterogeneous datasets. We show that quite a dramatic parameter reduction can be achieved in both cases. The advantage of Bayesian model averaging for improving uncertainty quantification is demonstrated. The statistical approaches used are pedagogically described; in this context this work can serve as a guide for future applications.
CRIS-Team Scopus Importer:2020-09-04

Supersymmetric quantum mechanics requires g=2 for vector bosons - Junker, Georg (EPL - Europhysics Letters, 2020)
Relativistic arbitrary spin Hamiltonians are shown to obey the algebraic structure of supersymmetric quantum system if their odd and even parts commute. This condition is identical to that required for the exactness of the Foldy-Wouthuysen transformation. Applied to a massive charged spin-1 particle in a constant magnetic field, supersymmetric quantum mechanics necessarily requires a gyromagnetic factor g = 2.
CRIS-Team Scopus Importer:2020-06-23

Supersymmetry of relativistic hamiltonians for arbitrary spin - Junker, Georg (Symmetry, 2020)
Hamiltonians describing the relativistic quantum dynamics of a particle with an arbitrary but fixed spin are shown to exhibit a supersymmetric structure when the even and odd elements of the Hamiltonian commute. Here, the supercharges transform between energy eigenstates of positive and negative energy. For such supersymmetric Hamiltonians, an exact Foldy-Wouthuysen transformation exists which brings it into a block-diagonal form separating the positive and negative energy subspaces. The relativistic dynamics of a charged particle in a magnetic field are considered for the case of a scalar (spin-zero) boson obeying the Klein-Gordon equation, a Dirac (spin one-half) fermion and a vector (spin-one) boson characterised by the Proca equation. In the latter case, supersymmetry implies for the Landé g-factor g = 2.
CRIS-Team Scopus Importer:2020-10-23
Keywords: Dirac equation; Klein-Gordon equation; Proca equation; Relativistic wave equation; Supersymmetry

The Axial Hartree-Fock plus BCS Code SkyAx - Reinhard, Paul-Gerhard; Schuetrumpf, B.; Maruhn, J. A. (Computer Physics Communications, 2021)
The nuclear mean-field model based on Skyrme forces can predict a variety of properties of nuclear ground states. We present the Code SKYAx solving the Hartree-Fock equations in two spatial dimensions assuming axial symmetry. Pairing can be included in the BCS approximation. The code is implemented with a view on computational speed.
CRIS-Team WoS Importer:2020-11-27