3D structural analysis: sensitivity of Minkowski functionals - Arns, C. H.; Knackstedt, M. A.; Mecke, Klaus (Journal of Microscopy, 2010)
The Minkowski functionals, a family of statistical measures based on the Euler-Poincare characteristic of n-dimensional space, are the complete set of additive morphological measures and can be simply calculated from local contributions. As such, they have found a wide range of applications. We consider the effects of distortions (drift, noise and blurring) on the morphological properties of complex random models, representative of a wide range of structure. Such distortions arise experimentally in imaging techniques due to diffraction, absorption and sample drift. The question is asked, how critically these distortions effect image quality as measured by the Minkowski functionals. Defining a length scale based on the two-point correlation function, we consider how distortion at different scales can lead to quantitative errors in morphological measures.
Keywords: Morphology; complex materials; porous media; Minkowski functional

Abelian Chern-Simons theory, Stokes' theorem, and generalized connections - Sahlmann, Hanno; Thiemann, Thomas (Journal of Geometry and Physics, 2012)
Generalized connections and their calculus have been developed in the context of quantum gravity. Here we apply them to abelian Chern-Simons theory. We derive the expectation values of holonomies in U(1) Chern-Simons theory using Stokes' theorem, flux operators and generalized connections. A framing of the holonomy loops arises in our construction, and we show how, by choosing natural framings, the resulting expectation values nevertheless define a functional over gauge invariant cylindrical functions.The abelian theory considered in the present article is the test case for our method. It can also be applied to the non-abelian theory. Results will be reported in a companion article. © 2011 Elsevier B.V.
Keywords: Abelian Chern-Simons theory; Generalized connections; Loop quantum gravity

Absence of circular polarisation in reflections of butterfly wing scales with chiral Gyroid structure - Saba, Matthias; Wilts, Bodo D.; Hielscher, Johannes; Schröder-Turk, Gerd (Materials Today, 2014)
The single Gyroid, a triply-periodic ordered chiral network of cubic symmetry, appears as a nanostructure in the green-colored wing scales of various butterflies. In lossless and perfectly ordered single Gyroid materials, the structural chirality leads to circularly polarized reflections from crystals oriented in the [100] direction. Here we report a circular polarisation study of the macroscopic reflections of the wing scales of Callophrys rubi and Teinopalpus imperialis that reveals no circular dichroism, that is, we find no significant difference in the reflectance values for left- and right-circularly polarized light. The reasons for the absence of circularly polarized reflections is likely to be a compound effect of various factors, including crystallite orientation, presence of both leftand right-handed single Gyroid enantiomers, and structural disorder. Each of these factors weakens, but does not fully extinguish, the circular polarisation signal. We further find a substantial amount of blue-absorbing pigment in those wing scales of C. rubi that are structured according to the single Gyroid. Numerical simulations demonstrate that absorption, while evidently reducing overall reflectance, does generally not reduce the circular dichroism strength. The experimental findings of this paper, however, clearly demonstrate that circular dichroism is absent from the reflections of the butterfly wing scale. Henceforth, the chiro-optical response of the idealised structure does not fulfil a biological photonic function.
EAM Import::2019-03-13

Accelerated gap collapse in a Slater antiferromagnet - Picano, Antonio; Eckstein, Martin (Physical Review B, 2021)
We study the melting of long-range antiferromagnetic order in the Hubbard model after an interaction quench, using nonequilibrium dynamical mean-field theory. From previous studies, the system is known to quickly relax into a prethermal symmetry-broken state. Using a convergent truncation of the memory integrals in the Kadanoff Baym equations, we unravel the subsequent relaxation dynamics of this state over several orders of magnitude in time. At long times, the prethermal state can be characterized by a single slow variable, which is related to the conduction band population. The dynamics of this variable is highly nonlinear, with a pronounced speedup once the gap falls below a certain value. This behavior indicates that nonthermal order can be self-sustained on some timescale because of a thermalization bottleneck is provided by the gap opened by the long-range order itself. These results cannot be reproduced using simple Fermi's golden rule estimate for the evolution of the conduction band population, and are distinct from the conventional scenario for the relaxation of prethermal states, in which a slow thermalization follows a rapid prethermalization.
CRIS-Team Scopus Importer:2021-04-30

A Chiral Beamsplitter Inspired by Butterfly Nanostructures - Turner, Mark D.; Saba, Matthias; Zhang, Qiming; Cumming, Benjamin P.; Schröder-Turk, Gerd; Gu, Min (Frontiers in Optics 2013, 2013)
Here we present a chiral analog of the linearly polarizing beamsplitter. This microscopic device was built from a biomimetic chiral photonic crystal and separates left and right-handed circularly polarized light at near-infrared wavelengths.
EAM Import::2019-03-13

A colloidal model system with tunable disorder: Solid-fluid transition and discontinuities in the limit of zero disorder - Richter, Christian; Schmiedeberg, Michael; Stark, Holger (European Physical Journal E, 2011)
We study a colloidal model system where disorder can be continuously tuned from no disorder-corresponding to a system that can crystallize-to large disorder where geometrical frustration occurs. The model system consists of colloidal particles with screened electrostatic repulsion. They can only move on single lines which are parallel and equidistant to each other. We introduce disorder by modulating the particle line density. The system exhibits a solid-to-fluid transition which we study by the structure factor and the temporal evolution of the mean-square distance of nearest neighbors on neighboring lines. A determining feature is the occurrence of discontinuities when disorder is tuned to zero. We observe that the peak height of the pair correlation function in the solid phase does not extrapolate to the value of the perfect crystal. Similarly, the mean interaction energy and the screening length at which the solid-fluid transition occurs seem to be discontinuous when the limit of zero disorder is approached.

A comparison of different choices of clocks in a reduced phase space quantization in loop quantum cosmology with an inflationary potential using effective techniques - Giesel, Kristina; Li, Bao-Fei; Singh, Parampreet (The Sixteenth Marcel Grossmann Meeting, pp. 4228-4233 (2023), 2023)
We review recent results that apply 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. All three models that we consider are two-fluid models and they differ by their choice of global clock which are chosen to be either Gaussian dust, Brown-Kuchař dust or a massless Klein-Gordon scalar field. Although two-fluid models are more complicated than models involving the inflaton only, it turns out that some of the technical hurdles in conventional quantum cosmological models can be bypassed in these models. Using the effective dynamics resulting from the reduced phase space quantization we discuss some phenomenological implications of these models including the resolution of the big bang singularity via a quantum bounce and in addition address the question whether different choices of clocks can leave an imprint on the inflationary dynamics.<br

A Comprehensive Brownian Dynamics Approach for the Determination of Non-ideality Parameters from Analytical Ultracentrifugation - Uttinger, Maximilian; Wawra, Simon; Guckeisen, Tobias; Walter, Johannes; Bear, Andreas; Thajudeen, Thaseem; Sherwood, Peter J.; Smith, Ana-Suncana; Wagemans, Anja M.; Stafford, Walter F.; Peukert, Wolfgang (Langmuir, 2019)
Brownian dynamics (BD) has been applied as a comprehensive tool to model sedimentation and diffusion of nanoparticles in analytical ultracentrifugation (AUC) experiments. In this article, we extend the BD algorithm by considering space-dependent diffusion and solvent compressibility. With this, the changes in the sedimentation and diffusion coefficient from altered solvent properties at increased pressures are accurately taken into account. Moreover, it is demonstrated how the concept of space-dependent diffusion is employed to describe concentration-dependent sedimentation and diffusion coefficients, in particular, through the Gralen coefficient and the second virial coefficient. The influence of thermodynamic nonideality on diffusional properties can be accurately simulated and agree with well-known evaluation tools. BD simulations for sedimentation equilibrium and sedimentation velocity (SV) AUC experiments including effects of hydrodynamic and thermodynamic nonideality are validated by global evaluation in SEDANAL. The interplay of solvent compressibility and retrieved nonideality parameters can be studied utilizing BD. Finally, the second virial coefficient is determined for lysozyme from SV AUC experiments and BD simulations and compared to membrane osmometry. These results are in line with DLVO theory. In summary, BD simulations are established for the validation of nonideal sedimentation in AUC providing a sound basis for the evaluation of complex interactions even in polydisperse systems.
CRIS-Team Scopus Importer:2019-09-17

Active Brownian and inertial particles in disordered environments: Short-time expansion of the mean-square displacement - Breoni, Davide; Schmiedeberg, Michael; Löwen, Hartmut (Physical Review E, 2020)
We consider an active Brownian particle moving in a disordered two-dimensional energy or motility landscape. The averaged mean-square displacement (MSD) of the particle is calculated analytically within a systematic short-time expansion. As a result, for overdamped particles, both an external random force field and disorder in the self-propulsion speed induce ballistic behavior adding to the ballistic regime of an active particle with sharp self-propulsion speed. Spatial correlations in the force and motility landscape contribute only to the cubic and higher-order powers in time for the MSD. Finally, for inertial particles two superballistic regimes are found where the scaling exponent of the MSD with time is α=3 and α=4. We confirm our theoretical predictions by computer simulations. Moreover, they are verifiable in experiments on self-propelled colloids in random environment
Keywords: Active particles; Brownian motion; Inertia

Active phase field crystal systems with inertial delay and underdamped dynamics - Arold, Dominic; Schmiedeberg, Michael (European Physical Journal E, 2020)
Active matter systems often are well approximated as overdamped, meaning that any inertial momentum is immediately dissipated by the environment. On the other hand, especially for macroscopic systems but also for many mesoscopic ones particle mass can become relevant for the dynamics. For such systems we recently proposed an underdamped continuum model which captures translationally inertial dynamics via two contributions. First, convection and second a damping time scale of inertial motion. In this paper, we ask how both of these features influence the collective behavior compared to overdamped dynamics by studying the example of the active phase field crystal model. We first focus on the case of suppressed convection to study the role of the damping time. We quantify that the relaxation process to the steady collective motion state is considerably prolonged with damping time due to the increasing occurrence of transient groups of circularly moving density peaks. Finally, we illustrate the fully underdamped case with convection. Instead of collective motion of density peaks we then find a coexistence of constant high and low density phases reminiscent of motility-induced phase separatio
Keywords: active matter; microswimmer; inertia; Phase Field Model; non-equillibrium

Adaptive optics enhanced direct laser writing of high refractive index gyroid photonic crystals in chalcogenide glass - Cumming, Benjamin P.; Turner, Mark D.; Schröder-Turk, Gerd; Debbarma, Sukanta; Luther-Davies, Barry; Gu, Min (Optics Express, 2014)
Chiral gyroid photonic crystals are fabricated in the high refractive index chalcogenide glass arsenic trisulfide with an adaptive optics enhanced direct laser writing system. The severe spherical aberration imparted when focusing into the arsenic trisulfide is mitigated with a defocus decoupled aberration compensation technique that reduces the level of aberration that must be compensated by over an order of magnitude. The fabricated gyroids are shown to have excellent uniformity after our adaptive optics method is employed, and the transmission spectra of the gyroids are shown to have good agreement with numerical simulations that are based on a uniform and diffraction limited fabrication resolution.
EAM Import::2019-03-13

Adhesion of biological membranes - Sengupta, Kheya; Smith, Ana-Suncana (Springer International Publishing, 2018)
One of the most important triggers of cell activity is adhesion, a process by which cells and their organelles interact and attach to substrates, internal scaffolds, external interfaces, or other cells. The physiological and pathological significance of cell adhesion is hard to exaggerate, and adhesion is ubiquitous in the living world. Adhesive contacts need to be able to function in widely varying circumstances and must be established in an extremely noisy environment. For these reasons, the control mechanisms of adhesion have had to develop so as to be able to permanently monitor and correct cellular performance. While a lot of effort has been invested into understanding the biochemical aspect of these processes, the underlying physical principles of adhesion regulation have obtained significantly less appreciation. Only in recent years have these two approaches begun to converge in a unified view. Due to the strong coupling of the biochemical reactions to the spatial coordination provided by membranes and the cytoskeleton, biological signaling is subject to a plethora of physical constraints. Indeed, many signaling pathways, particularly those involving the adhesion, involve protein diffusion and aggregation guided by membranes. It is these aspects of adhesion that can be understood in the framework of statistical physics, as we intend to demonstrate in this short review. Here we summarize the developments in understanding cell and membrane adhesion from a theoretical point of view and support it with experiments in model systems as well as with living cells.
CRIS-Team Scopus Importer:2022-06-05
Keywords: Artificial cell; Cell adhesion; Competitive binding; Ligand-receptor interactions; Multiscale approaches to adhesion

Adhesion of giant vesicles mediated by weak binding of Sialyl-Lewis(x) to E-selectin in the presence of repelling poly(ethylene glycol) molecules - Lorz, Barbara Gertrud; Smith, Ana-Suncana; Gege, Christian; Sackmann, Erich (Langmuir, 2007)
Prior to establishing tight contact with the endothelium, cells such as leukocytes or cancer cells use the recognition between sialyl-Lewis(X) ligands and E-selectin receptors to establish weak, reversible adhesion and to roll along the vessel wall. We study the physical aspects of this process by constructing a mimetic system that consists of a giant fluid vesicle with incorporated lipid-anchored sialyl-Lewis(X) molecules that bind to E-selectin that is immobilized on the flat substrate. The vesicles also carry a certain fraction of repelling PEG2000 molecules. We analyze the equilibrium state of adhesion in detail by means of reflection interference contrast microscopy and find that the adhesion process relies purely on the formation of one or more adhesion domains within the vesicle-substrate contact zone. We find that the content of ligands in the vesicle must be above 5 mol % to establish specific contacts. All concentrations of sialyl-Lewis(X) above 8 mot % provide a very similar final state of adhesion. However, the size and shape of the adhesion domains strongly depend on both the concentrations of E-selectin (0-3500 molecules/mu m(2)) and PEG2000 (0-5 mol %). At 3500 E-selectin molecules/mu m(2) and small concentrations of PEG2000, the vesicle-substrate contact is maximized and fully occupied by a single adhesion domain. At concentrations of 5 mol %, PEG2000 completely impedes the specific binding to any substrate. Lastly, an increase in the adhesion strength is observed in systems with identical compositions if the reduced volume of the vesicles is larger.
Keywords: CELL-ADHESION; FLUID MEMBRANES; MODEL MEMBRANE; P-SELECTIN; LIGAND; LIPOSOMES; FORCE; FLOW; GLYCOSPHINGOLIPIDS; GLYCOLIPOSOMES

Adiabatic Preparation of a Correlated Symmetry-Broken Initial State with the Generalized Kadanoff–Baym Ansatz - Tuovinen, Riku; Golež, Denis; Schüler, Michael; Werner, Philipp; Eckstein, Martin; Sentef, Michael A. (physica status solidi (b), 2019)
A fast time propagation method for nonequilibrium Green's functions (NEGF) based on the generalized Kadanoff–Baym Ansatz (GKBA) is applied to a lattice system with a symmetry-broken equilibrium phase, namely an excitonic insulator (EI). The adiabatic preparation of a correlated symmetry-broken initial state from a Hartree–Fock wave function within GKBA is assessed by comparing with a solution of the imaginary-time Dyson equation. It is found that it is possible to reach a symmetry-broken correlated initial state with nonzero excitonic order parameter by the adiabatic switching (AS) procedure. It is discussed under which circumstances this is possible in practice within reasonably short switching times.
CRIS-Team Scopus Importer:2019-08-06
Keywords: excitonic insulators; generalized Kadanoff–Baym Ansatz; nonequilibrium Green's function; time propagation

A digital twin to overcome long-time challenges in photovoltaics - Lüer, Larry; Peters, Ian Marius; Smith, Ana-Suncana; Dorschky, Eva; Eskofier, Björn; Liers-Bergmann, Frauke; Franke, Jörg; Sjarov, Martin; Brossog, Matthias; Guldi, Dirk Michael; Maier, Andreas; Brabec, Christoph (Joule, 2024)
The recent successes of emerging photovoltaics (PVs) are largely driven by innovations in material science. However, closing the gap to commercialization still requires significant progress to match contradicting requirements, such as performance, longevity, and recyclability. In this perspective, we envision the layout of a digital twin for PV materials able to provide the necessary acceleration of innovation. The layout combines machine learning approaches, as performed in materials acceleration platforms (MAPs), with physical models and digital twin concepts used in engineering. This layout will allow the use of high-throughput (HT) experimentation in MAPs to improve the parametrization of quantum chemical and solid-state models. In turn, the improved and generalized models can be used to obtain the crucial structural parameters. HT experimentation will thus yield a detailed understanding of generally valid structure-property relationships. We discuss the current state of the enabling technologies, namely fast scale-bridging surrogate models and large-scale optimization under uncertainty, and identify promising approaches for further development.<br
Keywords: digital twin; emerging photovoltaics; artificial intelligence; multi-scale simulations; optimization; high-throughput methods; materials acceleration platforms

Adsorption isotherms and thermal fluctuations - Mecke, Klaus; et al. (Physical Review B, 1996)
The influence of thermal fluctuations on adsorption isotherms is calculated within the context of a self consistent mean-held theory, and it is found that such fluctuations cannot be neglected in the analysis of adsorption data. This result arises from our observation that substrate-induced hindrance of thermal fluctuations can significantly alter the form of an adsorption isotherm, particularly in the thin-film regime (approximate to 0-5 nm) which is most commonly probed by adsorption experiments. Previous experiments involving room-temperature adsorption on flat surfaces have been reanalyzed, and the reported discrepancies with the Lifshitz theory of van der Waals forces are found to be much reduced when thermal fluctuations of the thickness of the adsorbed layer are taken into account. Recently published data for nitrogen adsorption on rough substrates have also been reanalyzed, and the thermal fluctuations are found to be more important for thin adsorbed layers than undulations of the film induced by the roughness of the substrate. The analysis reveals that, as long suspected, the scaling regime of asymptotic divergence has not yet been reached for film thicknesses remaining below 5 nm.

Adsorption isotherms of hydrogen: The role of thermal fluctuations - Mecke, Klaus; et al. (Physical Review Letters, 2001)
It is shown that experimentally obtained isotherms of adsorption on solid substrates may be completely reconciled with Lifshitz theory when thermal fluctuations of the free film surface are taken into account. This is demonstrated for hydrogen adsorbed on gold as a model system. Analysis of the fluctuation contributions allows one to determine the surface tension of the free hydrogen film as a function of film thickness. It is found to decrease sharply for film thicknesses below seven atomic layers.

Adsorption of Aspartate Derivatives to Calcite Surfaces in Aqueous Environment - Stepic, Robert; Jurković, Lara; Klementyeva, Ksenia; Ukrainczyk, Marko; Gredičak, Matija; Smith, David M.; Kralj, Damir; Smith, Ana-Suncana (Crystal Growth and Design, 2020)
In many living organisms, biomolecules interact favorably with various surfaces of calcium carbonate. In this work, we considered the interactions of aspartate (Asp) derivatives, as models of complex biomolecules, with calcite. Using kinetic growth experiments, we investigated the inhibition of calcite growth by Asp, Asp2, and Asp3.This entailed the determination of a step-pinning growth regime as well as the evaluation of the adsorption constants and binding free energies for the three species to calcite crystals. These latter values are compared to free energy profiles obtained from fully atomistic molecular dynamics simulations. When a flat (104) calcite surface is used in the models, the measured trend of binding energies is poorly reproduced. However, a more realistic model comprised of a surface with an island containing edges and corners yields binding energies that compare very well with experiments. Surprisingly, we find that most binding modes involve the positively charged ammonium group. Moreover, while attachment of the negatively charged carboxylate groups is also frequently observed, it is always balanced by the aqueous solvation of an equal or greater number of carboxylates. These effects are observed on all calcite features including edges and corners, the latter being associated with dominant affinities to Asp derivatives. As these features are also precisely the active sites for crystal growth, the experimental and theoretical results point strongly to a growth inhibition mechanism, whereby these sites become blocked, preventing further attachment of dissolved ions and halting further growth.
CRIS-Team Scopus Importer:2020-05-26

A first search for coincident gravitational waves and high energy neutrinos using LIGO, Virgo and ANTARES data from 2007 - Adrian-Martinez, S.; Al Samarai, I.; Albert, A.; Andre, Michel; Anghinolfi, M.; Anton, Gisela; Anvar, S.; Ardid, Miguel; Astraatmadja, T.; Aubert, J-J.; Baret, B.; Basa, S.; et al. (Journal of Cosmology and Astroparticle Physics, 2013)
We present the results of the first search for gravitational wave bursts associated with high energy neutrinos. Together, these messengers could reveal new, hidden sources that are not observed by conventional photon astronomy, particularly at high energy. Our search uses neutrinos detected by the underwater neutrino telescope ANTARES in its 5 line configuration during the period January - September 2007, which coincided with the fifth and first science runs of LIGO and Virgo, respectively. The LIGO-Virgo data were analysed for candidate gravitational-wave signals coincident in time and direction with the neutrino events. No significant coincident events were observed. We place limits on the density of joint high energy neutrino - gravitational wave emission events in the local universe, and compare them with densities of merger and core-collapse events.
Keywords: gravitational waves / experiments; neutrino astronomy

A fluid floating bilayer - Mecke, Klaus; et al. (EPL - Europhysics Letters, 2001)
A highly hydrated lipid bilayer, floating a few A above another one adsorbed on a smooth solid substrate, was prepared at room temperature where lipids (DSPC) are in gel phase, then heated at several temperatures including the pretransitional temperature T-p and the chain melting temperature T-m. A precise in situ characterization by neutron reflectivity led to two new results. First, even when in fluid phase the floating bilayer was structured and stable; it thus provides a flexible model system for physical and biophysical studies of membranes. Second, at intermediate temperatures, a spectacular maximum in both inter-bilayer distance and bilayer roughness was simultaneously observed. It might be a, direct observation of the balance between energy minimization and entropic repulsion, leading to an estimation of the dimensionless parameter (k(B)T)(2)/A(kappa), where A is the Hamacker constant; and kappa the bending modulus.

A generalized perspective on non-perturbative linked-cluster expansions - Coester, K.; Clever, S.; Herbst, F.; Capponi, Sylvain; Schmidt, Kai Phillip (EPL - Europhysics Letters, 2015)
We identify a fundamental challenge for any non-perturbative approach based on finite clusters resulting from the reduced symmetry on graphs, most importantly the breaking of translational symmetry, when targeting the properties of excited states. This can be traced back to the appearance of intruder states in the low-energy spectrum, which represent a major obstacle in quasi-degenerate perturbation theory. Here a generalized notion of cluster additivity is introduced, which is used to formulate an optimized scheme of graph-based continuous unitary transformations allowing to solve and to physically understand this major issue. Most remarkably, our improved scheme demands to go beyond the paradigm of using the exact eigenvectors on graphs. Copyright (C) EPLA, 2015

A general perturbative approach for bead-based microswimmers reveals rich self-propulsion phenomena - Ziegler, Sebastian; Hubert, Maxime; Vandewalle, Nicolas; Harting, Jens; Smith, Ana-Suncana (New Journal of Physics, 2019)
Studies of model microswimmers have significantly contributed to the understanding of the principles of self-propulsion we have today. However, only a small number of microswimmer types have been amenable to analytic modeling, and further development of such approaches is necessary to identify the key features of these active systems. Here we present a general perturbative calculation scheme for swimmers composed of beads interacting by harmonic potentials and via hydrodynamics, driven by an arbitrary force protocol. The approach can be used with mobility matrices of arbitrary accuracy, and we illustrate it with the Oseen and Rotne-Prager approximations. We validate our approach by using 3 bead assemblies and comparing the results with the numerically obtained full-solutions of the governing equations of motion, as well as with existing analytic models for the linear and the triangular swimmer geometry. While recovering the relation between the force and swimming velocity, our detailed analysis and the controlled level of approximation allow us to find qualitative differences already in the far field flow of the devices. Consequently, we are able to identify a behavior of the swimmer that is richer than predicted in previous models. Given its generality, the framework can be applied to any swimmer geometry, driving protocol and bead interactions, as well as in problems involving many swimmers.
CRIS-Team Scopus Importer:2019-12-10
Keywords: active matter; low Reynolds number dynamics; microswimmers; perturbation theory; self-propulsion

A gravitationally induced decoherence model using Ashtekar variables - Fahn, Max Joseph; Giesel, Kristina; Kobler, Michael (Classical and Quantum Gravity, 2023)
We consider the coupling of a scalar field to linearised gravity and derive a relativistic gravitationally induced decoherence model using Ashtekar variables. The model is formulated at the gauge invariant level using suitable geometrical clocks in the relational formalism, broadening existing gauge invariant formulations of decoherence models. For the construction of the Dirac observables we extend the known observable map by a kind of dual map where the role of clocks and constraints is interchanged. We also discuss a second choice of geometrical clocks existing in the ADM literature. Then we apply a reduced phase space quantisation on Fock space and derive the final master equation choosing a Gibbs state for the gravitational environment and using the projection operator technique. The resulting master equation is not automatically of Lindblad type, a starting point sometimes assumed for phenomenological models, but still involves a residual time dependence at the level of the effective operators in the master equation due to the form of the correlation functions that we express in terms of thermal Wightman functions. Furthermore, we discuss why in the model analysed here the application of a second Markov approximation in order to obtain a set of time independent effective system operators is less straightforward than in some of the quantum mechanical models.
CRIS-Team Scopus Importer:2023-04-28
Keywords: decoherence; Dirac observables; geometrical clocks; linearised gravity; master equation

A hierarchical approach for the dynamics of na clusters in contact with an ar substrate - Dinh, P. M.; Douady, J.; Fehrer, F.; Gervais, B.; Giglio, E.; Ipatov, A.; Reinhard, Paul-Gerhard; Suraud, E. (Theoretical and Computational Developments in Modern Density Functional Theory, 2012)
We present a short review on recent theoretical studies of the dynamics of small Na clusters in contact with Ar matrices, in some cases also on NaCl surfaces. We discuss the hierarchical modeling which treats the cluster fully quantum-mechanically and the substrate at a classical level. The dynamical polarizability of the substrate is properly taken into account. We present results on the structural properties, on dynamics in the regime of linear response, and on fully non-linear dynamics induced by strong femtosecond laser pulses. © 2012 by Nova Science Publishers, Inc. All rights reserved.
CRIS-Team Scopus Importer:2023-07-24
Keywords: Hierarchical approach; Interface interaction; Qm/mm; Rare-gas environment; Structure and dynamics of embedded clusters; Time-dependent density functional theory

A kinetic-Monte Carlo perspective on active matter - Klamser, Juliane U.; Kapfer, Sebastian; Krauth, Werner (Journal of Chemical Physics, 2019)
We study non-equilibrium phases for interacting two-dimensional self-propelled particles with isotropic pair-wise interactions using a persistent kinetic Monte Carlo approach. We establish the quantitative phase diagram, including the motility-induced phase separation (MIPS) that is a commonly observed collective phenomenon in active matter. In addition, we demonstrate for several different potential forms the presence of two-step melting, with an intermediate hexatic phase, in regions far from equilibrium. Increased activity can melt a two-dimensional solid and the melting lines remain disjoint from MIPS. We establish this phase diagram for a range of the inter-particle potential stiffnesses and identify the MIPS phase even in the hard-disk limit. We establish that the full description of the phase behavior requires three independent control parameters.
CRIS-Team WoS Importer:2019-04-30

A length operator for canonical quantum gravity - Thiemann, Thomas (Journal of Mathematical Physics, 1998)
We construct an operator that measures the length of a curve in four-dimensional Lorentzian vacuum quantum gravity. We work in a representation in which an SU(2) connection is diagonal and it is therefore surprising that the operator obtained after regularization is densely defined, does not suffer from factor ordering singularities, and does not require any renormalization. We show that the length operator admits self-adjoint extensions and compute part of its spectrum which, like its companions, the volume and area operators already constructed in the literature, is purely discrete and roughly quantized in units of the Planck length. The length operator contains full and direct information about all the components of the metric tensor which facilitates the construction of so-called weave states which approximate a given classical three-geometry. (C) 1998 American Institute of Physics.

Algebraic Loop Quantisation of the Gowdy Model: The Master Constraint - Alex, Nils; Giesel, Kristina (Friedrich-Alexander-Universität Erlangen-Nürnberg, 2015)

Algebraic quantum gravity (AQG): I. Conceptual setup - Giesel, Kristina; Thiemann, Thomas (Classical and Quantum Gravity, 2007)
We introduce a new top down approach to canonical quantum gravity, called algebraic quantum gravity (AQG). The quantum kinematics of AQG is determined by an abstract *-algebra generated by a countable set of elementary operators labelled by an algebraic graph. The quantum dynamics of AQG is governed by a single master constraint operator. While AQG is inspired by loop quantum gravity (LQG), it differs drastically from it because in AQG there is fundamentally no topology or differential structure. A natural Hilbert space representation acquires the structure of an infinite tensor product (ITP) whose separable strong equivalence class Hilbert subspaces (sectors) are left invariant by the quantum dynamics. The missing information about the topology and differential structure of the spacetime manifold as well as about the background metric to be approximated is supplied by coherent states. Given such data, the corresponding coherent state defines a sector in the ITP which can be identified with a usual QFT on the given manifold and background. Thus, AQG contains QFT on all curved spacetimes at once, possibly has something to say about topology change and provides the contact with the familiar low energy physics. In particular, in two companion papers we develop semiclassical perturbation theory for AQG and LQG and thereby show that the theory admits a semiclassical limit whose infinitesimal gauge symmetry agrees with that of general relativity. In AQG everything is computable with sufficient precision and no UV divergences arise due to the background independence of the fundamental combinatorial structure. Hence, in contrast to lattice gauge theory on a background metric, no continuum limit has to be taken. There simply is no lattice regulator that must be sent to zero.

Algebraic quantum gravity (AQG): III. Semiclassical perturbation theory - Giesel, Kristina; Thiemann, Thomas (Classical and Quantum Gravity, 2007)
In the two previous papers of this series we defined a new combinatorial approach to quantum gravity, algebraic quantum gravity (AQG). We showed that AQG reproduces the correct infinitesimal dynamics in the semiclassical limit, provided one incorrectly substitutes the non-Abelian group SU(2) by the Abelian group U(1)(3) in the calculations. The mere reason why that substitution was performed at all is that in the non-Abelian case the volume operator, pivotal for the definition of the dynamics, is not diagonizable by analytical methods. This, in contrast to the Abelian case, so far prohibited semiclassical computations. In this paper, we show why this unjustified substitution nevertheless reproduces the correct physical result. Namely, we introduce for the first time semiclassical perturbation theory within AQG ( and LQG) which allows us to compute expectation values of interesting operators such as the master constraint as a power series in h with error control. That is, in particular, matrix elements of fractional powers of the volume operator can be computed with extremely high precision for sufficiently large power of h in the h expansion. With this new tool, the non-Abelian calculation, although technically more involved, is then exactly analogous to the Abelian calculation, thus justifying the Abelian analysis in retrospect. The results of this paper turn AQG into a calculational discipline.

Algebraic quantum gravity (AQG): II. Semiclassical analysis - Giesel, Kristina; Thiemann, Thomas (Classical and Quantum Gravity, 2007)
In the previous paper (Giesel and Thiemann 2006 Conceptual setup Preprint gr-qc/0607099) a new combinatorial and thus purely algebraical approach to quantum gravity, called algebraic quantum gravity (AQG), was introduced. In the framework of AQG, existing semiclassical tools can be applied to operators that encode the dynamics of AQG such as the master constraint operator. In this paper, we will analyse the semiclassical limit of the (extended) algebraic master constraint operator and show that it reproduces the correct infinitesimal generators of general relativity. Therefore, the question of whether general relativity is included in the semiclassical sector of the theory, which is still an open problem in LQG, can be significantly improved in the framework of AQG. For the calculations, we will substitute SU(2) with U(1)(3). That this substitution is justified will be demonstrated in the third paper ( Giesel and Thiemann 2006 Semiclassical perturbation theory Preprint gr-qc/0607101) of this series.

Algebraic quantum gravity (AQG): IV. Reduced phase space quantization of loop quantum gravity - Giesel, Kristina; Thiemann, Thomas (Classical and Quantum Gravity, 2010)
We perform a canonical, reduced phase space quantization of general relativity by loop quantum gravity (LQG) methods. The explicit construction of the reduced phase space is made possible by the combination of (a) the Brown-Kuchar mechanism in the presence of pressure-free dust fields which allows to deparametrize the theory and (b) Rovelli's relational formalism in the extended version developed by Dittrich to construct the algebra of gauge-invariant observables. Since the resulting algebra of observables is very simple, one can quantize it using the methods of LQG. Basically, the kinematical Hilbert space of non-reduced LQG now becomes a physical Hilbert space and the kinematical results of LQG such as discreteness of spectra of geometrical operators now have physical meaning. The constraints have disappeared; however, the dynamics of the observables is driven by a physical Hamiltonian which is related to the Hamiltonian of the standard model (without dust) and which we quantize in this paper.

Algorithmic approach to cosmological coherent state expectation values in loop quantum gravity - Liegener, Klaus; Rudnicki, Lukasz (Classical and Quantum Gravity, 2021)
Within the lattice approach to loop quantum gravity on a fixed graph, explicit calculations tend to be involved and are rarely analytically manageable. However, being focused on a particularly interesting setting, concerned with expectation values with respect to coherent states on the lattice (sharply peaked on isotropic and flat cosmology), we are able to provide several simplifications which can facilitate approaching beyond-state-of-the-art problems. We present a step-by-step algorithm resulting in an analytical expression including up-to-first-order corrections in the spread of the coherent state. The algorithm is developed in such a way that it makes the computation straightforward and easily implementable.
CRIS-Team WoS Importer:2021-10-08

Alleviating the H0 tension with new gravitational scalar tensor theories - Banerjee, Shreya; Petronikolou, Maria; Saridakis, Emmanuel N. (Physical Review D, 2023)
We investigate the cosmological applications of new gravitational scalar-tensor theories, and we analyze them in the light of H0 tension. In these theories, the Lagrangian contains the Ricci scalar and its first and second derivatives in a specific combination that makes them free of ghosts, thus corresponding to healthy biscalar extensions of general relativity. We examine two specific models, and for particular choices of the model parameters, we find that the effect of the additional terms is negligible at high redshifts, obtaining a coincidence with ΛCDM cosmology; however, as time passes, the deviation increases, and thus, at low redshifts the Hubble parameter acquires increased values (H0≈74 km/s/Mpc) in a controlled way. The mechanism behind this behavior is the fact that the effective dark-energy equation-of-state parameter exhibits phantom behavior, which implies faster expansion, which is one of the sufficient conditions that are capable of alleviating the H0 tension. Lastly, we confront the models with cosmic chronometer (CC) data, showing full agreement within 1σ confidence level.
CRIS-Team Scopus Importer:2023-07-28

All good things come in threes - Three beads learn to swim with lattice Boltzmann and a rigid body solver - Pickl, Kristina; Götz, Jan; Iglberger, Klaus; Pande, Jayant; Mecke, Klaus; Smith, Ana-Suncana; Rüde, Ulrich (Journal of Computational Science, 2012)
We simulate the self-propulsion of devices in a fluid in the regime of low Reynolds numbers. Each device consists of three bodies (spheres or capsules) connected with two damped harmonic springs. Sinusoidal driving forces compress the springs which are resolved within a rigid body physics engine. The latter is consistently coupled to a 3D lattice Boltzmann framework for the fluid dynamics. In simulations of three-sphere devices, we find that the propulsion velocity agrees well with theoretical predictions. In simulations where some or all spheres are replaced by capsules, we find that the asymmetry of the design strongly affects the propelling efficiency.
UnivIS-Import:2015-03-09:Pub.2011.tech.IMMD.lsinfs.allgoo

All-optical generation of antiferromagnetic magnon currents via the magnon circular photogalvanic effect - Bostroem, Emil Vinas; Parvini, Tahereh Sadat; Mciver, James W.; Rubio, Angel; Viola Kusminskiy, Silvia; Sentef, Michael A. (Physical Review B, 2021)
We introduce the magnon circular photogalvanic effect enabled by two-magnon Raman scattering. This provides an all-optical pathway to the generation of directed magnon currents with circularly polarized light in honeycomb antiferromagnetic insulators. The effect is the leading order contribution to magnon photocurrent generation via optical fields. Control of the magnon current by the polarization and angle of incidence of the laser is demonstrated. Experimental detection by sizable inverse spin Hall voltages in platinum contacts is proposed.
CRIS-Team Scopus Importer:2021-09-17

Alternative k=-1 loop quantum cosmology - Yang, Jinsong; Zhang, Cong; Zhang, Xiangdong (Physical Review D, 2023)
An alternative quantization of the gravitational Hamiltonian constraint of the k=-1 Friedmann-Robertson-Walker model is proposed by treating the Euclidean term and the Lorentzian term independently, mimicking the treatment of full-loop quantum gravity. The resulting Hamiltonian constraint operator for the k=-1 model with a massless scalar field is successfully constructed, and it is shown to have the corrected classical limit. Compared to the former quantization schemes in the literature where only the Euclidean term is quantized, the new quantum dynamics of the k=-1 model with a massless scalar field indicates that the classical big bang singularity is replaced by an asymmetric quantum bounce.
CRIS-Team Scopus Importer:2023-05-12

A manifestly gauge-invariant approach to quantum theories of gauge fields - Ashtekar, Abhay; Lewandowski, Jerzy; Marolf, Donald; Mourao, José Manuel; Thiemann, Thomas (1995)

A method for detection of muon induced electromagnetic showers with the ANTARES detector - Aguilar, J. A.; Al Samarai, I.; Albert, A.; Andre, Michel; Anghinolfi, M.; Anton, Gisela; Anvar, S.; Ardid, Miguel; Jesus, A. C. Assis; Astraatmadja, T.; Auberth, J. J.; Baret, B.; et al. (Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2012)
The primary aim of ANTARES is neutrino astronomy with upward going muons created in charged current muon neutrino interactions in the detector and its surroundings. Downward going muons are background for neutrino searches. These muons are the decay products of cosmic-ray collisions in the Earth's atmosphere far above the detector. This paper presents a method to identify and count electromagnetic showers induced along atmospheric muon tracks with the ANTARES detector. The method is applied to both cosmic muon data and simulations and its applicability to the reconstruction of muon event energies is demonstrated. (C) 2012 Elsevier B.V. All rights reserved.
Keywords: Neutrino telescope; Electromagnetic shower identification; High energy muons; Energy reconstruction

A microscopic view on the Mott transition in chromium-doped V2O3 - Lupi, S.; Baldassarre, Leonetta; Mansart, B.; Perucchi, A.; Barinov, Alexey; Dudin, Pavel; Papalazarou, E.; Rodolakis, Fanny; Rueff, J. -P.; Itie, J. -P.; Ravy, Sylvain; Nicoletti, D.; et al. (Nature Communications, 2010)
V2O3 is the prototype system for the Mott transition, one of the most fundamental phenomena of electronic correlation. Temperature, doping or pressure induce a metal-to-insulator transition (MIT) between a paramagnetic metal (PM) and a paramagnetic insulator. This or related MITs have a high technological potential, among others, for intelligent windows and field effect transistors. However the spatial scale on which such transitions develop is not known in spite of their importance for research and applications. Here we unveil for the first time the MIT in Cr-doped V2O3 with submicron lateral resolution: with decreasing temperature, microscopic domains become metallic and coexist with an insulating background. This explains why the associated PM phase is actually a poor metal. The phase separation can be associated with a thermodynamic instability near the transition. This instability is reduced by pressure, that promotes a genuine Mott transition to an eventually homogeneous metallic state.
CRIS-Team WoS Importer:2020-02-19

A morphological model for complex fluids - Mecke, Klaus (IOP PUBLISHING LTD, 1996)
A new model is proposed for the study of porous media and complex fluids using morphological measures to describe homogeneous spatial domains of the constituents. Under rather natural assumptions a general expression for the Hamiltonian can be given extending the model of Widom and Rowlinson for penetrable spheres. The Hamiltonian includes energy contributions related to the volume, surface area mean curvature, and the Euler characteristic of the configuration generated by overlapping sets of arbitrary shapes. Phase diagrams of the model are calculated and discussed. In particular, we find that the Euler characteristic in the Hamiltonian stabilizes a highly connected bicontinuous structure, resembling the middle phase in oil-water microemulsions.

Analysing (cosmological) singularity avoidance in loop quantum gravity using U (1)(3) coherent states and Kummer's functions - Giesel, Kristina; Winnekens, David (Classical and Quantum Gravity, 2023)
Using a new procedure based on Kummer's Confluent Hypergeometric Functions, we investigate the question of singularity avoidance in loop quantum gravity (LQG) in the context of U(1)(3) complexifier coherent states and compare obtained results with already existing ones. Our analysis focuses on the dynamical operators, denoted by (q) over cap (i0)(I0) (r), whose products are the analogue of the inverse scale factor in LQG and also play a pivotal role for other dynamical operators such as matter Hamiltonians or the Hamiltonian constraint. For graphs of cubic topology and linear powers in (q) over cap (i0)(I0) (r), we obtain the correct classical limit and demonstrate how higher order corrections can be computed with this method. This extends already existing techniques in the way how the involved fractional powers are handled. We also extend already existing formalisms to graphs with higher-valent vertices. For generic graphs and products of (q) over cap (i0)(I0) (r), using estimates becomes inevitable and we investigate upper bounds for these semiclassical expectation values. Compared to existing results, our method allows to keep fractional powers involved in (q) over cap (i0)(I0) (r) throughout the computations, which have been estimated by integer powers elsewhere. Similar to former results, we find a non-zero upper bound for the inverse scale factor at the initial singularity. Additionally, our findings provide some insights into properties and related implications of the results that arise when using estimates and can be used to look for improved estimates.
CRIS-Team WoS Importer:2023-04-21

Analytic continuation of spinfoam models - Han, Muxin; Liu, Hongguang (Physical Review D, 2022)
The Lorentzian Engle-Pereira-Rovelli-Livine/Freidel-Krasnov spinfoam model and the Conrady-Hnybida timelike-surface extension can be expressed in the integral form ∫eS. This work studies the analytic continuation of the spinfoam action S to the complexification of the integration domain. Our work extends our knowledge from the real critical points well-studied in the spinfoam large-j asymptotics to general complex critical points of S analytic continued to the complex domain. The complex critical points satisfy critical equations of the analytic continued S. In the large-j regime, the complex critical points give subdominant contributions to the spinfoam amplitude when the real critical points are present. But the contributions from the complex critical points can become dominant when the real critical points are absent. Moreover, the contributions from the complex critical points cannot be neglected when the spins j are not large. In this paper, we classify the complex critical points of the spinfoam amplitude and find a subclass of complex critical points that can be interpreted as four-dimensional simplicial geometries. In particular, we identify the complex critical points corresponding to the Riemannian simplicial geometries although we start with the Lorentzian spinfoam model. The contribution from these complex critical points of Riemannian geometry to the spinfoam amplitude give e-SRegge in analogy with the Euclidean path integral, where SRegge is the Riemannian Regge action on the simplicial complex.
CRIS-Team Scopus Importer:2022-01-21

Analyzing X-ray tomographies of granular packings - Weis, Simon; Schröter, Matthias (Review of Scientific Instruments, 2017)
Starting from three-dimensional volume data of a granular packing, as, e.g., obtained by X-ray Computed Tomography, we discuss methods to first detect the individual particles in the sample and then analyze their properties. This analysis includes the pair correlation function, the volume and shape of the Voronoi cells, and the number and type of contacts formed between individual particles. We mainly focus on packings of monodisperse spheres, but we will also comment on other monoschematic particles such as ellipsoids and tetrahedra. This paper is accompanied by a package of free software containing all programs (including source code) and an example three-dimensional dataset which allows the reader to reproduce and modify all examples given.
EAM Import::2019-03-13

An analysis of nonadiabatic ring-polymer molecular dynamics and its application to vibronic spectra - Richardson, Jeremy; Meyer, Philipp; Pleinert, Marc-Oliver; Thoss, Michael (Chemical Physics, 2017)
Nonadiabatic ring-polymer molecular dynamics employs the mapping approach to describe nonadiabatic effects within the ring-polymer ansatz. In this paper, it is generalized to allow for the nuclear and electronic degrees of freedom to be described by different numbers of ring-polymer beads. Analysis of the resulting method shows that as the number of electronic mapping variables increases, certain problems associated with the approach are removed, such as the non-unique choice of the mapping Hamiltonian and negative populations leading to inverted potential-energy surfaces. Explicit integration over cyclic variables reduces the sign problem for the initial distribution in the general case. A new application for the simulation of vibronic spectra is described and promising results are presented for a model system. (C) 2016 Elsevier B.V. All rights reserved.
Keywords: Vibronic spectra; Ring-polymer molecular dynamics; Nonadiabatic; Mapping approach

Anderson localization of composite excitations in disordered optomechanical arrays - Roque, Thales Figueiredo; Peano, Vittorio; Yevtushenko, Oleg; Marquardt, Florian (New Journal of Physics, 2017)
Optomechanical (OMA) arrays are a promising future platform for studies of transport, many-body dynamics, quantum control and topological effects in systems of coupled photon and phonon modes. We introduce disordered OMA arrays, focusing on features of Anderson localization of hybrid photon-phonon excitations. It turns out that these represent a unique disordered system, where basic parameters can be easily controlled by varying the frequency and the amplitude of an external laser field. We show that the two-species setting leads to a non-trivial frequency dependence of the localization length for intermediate laser intensities. This could serve as a convincing evidence of localization in a non-equilibrium dissipative situation.
CRIS-Team Scopus Importer:2023-03-27
Keywords: disordered structures; localization; optomechanics

A new benchmark of soft X-ray transition energies of Ne , CO 2 , and SF 6 : paving a pathway towards ppm accuracy - Stierhof, Jakob; Kühn, S.; Winter, Mirko; Micke, P.; Steinbrügge, R.; Shah, C.; Hell, N.; Bissinger, Matthias; Hirsch, Maria; Ballhausen, Ralf; Lang, Melanie; Gräfe, Christina; et al. (European Physical Journal D, 2022)
Abstract: A key requirement for the correct interpretation of high-resolution X-ray spectra is that transition energies are known with high accuracy and precision. We investigate the K-shell features of Ne , CO 2, and SF 6 gases, by measuring their photo ion-yield spectra at the BESSY II synchrotron facility simultaneously with the 1s–np fluorescence emission of He-like ions produced in the Polar-X EBIT. Accurate ab initio calculations of transitions in these ions provide the basis of the calibration. While the CO 2 result agrees well with previous measurements, the SF 6 spectrum appears shifted by ∼ 0.5 eV, about twice the uncertainty of the earlier results. Our result for Ne shows a large departure from earlier results, but may suffer from larger systematic effects than our other measurements. The molecular spectra agree well with our results of time-dependent density functional theory. We find that the statistical uncertainty allows calibrations in the desired range of 1–10 meV, however, systematic contributions still limit the uncertainty to ∼ 40–100 meV, mainly due to the temporal stability of the monochromator energy scale. Combining our absolute calibration technique with a relative energy calibration technique such as photoelectron energy spectroscopy will be necessary to realize its full potential of achieving uncertainties as low as 1–10 meV. Graphical abstract: [Figure not available: see fulltext.].
CRIS-Team Scopus Importer:2022-03-11

An Exact Algorithm to Detect the Percolation Transition in Molecular Dynamics Simulations of Cross-Linking Polymer Networks - Livraghi, Mattia; Höllring, Kevin; Wick, Christian; Smith, David M.; Smith, Ana-Suncana (Journal of Chemical Theory and Computation, 2021)
Periodic molecular dynamics simulations are developing to a routine tool for the investigation of complex, polymeric materials. A typical application is the simulation of the curing reaction of covalently cross-linked polymers, which provides detailed understanding of network formation at the molecular scale, with examples including gelation and glass transitions. In this article, we delineate the connection between percolation theory and gel-point detection in periodic polymeric networks. Specifically, we present an algorithm that can detect the onset of percolation during cross-linking of polymers in periodic molecular dynamic simulations. A sample implementation is provided at https://github.com/puls-group/percolation-analyzer. As an example, we apply the algorithm to simulations of an epoxy resin undergoing curing with an amine hardener. We also compare results with indirect gel point measurements obtained from monitoring the growth of the largest mass and the onset of secondary cycles.

Angular distributions of electrons basic solid state physics emitted from free and deposited Na8 clusters - Bär, Matthias; Dinh, Phuong Mai; Moskaleva, Lyudmila V.; Reinhard, Paul-Gerhard; Rösch, Notker; Suraud, Eric (physica status solidi (b), 2010)
We explore from a theoretical perspective angular distributions of electrons emitted from a Na8 cluster after excitation by a short laser pulse. The tool of the study is time-dependent density-functional theory (TDDFT) at the level of the localdensity approximation (LDA) augmented by a self-interaction correction (SIC) to put emission properties in order. We consider free Na8 and Na8 deposited on the surfaces MgO(001) or Ar(001). For the case of free Na8, we distinguish between a hypothetical situation of known cluster orientation and a more realistic ensemble of orientations. We also consider the angular distributions for emission from separate single-electron levels. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
CRIS-Team Scopus Importer:2023-07-24

Angular distributions of photoelectrons from free Na clusters - Wopperer, P.; Faber, Bernd; Dinh, P. M.; Reinhard, Paul-Gerhard; Suraud, E. (Physical Review A - Atomic, Molecular, and Optical Physics, 2010)
We explore, from a theoretical perspective, photoelectron angular distributions (PADs) of the Na clusters Na8, Na10, Na 12, Na18, Na3⊃+, Na11⊃+, Na13⊃+, and Na19⊃+. The basis of the description is the time-dependent local-density approximation (TDLDA), augmented by a self-interaction correction (SIC) to describe ionization properties correctly. The scheme is solved on a numerical grid in coordinate space with absorbing bounds. We assume for each cluster system an isotropic ensemble of free clusters and develop for the case of one-photon emission analytical formulas for computing the orientation-averaged PAD on the basis of a few TDLDA-SIC calculations for properly chosen reference orientations. It turns out that all the information in the averaged PAD is contained in one anisotropy parameter. We find that this parameter varies very little with system size, but as a whole is crucially influenced by the detailed ionic structure. We also make comparisons with direct orientation averaging and consider one example reaching outside the perturbative regime. © 2010 The American Physical Society.
CRIS-Team Scopus Importer:2023-07-24

Angular Momentum and Quantum Gravity - Reichert, Thorsten; Thiemann, Thomas (2010)

Anisotropic molecular diffusion in confinement II: A model for structurally complex particles applied to transport in thin ionic liquid films - Höllring, Kevin; Baer, Andreas; Vučemilović-Alagić, Nataša; Smith, David M.; Smith, Ana-Suncana (Journal of Colloid and Interface Science, 2024)
Hypothesis: Diffusion in confinement is an important fundamental problem with significant implications for applications of supported liquid phases. However, resolving the spatially dependent diffusion coefficient, parallel and perpendicular to interfaces, has been a standing issue and for objects of nanometric size, which structurally fluctuate on a similar time scale as they diffuse, no methodology has been established so far. We hypothesise that the complex, coupled dynamics can be captured and analysed by using a model built on the 2-dimensional Smoluchowski equation and systematic coarse-graining. Methods and simulations: For large, flexible species, a universal approach is offered that does not make any assumptions about the separation of time scales between translation and other degrees of freedom. The method is validated on Molecular Dynamics simulations of bulk systems of a family of ionic liquids with increasing cation sizes where internal degrees of freedom have little to major effects. Findings: After validation on bulk liquids, where we provide an interpretation of two diffusion constants for each species found experimentally, we clearly demonstrate the anisotropic nature of diffusion coefficients at interfaces. Spatial variations in the diffusivities relate to interface-induced structuring of the ionic liquids. Notably, the length scales in strongly confined ionic liquids vary consistently but differently at the solid–liquid and liquid–vapour interfaces.
CRIS-Team Scopus Importer:2023-12-15
Keywords: Anisotropic diffusion; Diffusion at interfaces; Diffusion in films; Ionic liquids; Molecular liquids; Transport coefficient

Anisotropic molecular diffusion in confinement I: Transport of small particles in potential and density gradients - Höllring, Kevin; Baer, Andreas; Vučemilović-Alagić, Nataša; Smith, David M.; Smith, Ana-Suncana (Journal of Colloid and Interface Science, 2023)
Hypothesis: Diffusion in confinement is an important fundamental problem with significant implications for applications of supported liquid phases. However, resolving the spatially dependent diffusion coefficient, parallel and perpendicular to interfaces, has been a standing issue. In the vicinity of interfaces, density fluctuations as a consequence of layering locally impose statistical drift, which impedes the analysis of spatially dependent diffusion coefficients even further. We hypothesise, that we can derive a model to spatially resolve interface-perpendicular diffusion coefficients based on local lifetime statistics with an extension to explicitly account for the effect of local drift using the Smoluchowski equation, that allows us to resolve anisotropic and spatially dependent diffusivity landscapes at interfaces. Methods and simulations: An analytic relation between local crossing times in system slices and diffusivity as well as an explicit term for calculating drift-induced systematic errors is presented. The method is validated on Molecular Dynamics simulations of bulk water and applied to simulations of water in slit pores. Findings: After validation on bulk liquids, we clearly demonstrate the anisotropic nature of diffusion coefficients at interfaces. Significant spatial variations in the diffusivities correlate with interface-induced structuring but cannot be solely attributed to the drift induced by local density fluctuations.
CRIS-Team Scopus Importer:2023-08-11
Keywords: Anisotropic diffusion; Diffusion at interfaces; Diffusion in pores; Drift and density gradient; Porous materials; Transport coefficient

Anisotropic pair correlations in binary and multicomponent hard-sphere mixtures in the vicinity of a hard wall: A combined density functional theory and simulation study - Härtel, Andreas; Kohl, Matthias; Schmiedeberg, Michael (Physical Review E, 2015)
The fundamental measure approach to classical density functional theory has been shown to be a powerful tool to predict various thermodynamic properties of hard-sphere systems. We employ this approach to determine not only one-particle densities but also two-particle correlations in binary and six-component mixtures of hard spheres in the vicinity of a hard wall. The broken isotropy enables us to carefully test a large variety of theoretically predicted two-particle features by quantitatively comparing them to the results of Brownian dynamics simulations. Specifically, we determine and compare the one-particle density, the total correlation functions, their contact values, and the force distributions acting on a particle. For this purpose, we follow the compressibility route and theoretically calculate the direct correlation functions by taking functional derivatives. We usually observe an excellent agreement between theory and simulations, except for small deviations in cases where local crystal-like order sets in. Our results set the course for further investigations on the consistency of functionals as well as for structural analysis on, e.g., the primitive model. In addition, we demonstrate that due to the suppression of local crystallization, the predictions of six-component mixtures are better than those in bidisperse or monodisperse systems. Finally, we are confident that our results of the structural modulations induced by the wall lead to a deeper understanding of ordering in anisotropic systems in general, the onset of heterogeneous crystallization, caging effects, and glassy dynamics close to a wall, as well as structural properties in systems with confinement.

Anisotropy and memory during cage breaking events close to a wall - Kohl, Matthias; Haertel, Andreas; Schmiedeberg, Michael (Journal of Physics: Condensed Matter, 2016)
The slow dynamics in a glassy hard-sphere system is dominated by cage breaking events, i.e. rearrangements where a particle escapes from the cage formed by its neighboring particles. We study such events for an overdamped colloidal system by the means of Brownian dynamics simulations. While it is difficult to relate cage breaking events to structural mean field results in bulk, we show that the microscopic dynamics of particles close to a wall can be related to the anisotropic two-particle density. In particular, we study cage-breaking trajectories, mean forces on a tracked particle, and the impact of the history of trajectories. Based on our simulation results, we further construct two different one-particle random-walk models-one without and one with memory incorporated-and find the local anisotropy and the history-dependence of particles as crucial ingredients to describe the escape from a cage. Finally, our detailed study of a rearrangement event close to a wall not only reveals the memory effect of cages, but leads to a deeper insight into the fundamental mechanisms of glassy dynamics.
Keywords: cage breaking event; glassy dynamics; structure of liquids; dynamic slowdown; particle memory; random-walk model

Anomalous Dirac point transport due to extended defects in bilayer graphene - Shallcross, Sam; Sharma, Sangeeta; Weber, Heiko B. (Nature Communications, 2017)
Charge transport at the Dirac point in bilayer graphene exhibits two dramatically different transport states, insulating and metallic, that occur in apparently otherwise indistinguishable experimental samples. We demonstrate that the existence of these two transport states has its origin in an interplay between evanescent modes, that dominate charge transport near the Dirac point, and disordered configurations of extended defects in the form of partial dislocations. In a large ensemble of bilayer systems with randomly positioned partial dislocations, the distribution of conductivities is found to be strongly peaked at both the insulating and metallic limits. We argue that this distribution form, that occurs only at the Dirac point, lies at the heart of the observation of both metallic and insulating states in bilayer graphene.

Anomalous transport of a tracer on percolating clusters - Spanner, Markus; Hoefling, Felix; Schröder-Turk, Gerd; Mecke, Klaus; Franosch, Thomas (Journal of Physics: Condensed Matter, 2011)
We investigate the dynamics of a single tracer exploring a course of fixed obstacles in the vicinity of the percolation transition for particles confined to the infinite cluster. The mean-square displacement displays anomalous transport, which extends to infinite times precisely at the critical obstacle density. The slowing down of the diffusion coefficient exhibits power-law behavior for densities close to the critical point and we show that the mean-square displacement fulfills a scaling hypothesis. Furthermore, we calculate the dynamic conductivity as a response to an alternating electric field. Last, we discuss the non-Gaussian parameter as an indicator for heterogeneous dynamics.

Anomaly-free formulation of non-perturbative, four-dimensional Lorentzian quantum gravity - Thiemann, Thomas (Physics Letters B, 1996)
The technique introduced here can also be used to produce a couple of completely well-defined regulated operators including but not exhausting (i) the Euclidean Wheeler-DeWitt operator, (ii) the generator of the Wick rotation transform that maps solutions to the Euclidean Hamiltonian constraint to solutions to the Lorentzian Hamiltonian constraint, (iii) length operators, (iv) Hamiltonian operators of the matter sector and (v) the generators of the asymptotic Poincare group including the quantum ADM energy.
Keywords: Lorentzian; four-dimensional quantum gravity; Wheeler-DeWitt constraint; generator of the Wick rotation; euclidean Hamiltonian

An Open Scattering Model in Polymerized Quantum Mechanics - Giesel, Kristina; Kobler, Michael (Mathematics, 2022)
We derive a quantum master equation in the context of a polymerized open quantum mechanical system for the scattering of a Brownian particle in an ideal gas environment. The model is formulated in a top-down approach by choosing a Hamiltonian with a coupling between the system and environment that is generally associated with spatial decoherence. We extend the existing work on such models by using a non-standard representation of the canonical commutation relations, inspired by the quantization procedure applied in loop quantum gravity, which yields a model in which position operators are replaced by holonomies. The derivation of the master equation in a top-down approach opens up the possibility to investigate in detail whether the assumptions, usually used in such models in order to obtain a tractable form of the dissipator, hold also in the polymerized case or whether they need to be dropped or modified. Furthermore, we discuss some physical properties of the master equation associated to effective equations for the expectation values of the fundamental operators and compare our results to the already existing models of collisional decoherence.
CRIS-Team Scopus Importer:2022-12-02
Keywords: decoherence models; open quantum systems; polymer quantum mechanics

Antagonist-induced deadhesion of specifically adhered vesicles - Smith, Ana-Suncana; Lorz, Barbara Gertrud; Seifert, Udo; Sackmann, Erich (Biophysical Journal, 2006)
By use of a model system consisting of giant vesicles adhering to. at substrates, we identified, both experimentally and theoretically, two new control mechanisms for antagonist-induced deadhesion. Adhesion is established by specific binding of surface-grafted E-selectin and vesicle-carrying oligosaccharide Lewis(X). Deadhesion is achieved by controlled titration of monoclonal antibodies against E-selectin. The first mechanism is characterized by a considerable retraction of the contact zone resulting in a loss of contact area between the vesicle and the substrate. Within the developed theoretical framework, the observed equilibrium state is understood as a balance between the spreading pressure of the vesicle and the antagonist-induced lateral pressure at the edge of the contact zone. In the second mechanism, the antibodies induce unbinding by penetrating the contact zone without significantly affecting its size. This process reveals the decomposition of the adhesion zone into microdomains of tight binding separated by strongly fluctuating sections of the membrane. Both experiment and theory show a sigmoidal decrease of the number of bound ligands as a function of the logarithm of antagonist concentration. The work presented herein also provides a new method for the determination of the receptor binding affinity of either the surface-embedded ligands or the competing antagonist molecules.
Keywords: CELL-ADHESION; L-SELECTIN; MEMBRANE; LIGAND; REPULSION; INTEGRIN; CYTOSKELETON; NEUTROPHILS; TRANSITION; MICROSCOPY

ANTARES constrains a blazar origin of two IceCube PeV neutrino events - Adrian-Martinez, S.; Albert, A.; Andre, M.; Anton, Gisela; Ardid, Miguel; Aubert, J. -J.; Baret, B.; Barrios, J.; Basa, S.; Bertin, V.; Biagi, Simone; Bogazzi, C.; et al. (Astronomy & Astrophysics, 2015)
Results. Both blazars predicted to be the most neutrino-bright in the TANAMI sample (1653-329 and 1714-336) have a signal flux fitted by the likelihood analysis corresponding to approximately one event. This observation is consistent with the blazar-origin hypothesis of the IceCube event IC 14 for a broad range of blazar spectra, although an atmospheric origin cannot be excluded. No ANTARES events are observed from any of the other four blazars, including the three associated with IceCube event IC20. This excludes at a 90% confidence level the possibility that this event was produced by these blazars unless the neutrino spectrum is flatter than -2.4.
Keywords: neutrinos; galaxies: active; quasars: general

Antiferromagnetic cavity optomagnonics - Parvini, T. S.; Bittencourt, V. A. S. V.; Viola Kusminskiy, Silvia (Physical Review Research, 2020)
Currently there is a growing interest in studying the coherent interaction between magnetic systems and electromagnetic radiation in a cavity, prompted partly by possible applications in hybrid quantum systems. We propose a multimode cavity optomagnonic system based on antiferromagnetic insulators, where optical photons couple coherently to the two homogeneous magnon modes of the antiferromagnet. These have frequencies typically in the THz range, a regime so far mostly unexplored in the realm of coherent interactions, and which makes antiferromagnets attractive for quantum transduction from THz to optical frequencies. We derive the theoretical model for the coupled system, and show that it presents unique characteristics. In particular, if the antiferromagnet presents hard-axis magnetic anisotropy, the optomagnonic coupling can be tuned by a magnetic field applied along the easy axis. This allows us to bring a selected magnon mode into and out of a dark mode, providing an alternative for a quantum memory protocol. The dynamical features of the driven system present unusual behavior due to optically induced magnon-magnon interactions, including regions of magnon heating for a red-detuned driving laser. The multimode character of the system is evident in a substructure of the optomagnonically induced transparency window.
CRIS-Team WoS Importer:2021-01-22

Appearance of the single gyroid network phase in "nuclear pasta" matter - Schuetrumpf, Bastian; Klatt, Michael; Iida, K.; Schröder-Turk, Gerd; Maruhn, J. A.; Mecke, Klaus; Reinhard, Paul-Gerhard (Physical Review C - Nuclear Physics, 2015)
Nuclear matter under the conditions of a supernova explosion unfolds into a rich variety of spatially structured phases, called nuclear pasta. We investigate the role of periodic networklike structures with negatively curved interfaces in nuclear pasta structures, by static and dynamic Hartree-Fock simulations in periodic lattices. As the most prominent result, we identify for the first time the single gyroid network structure of cubic chiral I4(1)23 symmetry, a well-known configuration in nanostructured soft-matter systems, both as a dynamical state and as a cooled static solution. Single gyroid structures form spontaneously in the course of the dynamical simulations. Most of them are isomeric states. The very small energy differences from the ground state indicate its relevance for structures in nuclear pasta.

Approximating the physical inner product of loop quantum cosmology - Bahr, Benjamin; Thiemann, Thomas (Classical and Quantum Gravity, 2007)
In this paper, we investigate the possibility of approximating the physical inner product of constrained quantum theories. In particular, we calculate the physical inner product of a simple cosmological model in two ways: firstly, we compute it analytically via a trick; secondly, we use the complexifier coherent states to approximate the physical inner product defined by the master constraint of the system. We find that the approximation is able to recover the analytic solution of the problem, which consolidates hopes that coherent states will help to approximate solutions of more complicated theories, like loop quantum gravity.

Archimedean-like colloidal tilings on substrates with decagonal and tetradecagonal symmetry - Schmiedeberg, Michael; Mikhael, Jules; Rausch, Sebastian; Roth, Johannes; Helden, Laurent; Bechinger, Clemens; Stark, Holger (European Physical Journal E, 2010)
Two-dimensional colloidal suspensions subjected to laser interference patterns with decagonal symmetry can form an Archimedean-like tiling phase where rows of squares and triangles order aperiodically along one direction (J. Mikhael et al., Nature 454, 501 (2008)). In experiments as well as in Monte Carlo and Brownian dynamics simulations, we identify a similar phase when the laser field possesses tetradecagonal symmetry. We characterize the structure of both Archimedean-like tilings in detail and point out how the tilings differ from each other. Furthermore, we also estimate specific particle densities where the Archimedean-like tiling phases occur. Finally, using Brownian dynamics simulations we demonstrate how phasonic distortions of the decagonal laser field influence the Archimedean-like tiling. In particular, the domain size of the tiling can be enlarged by phasonic drifts and constant gradients in the phasonic displacement. We demonstrate that the latter occurs when the interfering laser beams are not ideally adjusted.

A resource theory of quantum measurements - Guff, Thomas; McMahon, Nathan; Sanders, Yuval R.; Gilchrist, Alexei (Journal of Physics A: Mathematical and Theoretical, 2021)
Resource theories are broad frameworks that capture how useful objects are in performing specific tasks. In this paperwe devise a formal resource theory quantum measurements, focusing on the ability of a measurement to acquire information. The objects of the theory are equivalence classes of positive operatorvalued measures, and the free transformations are changes to a measurement device that can only deteriorate its ability to report information about a physical system. We show that catalysis and purification, protocols that are possible in other resource theories, are impossible in our resource theory for quantum measurements. Standard measures of information gain are shown to be resource monotones, and the resource theory is applied to the task of quantum state discrimination.
CRIS-Team Scopus Importer:2021-05-14
Keywords: Catalysis; Measurement; Quantum; Quantum information; Resource; State discrimination; Theory

Are the spectra of geometrical operators in loop quantum gravity really discrete? - Dittrich, Bianca; Thiemann, Thomas (Journal of Mathematical Physics, 2009)
One of the celebrated results of loop quantum gravity (LQG) is the discreteness of the spectrum of geometrical operators such as length, area, and volume operators. This is an indication that the Planck scale geometry in LQG is discontinuous rather than smooth. However, there is no rigorous proof thereof at present. Because the aforementioned operators are not gauge invariant, they do not commute with the quantum constraints. The relational formalism in the incarnation of Rovelli's partial and complete observables provides a possible mechanism for turning a non-gauge-invariant operator into a gauge invariant one. In this paper we investigate whether the spectrum of such a physical, that is, gauge invariant, observable can be predicted from the spectrum of the corresponding gauge variant observables. We will not do this in full LQG but rather consider much simpler examples where field theoretical complications are absent. We find, even in those simpler cases, that kinematical discreteness of the spectrum does not necessarily survive at the gauge invariant level. Whether or not this happens depends crucially on how the gauge invariant completion is performed. This indicates that "fundamental discreteness at the Planck scale in LQG" is far from established. To prove it, one must provide the detailed construction of gauge invariant versions of geometrical operators.
Keywords: geometry; mathematical operators; quantum gravity

Artificial intelligence and machine learning for quantum technologies - Krenn, Mario; Landgraf, Jonas; Fösel, Thomas; Marquardt, Florian (Physical Review A, 2023)
In recent years the dramatic progress in machine learning has begun to impact many areas of science and technology significantly. In the present perspective article, we explore how quantum technologies are benefiting from this revolution. We showcase in illustrative examples how scientists in the past few years have started to use machine learning and more broadly methods of artificial intelligence to analyze quantum measurements, estimate the parameters of quantum devices, discover new quantum experimental setups, protocols, and feedback strategies, and generally improve aspects of quantum computing, quantum communication, and quantum simulation. We highlight open challenges and future possibilities and conclude with some speculative visions for the next decade.
CRIS-Team Scopus Importer:2023-01-20

A search for neutrino emission from the Fermi bubbles with the ANTARES telescope - Adrian-Martinez, S.; Albert, A.; Al Samarai, I.; Andre, Michel; Anton, Gisela; Anvar, S.; Ardid, Miguel; Astraatmadja, T.; Aubert, J-J; Baret, B.; Barrios-Marti, J.; Basa, S.; et al. (European Physical Journal C, 2014)
Analysis of the Fermi-LAT data has revealed two extended structures above and below the Galactic Centre emitting gamma rays with a hard spectrum, the so-called Fermi bubbles. Hadronic models attempting to explain the origin of the Fermi bubbles predict the emission of high-energy neutrinos and gamma rays with similar fluxes. The ANTARES detector, a neutrino telescope located in the Mediterranean Sea, has a good visibility to the Fermi bubble regions. Using data collected from 2008 to 2011 no statistically significant excess of events is observed and therefore upper limits on the neutrino flux in TeV range from the Fermi bubbles are derived for various assumed energy cutoffs of the source.

“‘A Sermon in the Midst of a Smutty Tale’: Blending in Genres of Speech, Writing and Literature.” - Sinding, Michael (Cognitive Literary Studies: Current Themes and New Directions, 2012)
Genre mixture is central to genre theory, in both literary and discourse studies. I characterize genres as multidimensional schemas integrating frames for action, situation, and discourse. Blending theory, as a model of the interaction of schemas, can clarify genre mixture and help to connect these fields. Laurence Sterne’s Tristram Shandy exemplifies Bakhtin’s view of the early novel cannibalizing extraliterary genres. Sterne framed his famous “Abuses of Conscience” sermon in four genre contexts: first, as the annual assize sermon at York Minster (1750); second, as a six-penny pamphlet (1750); third, integrated into Tristram Shandy (1760); fourth, in a collection of his sermons (1760). These productions vary considerably in purpose, rhetorical situation, and audience response. Notably, Tristram and Sterne’s sermons were great but mixed successes. The clash of the pious genre in the profane story, and Sterne’s attribution of his sermons to his comic alter-ego Mr. Yorick, prompted both denunciation and praise. I compare the blending and framing involved in the novel with the other cases, to examine how different conceptual networks produce different categorizations, meanings, and responses. I reflect on principles of genre blending relating the sermon to the book as a whole.
Keywords: Sterne; genre mixture; conceptual blending; sermon; novel

Assessing the Precision of Quantum Simulation of Many-Body Effects in Atomic Systems Using the Variational Quantum Eigensolver Algorithm - Sumeet, Sumeet; Srinivasa Prasannaa, V.; Das, Bhanu Pratap; Sahoo, Bijaya Kumar (Quantum Reports, 2022)
The emerging field of quantum simulation of many-body systems is widely recognized as a very important application of quantum computing. A crucial step towards its realization in the context of many-electron systems requires a rigorous quantum mechanical treatment of the different interactions. In this pilot study, we investigate the physical effects beyond the mean-field approximation, known as electron correlation, in the ground state energies of atomic systems using the classical-quantum hybrid variational quantum eigensolver algorithm. To this end, we consider three isoelectronic species, namely Be, Li−, and B+ . This unique choice spans three classes—a neutral atom, an anion, and a cation. We have employed the unitary coupled-cluster ansätz to perform a rigorous analysis of two very important factors that could affect the precision of the simulations of electron correlation effects within a basis, namely mapping and backend simulator. We carry out our all-electron calculations with four such basis sets. The results obtained are compared with those calculated by using the full configuration interaction, traditional coupled-cluster and the unitary coupled-cluster methods, on a classical computer, to assess the precision of our results. A salient feature of the study involves a detailed analysis to find the number of shots (the number of times a variational quantum eigensolver algorithm is repeated to build statistics) required for calculations with IBM Qiskit’s QASM simulator backend, which mimics an ideal quantum computer. When more qubits become available, our study will serve as among the first steps taken towards computing other properties of interest to various applications such as new physics beyond the Standard Model of elementary particles and atomic clocks using the variational quantum eigensolver algorithm.
CRIS-Team Scopus Importer:2022-05-13
Keywords: atomic systems; electron correlation; precision; variational quantum eigensolver algorithm

Association Rates of Membrane-Coupled Cell Adhesion Molecules - Bihr, Timo; Fenz, Susanne; Sackmann, Erich; Merkel, Rudolf; Seifert, Udo; Sengupta, Kheya; Smith, Ana-Suncana (Biophysical Journal, 2014)
Thus far, understanding how the confined cellular environment affects the lifetime of bonds, as well as the extraction of complexation rates, has been a major challenge in studies of cell adhesion. Based on a theoretical description of the growth curves of adhesion domains, we present a new (to our knowledge) method to measure the association rate k(on) of ligand-receptor pairs incorporated into lipid membranes. As a proof of principle, we apply this method to several systems. We find that the k(on) for the interaction of biotin with neutravidin is larger than that for integrin binding to RGD or sialyl Lewis(x) to E-selectin. Furthermore, we find k(on) to be enhanced by membrane fluctuations that increase the probability for encounters between the binders. The opposite effect on k(on) could be attributed to the presence of repulsive polymers that mimic the glycocalyx, which points to two potential mechanisms for controlling the speed of protein complexation during the cell recognition process.
Keywords: GIANT VESICLES; BINDING; BONDS; KINETICS; MODEL; DYNAMICS; SELECTIN; LIFETIME; SYSTEM; FORCE

A study of color centers in MgO using a hierarchical model - Berger, D.; Dinh, P. M.; Reinhard, Paul-Gerhard; Suraud, E. (European Physical Journal D, 2012)
Neutral and charged color centers in MgO, both in bulk and at the surface, are investigated from a theoretical perspective. We use a hierarchical approach which combines quantum-mechanical description with molecular mechanics (QM/MM) taking care to include dynamical polarizability of the MgO substrate. The performance of the QM/MM approach is compared with ab-initio calculations and with experimental data where available. We obtain fair acceptable agreement which lies well in the span of the different predictions. The particular efficiency of the QM/MM approach allows to vary the system size more freely and so to test the impact of the (necessarily) finite sample. We find that the sampling size plays a crucial role for color centers at MgO surface because the surface deformation can extend over several grid points. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2012.
CRIS-Team Scopus Importer:2023-07-24

Asymptotically Flat Boundary Conditions for the U(1)(3) Model for Euclidean Quantum Gravity - Bakhoda, Sepideh; Shojaie, Hossein; Thiemann, Thomas (Universe, 2021)
A generally covariant U(1)3 gauge theory describing the GN -> 0 limit of Euclidean general relativity is an interesting test laboratory for general relativity, specially because the algebra of the Hamiltonian and diffeomorphism constraints of this limit is isomorphic to the algebra of the corresponding constraints in general relativity. In the present work, we the study boundary conditions and asymptotic symmetries of the U(1)3 model and show that while asymptotic spacetime translations admit well-defined generators, boosts and rotations do not. Comparing with Euclidean general relativity, one finds that the non-Abelian part of the SU(2) Gauss constraint, which is absent in the U(1)3 model, plays a crucial role in obtaining boost and rotation generators.
CRIS-Team WoS Importer:2021-04-16

Asymptotic analysis of spin foam amplitude with timelike triangles - Liu, Hongguang; Han, Muxin (Physical Review D, 2019)
The large-j asymptotic behavior of the four-dimensional spin foam amplitude is investigated for the extended spin foam model (Conrady-Hnybida extension) on a simplicial complex. We study the most general situation in which timelike tetrahedra with timelike triangles are taken into account. The large-j asymptotic behavior is determined by the critical configurations of the amplitude. We identify the critical configurations that correspond to the Lorentzian simplicial geometries with timelike tetrahedra and triangles. Their contributions to the amplitude are asymptotic phases, whose exponents equal the Regge action of gravity. The amplitude may also contains critical configurations corresponding to nondegenerate split signature 4-simplices and degenerate vector geometries. But vertex amplitudes containing at least one timelike and one spacelike tetrahedra only give Lorentzian 4-simplices, while the split signature or degenerate 4-simplex does not appear.
CRIS-Team Scopus Importer:2019-05-14

Asymptotic analysis of the EPRL model with timelike tetrahedra - Sahlmann, Hanno; Kaminski, Wojciech; Kisielowski, Marcin (Classical and Quantum Gravity, 2018)
We study the asymptotic behaviour of the vertex amplitude for the EPRL spin foam model extended to include timelike tetrahedra. We analyze both, tetrahedra of signature — (standard EPRL), as well as of signature +– (Hnybida-Conrady extension), in a unified fashion. However, we assume all faces to be of signature –. We find that the critical points of the extended model are described again by 4-simplices and the phase of the amplitude is equal to the Regge action. Interestingly, in addition to the Lorentzian and Euclidean sectors there appear also split signature 4-simplices.
Keywords: quantum gravity; spin foam; EPRL model; stationary phase analysis; large spin limit

Asymptotic conformal symmetry at spatial infinity - Bakhoda, Sepideh; Mahdieh, Fatemeh; Shojaie, Hossein (Physical Review D, 2019)
In this paper, the effects of adding spatial conformal symmetry to the asymptotic symmetry group of an asymptotically conformally flat spacetime are studied. It is shown that, in addition to the BMS group, only the dilations of the spatial conformal generators keep the corresponding boundary conditions conformally invariant under hypersurface deformations. We prove that in order to attain (i) a well-defined symplectic structure and (ii) a finite and (iii) integrable conserved charge, these conditions are satisfied simultaneously when admitting Regge-Teitelboim and twisted Henneaux-Troessaert parity conditions, where the latter also contain supertranslation invariance. The conserved dilation charge contains nonzero terms independent of the field variables, giving a nonvanishing effect on the boundary. The dilation symmetry also modifies the ADM mass, which is another physical effect of the conformal symmetry.
CRIS-Team WoS Importer:2020-01-07

Asymptotic diagonalization of the fermionic Hamiltonian in hybrid loop quantum cosmology - Elizaga de Navascués, Beatriz; Mena Marugan, Guillermo A.; Prado, Santiago (Physical Review D, 2019)
We use the freedom available in hybrid loop quantum cosmology to split the degrees of freedom between the geometry and the matter fields so as to build a quantum field theory for the matter content with good quantum properties. We investigate this issue in an inflationary, flat cosmology with inhomogeneous perturbations, and focus the discussion on a Dirac field, minimally coupled to the cosmological background and treated as a perturbation. After truncating the action at the lowest nontrivial order in perturbations, one must define canonical variables for the matter content, for which one generally employs canonical transformations that mix the homogeneous background and the perturbations. Each of these possible definitions comes associated with a different matter contribution to the Hamiltonian of the complete system, that may, in general, contain terms that are quadratic in creation like variables, and in annihilation like variables, with the subsequent production and destruction of pairs of fermionic particles and antiparticles. We determine a choice of the fermionic canonical variables for which the interaction part of the Hamiltonian can be made as negligible as desired in the asymptotic regime of large particle/ antiparticle wave numbers. Finally, we study the quantum dynamics for this choice, imposing the total Hamiltonian constraint on the quantum states and assuming that their gravitational part is not affected significantly by the presence of fermions. In this way, we obtain a Schrodinger equation for the fermionic degrees of freedom in terms of quantum expectation values of the geometry that leads to asymptotically diagonal Heisenberg relations and Bogoliubov evolution transformations, with no divergences in the associated normal-ordered Hamiltonian.
CRIS-Team WoS Importer:2019-04-12

Asymptotic expansions of the hypergeometric function with two large parameters-application to the partition function of a lattice gas in a field of traps - Cvitkovic, Mislav; Smith, Ana-Suncana; Pande, Jayant (Journal of Physics A: Mathematical and Theoretical, 2017)
The canonical partition function of a two-dimensional lattice gas in a field of randomly placed traps, like many other problems in physics, evaluates to the Gauss hypergeometric function F-2(1)(a, b; c; z) in the limit when one or more of its parameters become large. This limit is difficult to compute from first principles, and finding the asymptotic expansions of the hypergeometric function is therefore an important task. While some possible cases of the asymptotic expansions of 2F1(a, b; c; z) have been provided in the literature, they are all limited by a narrow domain of validity, either in the complex plane of the variable or in the parameter space. Overcoming this restriction, we provide new asymptotic expansions for the hypergeometric function with two large parameters, which are valid for the entire complex plane of z except for a few specific points. We show that these expansions work well even when we approach the possible singularity of 2F1(a, b; c; z), |z| = 1, where the current expansions typically fail. Using our results we determine asymptotically the partition function of a lattice gas in a field of traps in the different possible physical limits of few/many particles and few/many traps, illustrating the applicability of the derived asymptotic expansions of the HGF in physics.
Keywords: hypergeometric function; asymptotic expansion; method of steepest descent; large parameters; special functions; partition function

Athermal jamming vs. glassy dynamics for particles with exponentially decaying repulsive pair interaction potentials with a cutoff - Wohlleben, Nicolas; Schmiedeberg, Michael (EPL - Europhysics Letters, 2021)
We study athermal jamming as well as the thermal glassy dynamics in systems composed of spheres that interact according to repulsive interactions that exponentially decay as a function of distance. As usual, a cutoff is employed in the simulations. While the athermal jamming transition that is determined by trying to remove overlaps is found to depend on the arbitrary and therefore unphysical choice of the cutoff, we do not find any athermal jamming transition or crossover that only relies on the physical decay length. In contrast, the glassy dynamics mainly depends on the decay length. Our findings constitute another demonstration of the fact that the athermal jamming transition is not related to thermal glassy dynamics. In addition, we argue that interactions without sharp physical cutoff should be considered more often as a model system in jamming. The reevaluation of widely used theoretical approaches or methods of analysis in the field of jamming and the task to change them such that they do not depend on unphysical cutoffs will lead to deeper insights into the nature of athermal and thermal jammin
Keywords: Glass transition; Jamming; cutoff; exponential interaction

Atomic and itinerant effects at the transition-metal x-ray absorption K pre-edge exemplified in the case of V2O3 - Hansmann, Philipp; Haverkort, M. W.; Toschi, Alessandro; Sangiovanni, Giorgio; Rodolakis, Fanny; Rueff, J. P.; Marsi, Marino; Held, Karsten (Physical Review B, 2012)
X-ray absorption spectroscopy is a well-established tool for obtaining information about orbital and spin degrees of freedom in transition-metal and rare-earth compounds. For this purpose usually the dipole transitions of the L (2p to 3d) and M (3d to 4f) edges are employed, whereas higher order transitions such as quadrupolar 1s to 3d in the K edge are rarely studied in that respect. This is due to the fact that usually such quadrupolar transitions are overshadowed by dipole-allowed 1s to 4p transitions and, hence, are visible only as minor features in the pre-edge region. Nonetheless, these features carry a lot of valuable information, similar to the dipole L-edge transition, which is not accessible in experiments under pressure due to the absorption of the diamond anvil pressure cell. We recently performed a theoretical and experimental analysis of such a situation for the metal-insulator transition of (V(1-x)Crx)(2)O-3. Since the importance of the orbital degrees of freedom in this transition is widely accepted, a thorough understanding of quadrupole transitions of the vanadium K pre-edge provides crucial information about the underlying physics. Moreover, the lack of inversion symmetry at the vanadium site leads to on-site mixing of vanadium 3d and 4p states and related quantum mechanical interferences between dipole and quadrupole transitions. Here we present a theoretical analysis of experimental high-resolution x-ray absorption spectroscopy at the V K pre-edge measured in partial fluorescence yield mode for single crystals. We carried out density functional as well as configuration interaction calculations in order to capture effects coming from both itinerant and atomic limits.
CRIS-Team WoS Importer:2020-02-19

Atomistic configurational forces in crystalline fracture - Birang Oskouei, Seyedehelmira; Park, Harold S.; Smith, Ana-Suncana; Steinmann, Paul (Forces in Mechanics, 2021)
Configurational atomistic forces contribute to the configurational mechanics (i.e. non-equilibrium) problem that determines the release of total potential energy of an atomistic system upon variation of the atomistic positions relative to the initial atomic configuration. These forces drive energetically favorable irreversible re-organizations of the material body, and thus characterize the tendency of crystalline defects to propagate. In this work, we provide new expressions for the atomistic configurational forces for two realistic interatomic potentials, i.e. the embedded atom potential (EAM) for metals, and second generation reactive bond order (REBO-II) potential for hydrocarbons. We present a range of numerical examples involving quasistatic fracture for both FCC metals and mono and bi-layer graphene at zero Kelvin that demonstrate the ability to predict defect nucleation and evolution using the proposed atomistic configurational mechanics approach. Furthermore, we provide the contributions for each potential including two-body stretching, three-body mixed-mode stretching-bending, and four-body mixed-mode stretching-bending-twisting terms that make towards defect nucleation and propagation.
Keywords: Atomistic configurational mechanics; Configurational forces; Many-body potentials; Fracture mechanics

Atomistic two-, three- and four-body potentials. Spatial and material settings - Steinmann, Paul; Smith, Ana-Suncana; Birang Oskouei, Seyedehelmira; Birang, E.; McBride, A.; Javili, A. (Journal of the Mechanics and Physics of Solids, 2021)
In molecular dynamics or molecular statics (MD/MS) multi-body potentials empirically capture the energetic interactions in atomistic systems enabling the computation of the corresponding atomistic forces as energetic conjugates to the atomistic positions. We distinguish here between spatial and material atomistic positions and consequently between the corresponding spatial and material atomistic forces. In quasi-statics, i.e. MS, the former, also denoted as deformational atomistic forces, contribute to the classical deformational mechanics (i.e., equilibrium) problem that seeks to minimise the total potential energy of an atomistic system with respect to the atomistic positions relative to the ambient space. The latter, also denoted as configurational atomistic forces, contribute to the configurational mechanics (i.e., non-equilibrium) problem that determines the release of total potential energy of an atomistic system upon variation of the atomistic positions relative to the ambient material, i.e., due to perturbations of the material (initial) atomistic configuration. The importance of material atomistic forces is that they drive energetically favourable re-organisations of the material atomistic configuration, thereby characterising the tendency of generic atomistic defects to propagate. In this contribution we focus on two-, three-, and four-body potentials, whereby we distinguish between novel stretch- and classical angle-based potentials for the two latter cases. Taken together, as the main contribution, we derive expressions for the corresponding spatial and, for the first time, material atomistic forces and highlight their striking formal similarity. The derivations are detailed but the final expression compact and well-suited for numerical implementation.
CRIS-Team Scopus Importer:2021-06-25

“A Triple-Frame Model of Genre: Framing for Discourse Sequencing.” - Sinding, Michael (Genre in Language, Discourse and Cognition, 2016)
I propose a theoretical model of genre with parameters organized in terms of the concept of “frame.” The model has three nested frames: a sociocognitive action frame contains a rhetorical situation frame, which contains a discourse structure frame. I motivate the model in relation to existing research on both literary and extraliterary genres, sketch its advantages and the lines of research it suggests, then address challenges to it. First, to clarify how genre relates to language, discourse and cognition, and my investments in various research areas, I review major conundrums about genre identity. Second, I argue for an inclusive approach to genres, as against defining “genre” to suit assumptions and methods. Third, I discuss the broader conception of discourse structure and sequencing that this inclusive approach entails. Fourth, I investigate frame-based sequencing in text composition—specifically, how poets adapt everyday genre frames for literary purposes. I propose a theoretical model of genre with parameters organized in terms of the concept of “frame.” The model has three nested frames: a sociocognitive action frame contains a rhetorical situation frame, which contains a discourse structure frame. I motivate the model in relation to existing research on both literary and extraliterary genres, sketch its advantages and the lines of research it suggests, then address challenges to it. First, to clarify how genre relates to language, discourse and cognition, and my investments in various research areas, I review major conundrums about genre identity. Second, I argue for an inclusive approach to genres, as against defining “genre” to suit assumptions and methods. Third, I discuss the broader conception of discourse structure and sequencing that this inclusive approach entails. Fourth, I investigate frame-based sequencing in text composition—specifically, how poets adapt everyday genre frames for literary purposes.
Keywords: genre; cognition; frame; sociocognitive action; rhetorical situation; discourse structure

Auf der Suche nach Sprache. Ulrike Draesner im Dialog zu »Mitgift« und »Vorliebe« - Heydenreich, Aura; Mecke, Klaus (Physik und Poetik. Produktionsästhetik und Werkgenese. Autorinnen und Autoren im Dialog, 2015)

Automorphisms in loop quantum gravity - Bahr, Benjamin; Thiemann, Thomas (Classical and Quantum Gravity, 2009)
We investigate a certain distributional extension of the group of spatial diffeomorphisms in loop quantum gravity. This extension, which is given by the automorphisms Aut(P) of the path groupoid P, was proposed by Velhinho and is inspired by category theory. These automorphisms have much larger orbits than piecewise analytic diffeomorphisms. In particular, we will show that graphs with the same combinatorics but different generalized knotting classes can be mapped into each other. We describe the automorphism-invariant Hilbert space and comment on how a combinatorial formulation of LQG might arise.

A wavefunction description for a localized quantum particle in curved spacetimes - Perche, T. Rick; Neuser, Jonas (Classical and Quantum Gravity, 2021)
We reduce Dirac's spinor formalism for a spin 1/2 particle to a complex wavefunction description in curved spacetimes. We consider a localized fermionic particle in curved spacetimes and perform an expansion in terms of the acceleration and curvature around the center of mass of the system, generalizing the results of Parker (1980 Phys. Rev. Lett. 44 1559), Parker (1980 Phys. Rev. D 22 1922-1934). Under a non-relativistic approximation, one obtains a quantum description in a Hilbert space of complex wavefunctions defined in the rest space of the system. The wavefunction of the particle then evolves according to a modified Schrodinger equation associated with a symmetric Hamiltonian. When compared to the standard Schrodinger equation for a wavefunction, we obtain corrections in terms of the acceleration of the system's center of mass and curvature of spacetime along its trajectory. In summary, we provide a formalism for the use of a complex wavefunction to describe a localized quantum particle in curved spacetimes.
CRIS-Team WoS Importer:2021-08-20

Backreaction in Cosmology - Schander, S.; Thiemann, Thomas (Frontiers in Astronomy and Space Sciences, 2021)
In this review, we investigate the question of backreaction in different approaches to cosmological perturbation theory, and with a special focus on quantum theoretical aspects. By backreaction we refer here to the effects of matter field or cosmological inhomogeneities on the homogeneous dynamical background degrees of freedom of cosmology. We begin with an overview of classical cosmological backreaction which is ideally suited for physical situations in the late time Universe. We then proceed backwards in time, considering semiclassical approaches such as semiclassical or stochastic (semiclassical) gravity which take quantum effects of the perturbations into account. Finally, we review approaches to backreaction in quantum cosmology that should apply to the very early Universe where classical and semiclassical approximations break down. The main focus is on a recently proposed implementation of backreaction in quantum cosmology using a Born-Oppenheimer inspired method.
CRIS-Team WoS Importer:2021-08-20

Ballistic propagation of density correlations and excess wall forces in quenched granular media - Schindler, Thomas; Rohwer, Christian M. (Physical Review E, 2020)
We investigate a granular gas in a shaken quasi-two-dimensional box in molecular dynamics computer simulations. After a sudden change (quench) of the shaking amplitude, transient density correlations are observed orders of magnitude beyond the steady-state correlation length scale. Propagation of the correlations is ballistic, in contrast to recently investigated quenches of Brownian particles that show diffusive propagation [Rohwer et al., Phys. Rev. Lett. 118, 015702 (2017)PRLTAO0031-900710.1103/PhysRevLett.118.015702, Rohwer et al., Phys. Rev. E 97, 032125 (2018)2470-004510.1103/PhysRevE.97.032125]. At sufficiently strong cooling of the fluid the effect is overlaid by clustering instability of the homogeneous cooling state with different scaling behavior. We are able to identify different quench regimes. In each regime correlations exhibit remarkably universal position dependence. In simulations performed with side walls we find confinement effects for temperature and pressure in steady-state simulations and an additional transient wall pressure contribution when changing the shaking amplitude. The transient contribution is ascribed to enhanced relaxation of the fluid in the presence of walls. From incompatible scaling behavior we conclude that the observed effects with and without side walls constitute distinct phenomena.
CRIS-Team Scopus Importer:2020-12-11

Band Alignment and Charge Transfer in Complex Oxide Interfaces - Zhong, Zhicheng; Hansmann, Philipp (Physical Review X, 2017)
The synthesis of transition metal heterostructures is currently one of the most vivid fields in the design of novel functional materials. In this paper, we propose a simple scheme to predict band alignment and charge transfer in complex oxide interfaces. For semiconductor heterostructures, band-alignment rules like the well-known Anderson or Schottky-Mott rule are based on comparison of the work function or electron affinity of the bulk components. This scheme breaks down for oxides because of the invalidity of a single work-function approximation as recently shown in [Phys. Rev. B 93, 235116 (2016); Adv. Funct. Mater. 26, 5471 (2016)]. Here, we propose a new scheme that is built on a continuity condition of valence states originating in the compounds' shared network of oxygen. It allows for the prediction of sign and relative amplitude of the intrinsic charge transfer, taking as input only information about the bulk properties of the components. We support our claims by numerical density functional theory simulations as well as (where available) experimental evidence. Specific applications include (i) controlled doping of SrTiO3 layers with the use of 4d and 5d transition metal oxides and (ii) the control of magnetic ordering in manganites through tuned charge transfer.
CRIS-Team WoS Importer:2020-02-19

Bands, resonances, edge singularities and excitons in core level spectroscopy investigated within the dynamical mean-field theory - Haverkort, M. W.; Sangiovanni, Giorgio; Hansmann, Philipp; Toschi, Alessandro; Lu, Yi; Macke, S. (EPL - Europhysics Letters, 2014)
Using a recently developed impurity solver we exemplify how dynamical mean-field theory captures band excitations, resonances, edge singularities and excitons in core level x-ray absorption spectroscopy (XAS) and core level photo electron spectroscopy (cPES) on metals, correlated metals and Mott insulators. Comparing XAS at different values of the core-valence interaction shows how the quasiparticle peak in the absence of core-valence interactions evolves into a resonance of similar shape, but different origin. Whereas XAS is rather insensitive to the metal insulator transition, cPES can be used, due to nonlocal screening, to measure the amount of local charge fluctuation. Copyright (C) EPLA, 2014
CRIS-Team WoS Importer:2020-02-19

Baxter-Wu model in a transverse magnetic field - Capponi, Sylvain; Jahromi, Saeed S.; Alet, Fabien; Schmidt, Kai Phillip (Physical Review E, 2014)
We investigate the low-energy properties as well as quantum and thermal phase transitions of the Baxter-Wu model in a transverse magnetic field. Our study relies on stochastic series expansion quantum Monte Carlo and on series expansions about the low- and high-field limits at zero temperature using the quantum finite-lattice method on the triangular lattice. The phase boundary consists of a second-order critical line in the four-state Potts model universality class starting from the pure Baxter-Wu limit meeting a first-order line connected to the zero-temperature transition point (h approximate to 2.4, T = 0). Both lines merge at a tricritical point approximately located at (h approximate to 2.3J, T approximate to J).

Beating classical heuristics for the binary paint shop problem with the quantum approximate optimization algorithm - Streif, Michael; Yarkoni, Sheir; Skolik, Andrea; Neukart, Florian; Leib, Martin (Physical Review A, 2021)
The binary paint shop problem (BPSP) is an APX-hard optimization problem of the automotive industry. In this work, we show how to use the quantum approximate optimization algorithm (QAOA) to find solutions of the BPSP. We demonstrate that QAOA with constant depth is able to beat all known heuristics for the binary paint shop problem on average in the infinite size limit n→∞. We complete our studies by running first experiments of small-sized instances on a trapped-ion quantum computer through Amazon Braket.
CRIS-Team Scopus Importer:2021-07-16

BENDING RIGIDITY OF FLUCTUATING MEMBRANES - Mecke, Klaus (Zeitschrift für Physik B Condensed Matter, 1995)
lattice model of random surfaces is studied including configurations with arbitrary topologies, overhangs and bubbles. The Hamiltonian of the surface includes a term proportional to its area and a scale-invariant integral of the squared mean curvature. We propose a discretization of the curvature which ensures the scale-invariance of the bending energy on the lattice. Nonperturbative renormalization groups for the surface tension and the bending rigidity are applied, which are also valid at high temperatures and scales above the persistence length. We find at vanishing surface tensions a closed expression for the scale dependent rigidity including the usual logarithmic decay at low temperatures. Different scaling behaviours at non-vanishing tensions occur yielding characteristic length scales, which determine the structure of homogeneous droplet, lamellar, and microemulsion phases.

Beyond the percolation universality class: the vertex split model for tetravalent lattices - Nachtrab, Susan; Hoffmann, Matthias; Kapfer, Sebastian; Schröder-Turk, Gerd; Mecke, Klaus (New Journal of Physics, 2015)
We propose a statistical model defined on tetravalent three-dimensional lattices in general and the three-dimensional diamond network in particular where the splitting of randomly selected nodes leads to a spatially disordered network, with decreasing degree of connectivity. The terminal state, that is reached when all nodes have been split, is a dense configuration of self-avoiding walks on the diamond network. Starting from the crystallographic diamond network, each of the four-coordinated nodes is replaced with probability p by a pair of two edges, each connecting a pair of the adjacent vertices. For all values 0 <= p <= 1 the network percolates, yet the fraction f(p) of the system that belongs to a percolating cluster drops sharply at p(c) = 1 to a finite value f(p)(c). This transition is reminiscent of a percolation transition yet with distinct differences to standard percolation behaviour, including a finite mass f(p)(c) > 0 of the percolating clusters at the critical point. Application of finite size scaling approach for standard percolation yields scaling exponents for p -> p(c) that are different from the critical exponents of the second-order phase transition of standard percolation models. This transition significantly affects the mechanical properties of linear-elastic realizations (e.g. as custom-fabricated models for artificial bone scaffolds), obtained by replacing edges with solid circular struts to give an effective density phi. Finite element methods demonstrate that, as a low-density cellular structure, the bulk modulus K shows a cross-over from a compression-dominated behaviour, K(phi) alpha phi(k) with kappa approximate to 1, at p = 0 to a bending-dominated behaviour with kappa approximate to 2 at p = 1.
Keywords: percolation; networks in phase transitions; mechanical properties of solids

Bias-controlled selective excitation of vibrational modes in molecular junctions: a route towards mode-selective chemistry - Volkovich, Roie; Härtle, Rainer; Thoss, Michael; Peskin, Uri (Physical Chemistry Chemical Physics, 2011)
We show that individual vibrational modes in single-molecule junctions with asymmetric molecule-lead coupling can be selectively excited by applying an external bias voltage. Thereby, a non-statistical distribution of vibrational energy can be generated, that is, a mode with a higher frequency can be stronger excited than a mode with a lower frequency. This is of particular interest in the context of mode-selective chemistry, where one aims to break specific (not necessarily the weakest) chemical bond in a molecule. Such mode-selective vibrational excitation is demonstrated for two generic model systems representing asymmetric molecular junctions and/or scanning tunneling microscopy experiments. To this end, we employ two complementary theoretical approaches, a nonequilibrium Green's function approach and a master equation approach. The comparison of both methods reveals good agreement in describing resonant electron transport through a single-molecule contact, where differences between the approaches highlight the role of non-resonant transport processes, in particular co-tunneling and off-resonant electron-hole pair creation processes. © the Owner Societies 2011.
UnivIS-Import:2015-03-09:Pub.2011.nat.dphy.ITP.ppem.biasco

Bicontinuous minimal surface nanostructures for polymer blend solar cells - Kimber, Robin G. E.; Walker, Alison B.; Schröder-Turk, Gerd; Cleaver, Douglas J. (Physical Chemistry Chemical Physics, 2010)
This paper presents the first examination of the potential for bicontinuous structures such as the gyroid structure to produce high efficiency solar cells based on conjugated polymers. The solar cell characteristics are predicted by a simulation model that shows how the morphology influences device performance through integration of all the processes occurring in organic photocells in a specified morphology. In bicontinuous phases, the surface defining the interface between the electron and hole transporting phases divides the volume into two disjoint subvolumes. Exciton loss is reduced because the interface at which charge separation occurs permeates the device so excitons have only a short distance to reach the interface. As each of the component phases is connected, charges will be able to reach the electrodes more easily. In simulations of the current–voltage characteristics of organic cells with gyroid, disordered blend and vertical rod (rods normal to the electrodes) morphologies, we find that gyroids have a lower than anticipated performance advantage over disordered blends, and that vertical rods are superior. These results are explored thoroughly, with geminate recombination, i.e. recombination of charges originating from the same exciton, identified as the primary source of loss. Thus, if an appropriate materials choice could reduce geminate recombination, gyroids show great promise for future research and applications.
EAM Import::2019-03-13

Bilayer Kitaev models: Phase diagrams and novel phases - Seifert, Urban F. P.; Gritsch, Julian; Wagner, Erik; Joshi, Darshan G.; Brenig, Wolfram; Vojta, Matthias; Schmidt, Kai Phillip (Physical Review B, 2018)
Kitaev's honeycomb-lattice spin-1/2 model has become a paradigmatic example for Z(2) quantum spin liquids, both gapped and gapless. Here we study the fate of these spin-liquid phases in differently stacked bilayer versions of the Kitaev model. Increasing the ratio between the interlayer Heisenberg coupling J(perpendicular to) and the intralayer Kitaev couplings K-x,K-y,K-z destroys the topological spin liquid in favor of a paramagnetic dimer phase. We study phase diagrams as a function of J(perpendicular to)/K and Kitaev coupling anisotropies using Majorana-fermion mean-field theory, and we employ different expansion techniques in the limits of small and large J(perpendicular to)/K. For strongly anisotropic Kitaev couplings, we derive effective models for the different layer stackings that we use to discuss the quantum phase transition out of the Kitaev phase. We find that the phase diagrams depend sensitively on the nature of the stacking and anisotropy strength. While in some stackings and at strong anisotropies we find a single transition between the Kitaev and dimer phases, other stackings are more involved. Most importantly, we prove the existence of two novel macrospin phases, which can be understood in terms of Ising chains that can be either coupled ferromagnetically or remain degenerate, thus realizing a classical spin liquid. In addition, our results suggest the existence of a flux phase with spontaneous interlayer coherence. We discuss prospects for experimental realizations.

Bioinspired Hybrid White Light-Emitting Diodes - Weber, Michael; Niklaus, Lukas; Pröschel, Marlene; Brana Coto, Pedro; Sonnewald, Uwe; Costa Riquelme, Rubén Dario (Advanced Materials, 2015)
The first bioinspired hybrid white-light-emitting diodes (bio-HLEDs) featuring protein cascade coatings are presented. For easy fabrication a new strategy to stabilize proteins in rubber-like material was developed. The synergy between the excellent features of fluorescent proteins and the easily processed rubber produces bio-HLEDs with less than 10% loss in luminous efficiency over 100 hours.

Biomechanical of adhesion - from basic principles to mechanisms of immune cell activity - Smith, Ana-Suncana (EUROPEAN BIOPHYSICS JOURNAL WITH BIOPHYSICS LETTERS, 2021)
CRIS-Team WoS Importer:2021-08-13

Biomechanics as driver of aggregation of tethers in adherent membranes - Li, Long; Kamal, Mohammad Arif; Stumpf, Bernd Henning; Thibaudau, Franck; Sengupta, Kheya; Smith, Ana-Suncana (Soft Matter, 2021)
Cell adhesion is an important cellular process and is mediated by adhesion proteins residing on the cell membrane. Sometimes, two types of linker proteins are involved in adhesion, and they can segregate to form domains through a poorly understood size-exclusion process. We present an experimental and theoretical study of adhesion via linkers of two different sizes, realised in a mimetic model-system, based on giant unilamellar vesicles interacting with supported lipid bilayers. Here, adhesion is mediated by DNA linkers with two different lengths, but with the same binding enthalpy. We study the organisation of these linkers into domains as a function of relative fraction of long and short DNA constructs. Experimentally, we find that, irrespective of the composition, the adhesion domains are uniform with coexisting DNA bridge types, despite their relative difference in length of 9 nm. However, simulations suggest formation of nanodomains of the minority fraction at short length scales, which is below the optical resolution of the microscope. The nano-aggregation is more significant for long bridges, which are also more stable.
CRIS-Team WoS Importer:2021-11-12

BIOPHYSICS Alive and twitching - Smith, Ana-Suncana (2016)
Living systems are constantly being driven out of equilibrium by consuming energy. Studying fluctuations can tell us how they do so while maintaining order — and what this teaches us about non-equilibrium processes in general.

Bis-polyethylene glycol-functionalized imidazolium ionic liquids: A multi-method approach towards bulk and surface properties - Seidl, Vera; Bosch, Michael; Paap, Ulrike; Livraghi, Mattia; Zhai, Ziwen; Wick, Christian; Koller, Thomas Manfred; Wasserscheid, Peter; Maier, Florian; Smith, Ana-Suncana; Bachmann, Julien; Steinrück, Hans-Peter; et al. (Journal of Ionic Liquids, 2022)
Adding to the versatile class of ionic liquids, we report a new series of hydrophilic polyethylene glycol-functionalized room-temperature ionic liquids, offering interesting thermo- and electrochemical behavior as well as remarkable surface properties. A scalable and generally applicable synthetic procedure for the preparation of N,N’-bis(polyethylene glycol)imidazolium salts, generally abbreviated as [(mPEGn)2Im][A] (n = 2‒6, A = I‒, OMs‒, PF6‒, NTf2‒) was developed. These ionic liquids were studied concerning their thermo- and electrochemical properties, such as phase transition behavior, decomposition temperature, viscosity, and density, as well as electrical conductivity and electrochemical stability, using i.a. differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV). Additionally, the surface properties were investigated by angle-resolved X-ray photoelectron spectroscopy (ARXPS) and the pendant drop method. olecular dynamics simulations complement the studies and provide insight into the molecular structure of the ionic liquids and their specific orientation at the liquid-vacuum interface.<br

Bistability in a nonequilibrium quantum system with electron-phonon interactions - Wilner, Eli Y.; Wang, Haobin; Cohen, Guy; Thoss, Michael; Rabani, Eran (Physical Review B, 2013)
The existence of more than one steady state in a many-body quantum system driven out of equilibrium has been a matter of debate, both in the context of simple impurity models and in the case of inelastic tunneling channels. In this paper, we combine a reduced density matrix formalism with the multilayer multiconfiguration time-dependent Hartree method to address this problem. This allows us to obtain a converged numerical solution of the nonequilibrium dynamics. Considering a generic model for quantum transport through a quantum dot with electron-phonon interaction, we prove that a unique steady state exists regardless of the initial electronic preparation of the quantum dot, consistent with the converged numerical results. However, a bistability can be observed for different initial phonon preparations. The effects of the phonon frequency and strength of the electron-phonon couplings on the nonequilibrium dynamics and on the emergence of bistability is discussed. © 2013 American Physical Society.
UnivIS-Import:2015-03-09:Pub.2013.nat.dphy.ITP.ppem.bistab

Black hole horizons from within loop quantum gravity - Sahlmann, Hanno (Physical Review D - Particles, Fields, Gravitation and Cosmology, 2011)
In general relativity, the fields on a black hole horizon are obtained from those in the bulk by pullback and restriction. Similarly, in quantum gravity, the quantized horizon degrees of freedom should result from restricting, or pulling back, the quantized bulk degrees of freedom. This is not yet fully realized in the-otherwise very successful-quantization of isolated horizons in loop quantum gravity. In this work we outline a setting in which the quantum horizon degrees of freedom are simply components of the quantized bulk degrees of freedom. There is no need to quantize them separately. We present evidence that for a horizon of sphere topology, the resulting horizon theory is remarkably similar to what has been found before. © 2011 American Physical Society.
Keywords: black holes; quantum gravity; horizons; loop quantum gravity

“Blending in a baciyelmo: Don Quixote’s Genre Blending and the Invention of the Novel.” - Sinding, Michael (Blending and the Study of Narrative., 2012)
An account of Cervantes’ "invention of the novel"–by mixing the genres of chivalric romance and picaresque tale–in terms of Blending Theory has several related aims, concerning issues in several related areas. First, regarding this text and its genre, I want to clarify Don Quixote’s structure, and its decisive impact on the novel genre. I will describe how Cervantes created a new genre template that later writers adopted by blending two known genres. Second, regarding literary genre theory, I want to clarify how genre blending shapes and refracts through various textual levels. I suggest that the narrative level of genre structure, the level of typical characters and actions, is the most fundamental one, as it has more influence on the structure of other levels than they have on it. This level is also fundamental in the unfolding of genre blends. Third, I hope this case study will also clarify the benefits and challenges of using Blending Theory to model cognitive structures and processes subtending discourse. Literary genre blending highlights the potential variability, complexity, and depth of ‘full meaning construction’ in discourse, and the case of Don Quixote suggests that it may be unrealistic to expect a simple answer to the question of exactly whose mental processes are being modeled in blending analyses. Finally, genre blending analyses offer a vantage on the cognitive study of culture, because they can clarify the structure and history of the worldviews that are combined and transformed in genre mixtures such as Cervantes’.
Keywords: Genre mixture; conceptual blending; Don Quixote; chivalric romance; picaresque; narrative structure

Block Chemistry for Accurate Modeling of Epoxy Resins - Livraghi, Mattia; Pahi, Sampanna; Nowakowski, Piotr; Smith, David M.; Wick, Christian; Smith, Ana-Suncana (Journal of Physical Chemistry B, 2023)
Accurate molecular modeling of the physical and chemical behavior of highly cross-linked epoxy resins at the atomistic scale is important for the design of new property-optimized materials. However, a systematic approach to parametrizing and characterizing these systems in molecular dynamics is missing. We therefore present a unified scheme to derive atomic charges for amine-based epoxy resins, in agreement with the AMBER force field, based on defining reactive fragments─blocks─building the network. The approach is applicable to all stages of curing from pure liquid to gelation to fully cured glass. We utilize this approach to study DGEBA/DDS epoxy systems, incorporating dynamic topology changes into atomistic molecular dynamics simulations of the curing reaction with 127,000 atoms. We study size effects in our simulations and predict the gel point utilizing a rigorous percolation theory to recover accurately the experimental data. Furthermore, we observe excellent agreement between the estimated and the experimentally determined glass transition temperatures as a function of curing rate. Finally, we demonstrate the quality of our model by the prediction of the elastic modulus based on uniaxial tensile tests. The presented scheme paves the way for a broadly consistent approach for modeling and characterizing all amine-based epoxy resins.
CRIS-Team Scopus Importer:2023-09-15

Body of Vision: Northrop Frye and the Poetics of Mind - Sinding, Michael (University of Toronto Press, 2014)
In Body of Vision, Michael Sinding connects Northrop Frye’s groundbreaking contributions to our understanding of the human imagination with cognitive poetics – the cutting-edge school of literary criticism that applies the principles of cognitive science to the interpretation of literary texts and contexts. Sinding undertakes this task through analyses of the interplay of metaphoric and narrative schemas in several forms of cultural mythology. Sinding identifies the profound connections between cognitive views of language, literature, and culture and Frye’s views by exploring three related aspects of Frye’s work – meaning and thought, culture and society, and literary history. He investigates these connections through detailed studies of major cultural texts including Dante’s Divine Comedy, Hobbes’ Leviathan, Rousseau’s Social Contract, and Milton’s "Lycidas." By linking Frye’s classic studies to exciting recent approaches in the humanities and the cognitive revolution of the past few decades, Body of Vision casts Frye’s achievements in a fascinating new light.
Keywords: Northrop Frye; cognitive poetics; literature; culture; metaphor; narrative; genre

Boolean reconstructions of complex materials: Integral geometric approach - Arns, C. H.; Knackstedt, M. A.; Mecke, Klaus (Physical Review E, 2009)
We show that for the Boolean model of random composite media one can, from a single image of a system at any particle fraction, define a set of parameters which allows one to accurately reconstruct the medium for all other phase fractions. The morphological characterization is based on a family of measures known in integral geometry which provides powerful formulas for the Boolean model. The percolation thresholds of either phase are obtained with good accuracy. From the reconstructions one can subsequently predict property curves for the material across all phase fractions from the single three-dimensional image. We illustrate this for transport and mechanical properties of complex Boolean systems and for experimental sandstone samples.
Keywords: Boolean algebra; granular materials; integral equations; percolation; random media

Bounds on universal quantum computation with perturbed two-dimensional cluster states - Orus, Roman; Kalis, Henning; Bornemann, Marcel; Schmidt, Kai Phillip (Physical Review A, 2013)
Motivated by the possibility of universal quantum computation under noise perturbations, we compute the phase diagram of the two-dimensional (2D) cluster state Hamiltonian in the presence of Ising terms and magnetic fields. Unlike in previous analysis of perturbed 2D cluster states, we find strong evidence of a very well-defined cluster phase, separated from a polarized phase by a line of first-and second-order transitions compatible with the 3D Ising universality class and a tricritical end point. The phase boundary sets an upper bound for the amount of perturbation in the system so that its ground state is still useful for measurement-based quantum computation purposes. Moreover, we also compute the local fidelity with the unperturbed 2D cluster state. Besides a classical approximation, we determine the phase diagram by combining series expansion and variational infinite projected entangled-pair states methods. Our work constitutes an analysis of the nontrivial effect of few-body perturbations in the 2D cluster state, which is of relevance for experimental proposals.

Bound States and Field-Polarized Haldane Modes in a Quantum Spin Ladder - Ward, S.; Mena, M.; Bouillot, P.; Kollath, C.; Giamarchi, T.; Schmidt, Kai Phillip; Normand, B.; Kramer, K. W.; Biner, D.; Bewley, R.; Guidi, T.; Boehm, M.; et al. (Physical Review Letters, 2017)
The challenge of one-dimensional systems is to understand their physics beyond the level of known elementary excitations. By high-resolution neutron spectroscopy in a quantum spin-ladder material, we probe the leading multiparticle excitation by characterizing the two-magnon bound state at zero field. By applying high magnetic fields, we create and select the singlet (longitudinal) and triplet (transverse) excitations of the fully spin-polarized ladder, which have not been observed previously and are close analogs of the modes anticipated in a polarized Haldane chain. Theoretical modeling of the dynamical response demonstrates our complete quantitative understanding of these states.

Bound states in two-dimensional spin systems near the Ising limit: A quantum finite-lattice study - Dusuel, Sebastien; Kamfor, Michael; Schmidt, Kai Phillip; Thomale, Ronny; Vidal, Julien (Physical Review B, 2010)
We analyze the properties of low-energy bound states in the transverse-field Ising model and in the XXZ model on the square lattice. To this end, we develop an optimized implementation of perturbative continuous unitary transformations. The Ising model is studied in the small-field limit which is found to be a special case of the toric code model in a magnetic field. To analyze the XXZ model, we perform a perturbative expansion about the Ising limit in order to discuss the fate of the elementary magnon excitations when approaching the Heisenberg point.

Breakdown of a perturbed Z(N) topological phase - Schulz, Marc Daniel; Dusuel, Sebastien; Orus, Roman; Vidal, Julien; Schmidt, Kai Phillip (New Journal of Physics, 2012)
We studied the robustness of a generalized Kitaev's toric code with Z(N) degrees of freedom in the presence of local perturbations. For N = 2, this model reduces to the conventional toric code in a uniform magnetic field. A quantitative analysis was performed for the perturbed Z(3) toric code by applying a combination of high-order series expansions and variational techniques. We found strong evidence for first- and second-order phase transitions between topologically ordered and polarized phases. Most interestingly, our results also indicate the existence of topological multi-critical points in the phase diagram.

Brownian particles in random and quasicrystalline potentials: How they approach the equilibrium - Schmiedeberg, Michael; Roth, Johannes; Stark, Holger (European Physical Journal E, 2007)
We study the Brownian motion of an ensemble of single colloidal particles in a random square and a quasicrystalline potential when they start from non-equlibrium. For both potentials, Brownian dynamics simulations reveal a widespread subdiffusive regime before the diffusive long-time limit is reached in thermal equilibrium. We develop a random trap model based on a distribution for the depths of trapping sites that reproduces the results of the simulations in detail. Especially, it gives analytic formulas for the long-time diffusion constant and the relaxation time into the diffusive regime. Aside from detailed differences, our work demonstrates that quasicrystalline potentials can be used to mimic aspects of random potentials.

Brownian particles on rough substrates: Relation between intermediate subdiffusion and asymptotic long-time diffusion - Hanes, Richard; Schmiedeberg, Michael; Egelhaaf, Stefan (Physical Review E, 2013)
Brownian particles in random potentials show an extended regime of subdiffusive dynamics at intermediate times. The asymptotic diffusive behavior is often established at very long times and thus cannot be accessed in experiments or simulations. For the case of one-dimensional random potentials with Gaussian distributed energies, we present a detailed analysis of experimental and simulation data. It is shown that the asymptotic long-time diffusion coefficient can be related to the behavior at intermediate times, namely, the minimum of the exponent that characterizes subdiffusion and hence corresponds to the maximum degree of subdiffusion. As a consequence, investigating only the dynamics at intermediate times is sufficient to predict the order of magnitude of the long-time diffusion coefficient and the time scale at which the crossover from subdiffusion to diffusion occurs, i.e., when the long-time diffusive regime and hence thermal equilibrium is established. Inversely, theoretical predictions derived for the asymptotic long-time behavior can be used to quantitatively characterize the intermediate behavior, which hardly has been studied so far.

Calculating CD Spectra of Flexible Peptides: An Assessment of TD-DFT Functionals - Brkljaca, Zlatko; Malis, Momir; Smith, David M.; Smith, Ana-Suncana (Journal of Chemical Theory and Computation, 2014)
Calculations of CD spectra can provide important structural information for peptide systems. Although TD-DFT is an attractive method for these calculations, recent studies have pointed to problems with modeling charge transfer excitations. Motivated by these problems, we benchmarked the performances of CAM-B3LYP, wB97X-D, M06-2X, B3LYP, and PBE0 against high level ab initio RICC2 calculations for selected peptide structures. Furthermore, we compared the performance of the functionals with the experimentally available data. Our results show that long-range corrected functionals (CAM-B3LYP and wB97X-D) correlate relatively well with RICC2 calculations, as does the meta-hybrid M06-2X, while the global hybrid functionals (B3LYP and PBE0) exhibit the aforementioned charge-transfer artifacts. On the other hand, PBE0 and even more so M06-2X and B3LYP produce spectra in better agreement with the experimental data. We have clarified this apparent discrepancy by finding that the surplus charge-transfer excitations, exhibited by B3LYP and PBE0, seem to have a negligible contribution to the final spectra, once appropriate structural averaging is performed.
Keywords: CIRCULAR-DICHROISM SPECTRA; TRANSITION-METAL-COMPLEXES; HYBRID DENSITY FUNCTIONALS; MOLLER-PLESSET THEORY; IDENTITY APPROXIMATION; MOLECULAR-DYNAMICS; ABSORPTION-SPECTRA; AQUEOUS-SOLUTION; EXCITED-STATES; MODEL CC2

Calculation of the CD Spectrum of a Peptide from Its Conformational Phase Space: The Case of Met-enkephalin and Its Unnatural Analogue - Brkljaca, Zlatko; Condic-Jurkic, Karmen; Smith, Ana-Suncana; Smith, David M. (Journal of Chemical Theory and Computation, 2012)
We have investigated the conformational phase spaces of both Met-enkephalin and Ada-enkephalin in 2,2,2-trifluoroethanol in order to connect them to their respective CD spectra. To this end, we have characterized the conformational preferences of the zwitterionic and neutral forms of Met-enkephalin and of both the R- and the S-epimers of Ada-enkephalin, as obtained by classical molecular dynamics. The CD spectrum for each peptide was subsequently obtained with a procedure of successive averaging, which accounts for the behavior of the solvent, the side chains, and the backbone variations of the peptides. To make an appropriate comparison with experimental results, we have produced composite spectra that account for the appropriate contributions of the zwitterionic and neutral forms of the peptides as well as the expected epimeric ratio. Such a procedure results in theoretically obtained CD spectra that show significant promise in terms of reproducing their experimentally measured counterparts.
Keywords: DENSITY-FUNCTIONAL THEORY; ELECTRONIC CIRCULAR-DICHROISM; EXCHANGE MOLECULAR-DYNAMICS; AQUEOUS-SOLUTION; METHIONINE-ENKEPHALIN; ABSORPTION-SPECTRA; HELICAL PEPTIDES; EXCITED-STATES; FORCE-FIELD; AMINO-ACIDS

Canonical Analysis of Gravity Theories without the Time Gauge - Bodendorfer, Norbert; Thiemann, Thomas (Friedrich-Alexander-Universität Erlangen-Nürnberg, 2009)

Canonical path integral measures for Holst and Plebanski gravity: I. Reduced phase space derivation - Engle, Jonathan; Han, Muxin; Thiemann, Thomas (Classical and Quantum Gravity, 2010)
An important aspect in defining a path integral quantum theory is the determination of the correct measure. For interacting theories and theories with constraints, this is non-trivial, and is normally not the heuristic 'Lebesgue measure' usually used. There have been many determinations of a measure for gravity in the literature, but none for the Palatini or Holst formulations of gravity. Furthermore, the relations between different resulting measures for different formulations of gravity are usually not discussed. In this paper we use the reduced phase technique in order to derive the path-integral measure for the Palatini and Holst formulation of gravity, which is different from the Lebesgue measure up to local measure factors which depend on the spacetime volume element and spatial volume element. From this path integral for the Holst formulation of general relativity we can also give a new derivation of the Plebanski path integral and discover a discrepancy with the result due to Buffenoir, Henneaux, Noui and Roche whose origin we resolve. This paper is the first in a series that aims at better understanding the relation between canonical loop quantum gravity and the spin-foam approach.

CANONICAL QUANTIZATION OF A MINISUPERSPACE MODEL FOR GRAVITY USING SELF-DUAL VARIABLES - Thiemann, Thomas (1994)

CANONICAL QUANTIZATION OF SPHERICALLY SYMMETRICAL GRAVITY IN ASHTEKAR SELF-DUAL REPRESENTATION - Thiemann, Thomas; Kastrup, Hans (Nuclear Physics B, 1993)
We show that the quantization of spherically symmetric pure gravity can be carried out completely in the framework of Ashtekar's self-dual representation. Consistent operator orderings can be given for the constraint functionals yielding two kinds of solutions for the constraint equations, corresponding classically to globally nondegenerate or degenerate metrics. The physical state functionals can be determined by quadratures and the reduced hamiltonian system possesses two degrees of freedom, one of them corresponding to the classical Schwarzschild mass squared and the canonically conjugate one representing a measure for the deviation of the nonstatic field configurations from the static Schwarzschild one. There is a natural choice for the scalar product making the two fundamental observables self-adjoint. Finally, a unitary transformation is performed in order to calculate the triad-representation of the physical state functionals and to provide for a solution of the appropriately regularized Wheeler-DeWitt equation.

Canonical Quantum Gravity, Constructive QFT, and Renormalisation - Thiemann, Thomas (Frontiers in Physics, 2020)
The canonical approach to quantum gravity has been put on a firm mathematical foundation in the recent decades. Even the quantum dynamics can be rigorously defined, however, due to the tremendously non-polynomial character of the gravitational interaction, the corresponding Wheeler-DeWitt operator-valued distribution suffers from quantisation ambiguities that need to be fixed. In a very recent series of works, we have employed methods from the constructive quantum field theory in order to address those ambiguities. Constructive QFT trades quantum fields for random variables and measures, thereby phrasing the theory in the language of quantum statistical physics. The connection to the canonical formulation is made via Osterwalder-Schrader reconstruction. It is well known in quantum statistics that the corresponding ambiguities in measures can be fixed using renormalisation. The associated renormalisation flow can thus be used to define a canonical renormalisation programme. The purpose of this article was to review and further develop these ideas and to put them into context with closely related earlier and parallel programmes.
CRIS-Team WoS Importer:2021-01-29

Capillary assemblies in a rotating magnetic field - Grosjean, Galien; Hubert, Maxime; Collard, Ylona; Sukhov, Alexander; Harting, Jens; Smith, Ana-Suncana; Vandewalle, Nicolas (Soft Matter, 2019)
Small objects floating on a fluid have a tendency to aggregate due to capillary forces. This effect has been used, with the help of a magnetic induction field, to assemble submillimeter metallic spheres into a variety of structures, whose shape and size can be tuned. Under time-varying fields, these assemblies can propel themselves due to a breaking of time reversal symmetry in their adopted shapes. In this article, we study the influence of an in-plane rotation of the magnetic field on these structures. Various rotational modes have been observed with different underlying mechanisms. The magnetic properties of the particles cause them to rotate individually. Dipole-dipole interactions in the assembly can cause the whole structure to align with the field. Finally, non-reciprocal deformations can power the rotation of the assembly. Symmetry plays an important role in the dynamics, as well as the frequency and amplitude of the applied field. Understanding the interplay of these effects is essential, both to explain previous observations and to develop new functions for these assemblies.
CRIS-Team Scopus Importer:2019-11-22

Capillary waves of compressible fluids - Falk, Kerstin; Mecke, Klaus (Journal of Physics: Condensed Matter, 2011)
The interplay of thermal noise and molecular forces is responsible for surprising features of liquids on sub-micrometer lengths-in particular at interfaces. Not only does the surface tension depend on the size of an applied distortion and nanoscopic thin liquid films dewet faster than would be expected from hydrodynamics, but also the dispersion relation of capillary waves differ at the nanoscale from the familiar macroscopic behavior. Starting with the stochastic Navier-Stokes equation we study the coupling of capillary waves to acoustic surface waves which is possible in compressible fluids. We find propagating 'acoustic-capillary waves' at nanometer wavelengths where in incompressible fluids capillary waves are overdamped.

Cartan Geometries and Spin Network Quantisation - Bärenz, Manuel; Thiemann, Thomas (2011)

Catalytic flow with a coupled finite difference — Lattice Boltzmann scheme - Kulyk, Nadiia; Berger, Daniel; Smith, Ana-Suncana; Harting, Jens (Computer Physics Communications, 2020)
Many catalyst devices employ flow through porous structures, which leads to a complex macroscopic mass and heat transport. To unravel the detailed dynamics of the reactive gas flow, we present an all-encompassing model, consisting of thermal lattice Boltzmann model by Kang et al., used to solve the heat and mass transport in the gas domain, coupled to a finite differences solver for the heat equation in the solid via thermal reactive boundary conditions for a consistent treatment of the reaction enthalpy. The chemical surface reactions are incorporated in a flexible fashion through flux boundary conditions at the gas–solid interface. We scrutinize the thermal FD-LBM by benchmarking the macroscopic transport in the gas domain as well as conservation of the enthalpy across the solid–gas interface. We exemplify the applicability of our model by simulating the reactive gas flow through a microporous material catalyzing the so-called water-gas-shift reaction.
CRIS-Team Scopus Importer:2020-07-03
Keywords: Catalytic flow; Conjugated heat transfer; Reaction enthalpy; Thermal lattice Boltzmann method

Cavity-mediated thermal control of metal-to-insulator transition in 1T-TaS2 - Jarc, Giacomo; Mathengattil, Shahla Yasmin; Montanaro, Angela; Giusti, Francesca; Rigoni, Enrico Maria; Sergo, Rudi; Fassioli Olsen, Francesca; Winnerl, Stephan; Dal Zilio, Simone; Mihailovic, Dragan; Prelovšek, Peter; Eckstein, Martin; et al. (Nature, 2023)
Placing quantum materials into optical cavities provides a unique platform for controlling quantum cooperative properties of matter, by both weak and strong light–matter coupling1,2. Here we report experimental evidence of reversible cavity control of a metal-to-insulator phase transition in a correlated solid-state material. We embed the charge density wave material 1T-TaS2 into cryogenic tunable terahertz cavities3 and show that a switch between conductive and insulating behaviours, associated with a large change in the sample temperature, is obtained by mechanically tuning the distance between the cavity mirrors and their alignment. The large thermal modification observed is indicative of a Purcell-like scenario in which the spectral profile of the cavity modifies the energy exchange between the material and the external electromagnetic field. Our findings provide opportunities for controlling the thermodynamics and macroscopic transport properties of quantum materials by engineering their electromagnetic environment.
CRIS-Team Scopus Importer:2023-10-27

Cavity optomagnonics - Viola Kusminskiy, Silvia (Optomagnonic Structures: Novel Architectures For Simultaneous Control Of Light And Spin Waves, 2021)
In the recent years a series of experimental and theoretical efforts have centered around a new topic: the coherent, cavity-enhanced interaction between optical photons and solid state magnons. The resulting emerging field of Cavity Optomagnonics is of interest both at a fundamental level, providing a new platform to study light-matter interaction in confined structures, as well as for its possible relevance for hybrid quantum technologies. In this chapter, I introduce the basic concepts of Cavity Optomagnonics and review some theoretical developments.
CRIS-Team Scopus Importer:2021-07-16

Cavity optomechanics in a levitated helium drop - Childress, Lilian; Schmidt, M. P.; Kashkanova, A. D.; Brown, C. D.; Harris, G. I.; Aiello, Andrea; Marquardt, Florian; Harris, J. G. E. (Physical Review A, 2017)
We describe a proposal for a type of optomechanical system based on a drop of liquid helium that is magnetically levitated in vacuum. In the proposed device, the drop would serve three roles: its optical whispering-gallery modes would provide the optical cavity, its surface vibrations would constitute the mechanical element, and evaporation of He atoms from its surface would provide continuous refrigeration. We analyze the feasibility of such a system in light of previous experimental demonstrations of its essential components: magnetic levitation of mm-scale and cm-scale drops of liquid He, evaporative cooling of He droplets in vacuum, and coupling to high-quality optical whispering-gallery modes in a wide range of liquids. We find that the combination of these features could result in a device that approaches the single-photon strong-coupling regime, due to the high optical quality factors attainable at low temperatures. Moreover, the system offers a unique opportunity to use optical techniques to study the motion of a superfluid that is freely levitating in vacuum (in the case of He4). Alternatively, for a normal fluid drop of He3, we propose to exploit the coupling between the drop's rotations and vibrations to perform quantum nondemolition measurements of angular momentum.
CRIS-Team Scopus Importer:2023-03-27

Characterisation of irregular spatial structures by parallel sets and integral geometric measures - Mecke, Klaus; et al. (ELSEVIER SCIENCE BV, 2004)
A new method of characterising the morphology of disordered systems is presented based on the evolution of a family of integral geometric measures during erosion and dilation operations. The method is used to determine the accuracy of model reconstructions of random systems. It is shown that the use of erosion/dilation operations on the original image leads to an accurate discrimination of morphology. We consider the morphology of an experimental system and use the method to optimally match a reconstructed model morphology. (C) 2004 Elsevier B.V. All rights reserved.
Keywords: porous media; morphology; Euler number; Minkowski functional

Characterization of anisotropic Gaussian random fields by Minkowski tensors - Klatt, Michael; Hörmann, Max; Mecke, Klaus (Journal of Statistical Mechanics-Theory and Experiment, 2022)
Gaussian random fields are among the most important models of amorphous spatial structures and appear across length scales in a variety of physical, biological, and geological applications, from composite materials to geospatial data. Anisotropy in such systems can be sensitively and comprehensively characterized by the so-called Minkowski tensors (MTs) from integral geometry. Here, we analytically calculate expected MTs of arbitrary rank for the level sets of Gaussian random fields. The explicit expressions for interfacial MTs are confirmed in detailed simulations. We demonstrate how the MTs detect and characterize the anisotropy of the level sets, and we clarify which shape information is contained in the MTs of different rank. Using an irreducible representation of the MTs in the Euclidean plane, we show that higher-rank tensors indeed contain additional anisotropy information compared to a rank two tensor. Surprisingly, we can nevertheless predict this information from the second-rank tensor if we assume that the random field is Gaussian. This relation between tensors of different rank is independent of the details of the model. It is, therefore, useful for a null hypothesis test that detects non-Gaussianities in anisotropic random fields.
CRIS-Team WoS Importer:2022-05-06

Characterization of the dynamics of block copolymer microdomains with local morphological measures - Rehse, S.; Mecke, Klaus; Magerle, R. (Physical Review E, 2008)
We investigate the structure formation in thin films of cylinder forming block copolymers. With in situ scanning probe microscopy image sequences can be recorded with high temporal (2 min per frame) and spatial (10 nm) resolution. We compare different image processing methods for quantitative analysis of the large amount of data. Computing local Minkowski functionals yields local geometrical and morphological information about the observed structures and enables us to track their evolution with time. An alternative characterization method is to reduce the gray scale images to their skeleton and to classify and count the branching points of the skeletonized structure. We tracked the temporal evolution of these measures and computed correlation functions.

Charge-carrier transport in large-area epitaxial graphene - Kißlinger, Ferdinand; Popp, Matthias Albert; Jobst, Johannes; Shallcross, Sam; Weber, Heiko B. (Annalen Der Physik, 2017)

Charge-order-induced sharp Raman peak in Sr14Cu24O41 - Schmidt, Kai Phillip; et al. (Physical Review Letters, 2003)
In the two-leg S=1/2 ladders of Sr14Cu24O41, a modulation of the exchange coupling arises from the charge order within the other structural element, the CuO2 chains. In general, breaking translational invariance by modulation causes gaps within the dispersion of elementary excitations. We show that the gap induced by the charge order can drastically change the magnetic Raman spectrum, leading to the sharp peak observed in Sr14Cu24O41. This sharp Raman line gives insight into the charge-order periodicity and hence into the distribution of carriers. The much broader spectrum of La6Ca8Cu24O41 reflects the response of an undoped ladder in the absence of charge order.

Charge-transfer energy in iridates: A hard x-ray photoelectron spectroscopy study - Takegami, D.; Kasinathan, D.; Wolff, K. K.; Altendorf, S. G.; Chang, C. F.; Hoefer, K.; Melendez-Sans, A.; Utsumi, Y.; Meneghin, F.; Ha, T. D.; Yen, C. H.; Chen, K.; et al. (Physical Review B, 2020)
We have investigated the electronic structure of iridates in the double perovskite crystal structure containing either Ir4+ or Ir5+ using hard x-ray photoelectron spectroscopy. The experimental valence band spectra can be well reproduced using tight-binding calculations including only the Ir 5d, O 2p, and O 2s orbitals with parameters based on the downfolding of the density-functional band structure results. We found that, regardless of the A and B cations, the A(2)BIrO(6) iridates have essentially zero O 2p to Ir 5d charge-transfer energies. Hence double perovskite iridates turn out to be extremely covalent systems with the consequence being that the magnetic exchange interactions become very long ranged, thereby hampering the materialization of the long-sought Kitaev physics. Nevertheless, it still would be possible to realize a spin-liquid system using the iridates with a proper tuning of the various competing exchange interactions.
CRIS-Team WoS Importer:2020-07-24

Charge Transport in C60-based Single-Molecule Junctions with Graphene Electrodes - Leitherer, Susanne; Brana Coto, Pedro; Ullmann, Konrad; Weber, Heiko B.; Thoss, Michael (Nanoscale, 2017)
We investigate charge transport in C60-based single-molecule junctions with graphene electrodes employing a combination of density functional theory (DFT) electronic structure calculations and Landauer transport theory. In particular, the dependence of the transport properties on the conformation of the molecular bridge and the type of termination of the graphene electrodes is investigated. Furthermore, electron pathways through the junctions are analyzed using the theory of local currents. The results reveal, in agreement with previous experiments, a pronounced dependence of the transport properties on the bias polarity, which is rationalized in terms of the electronic structure of the molecule. It is also shown that the edge states of zigzag-terminated graphene induce additional transport channels, which dominate transport at low voltages. The importance of the edge states for transport depends profoundly on the interface geometry of the junctions.

Charge Transport in Pentacene–Graphene Nanojunctions - Pshenichnyuk, Ivan; Brana Coto, Pedro; Leitherer, Susanne; Thoss, Michael (Journal of Physical Chemistry Letters, 2013)
We investigate charge transport in pentacene-graphene nanojunctions employing density functional theory (DFT) electronic structure calculations and the Landauer transport formalism. The results show that the unique electronic properties of graphene strongly influence the transport in the nanojunctions. In particular, edge states in graphene electrodes with zigzag termination result in additional transport channels close to the Fermi energy, which deeply affects the conductance at small bias voltages. Investigating different linker groups as well as chemical substitution, we demonstrate how the transport properties are furthermore influenced by the molecule-lead coupling and the energy level lineup. © 2013 American Chemical Society.
UnivIS-Import:2015-04-14:Pub.2013.nat.dphy.ITP.ppem.charge
Keywords: Energy and Charge Transport; Energy Conversion and Storage

Chern-simons expectation values and quantum horizons from loop quantum gravity and the duflo map - Sahlmann, Hanno; Thiemann, Thomas (Physical Review Letters, 2012)
We report on a new approach to the calculation of Chern-Simons theory expectation values, using the mathematical underpinnings of loop quantum gravity, as well as the Duflo map, a quantization map for functions on Liealgebras. These new developments can be used in the quantum theory for certain types of black hole horizons, and they may offer new insights for loop quantum gravity, Chern-Simons theory and the theory of quantum groups. © 2012 American Physical Society.

Chern-Simons theory, Stokes' theorem, and the Duflo map - Sahlmann, Hanno; Thiemann, Thomas (Journal of Geometry and Physics, 2011)
We consider a novel derivation of the expectation values of holonomies in Chern-Simons theory, based on Stokes' Theorem and the functional properties of the Chern-Simons action. It involves replacing the connection by certain functional derivatives under the path integral. It turns out that ordering choices have to be made in the process, and we demonstrate that, quite surprisingly, the Duflo isomorphism gives the right ordering, at least in the simple cases that we consider. In this way, we determine the expectation values of unknotted, but possibly linked, holonomy loops for SU(2) and SU(3), and sketch how the method may be applied to more complicated cases. Our manipulations of the path integral are formal but well motivated by a rigorous calculus of integration on spaces of generalized connections which has been developed in the context of loop quantum gravity. © 2011 Elsevier B.V.
Keywords: Chern-Simons theory; Duflo map; Loop quantum gravity

Chiral d-wave superconductivity in a triangular surface lattice mediated by long-range interaction - Cao, Xiaodong; Ayral, Thomas; Zhong, Zhicheng; Parcollet, Olivier; Manske, Dirk; Hansmann, Philipp (Physical Review B, 2018)
Adatom systems on the Si(111) surface have recently attracted an increasing attention as strongly correlated systems with a rich phase diagram. We study these materials by a single band model on the triangular lattice, including 1/r long-range interaction. Employing the recently proposed TRILEX method, we find an unconventional superconducting phase of chiral d-wave symmetry in hole-doped systems. Contrary to usual scenarios where charge and spin fluctuations are seen to compete, here the superconductivity is driven simultaneously by both charge and spin fluctuations and crucially relies on the presence of the long-range tail of the interaction. We provide an analysis of the relevant collective bosonic modes and predict how a cumulative charge and spin paring mechanism leads to superconductivity in doped silicon adatom materials.
CRIS-Team WoS Importer:2020-02-19

Chiral Photonic Crystals with 4-fold Symmetry: Band Structure and S-Parameters of Eight-Fold Intergrown Gyroid Nets - Saba, Matthias; Turner, Mark D.; Gu, Min; Mecke, Klaus; Schröder-Turk, Gerd (Proceedings of SPIE, 2013)
The Single Gyroid, or srs, nanostructure has attracted interest as a circular-polarisation sensitive photonic material. We develop a group theoretical and scattering matrix method, applicable to any photonic crystal with symmetry I432, to demonstrate the remarkable chiral-optical properties of a generalised structure called 8-srs, obtained by intergrowth of eight equal-handed srs nets. Exploiting the presence of four-fold rotations, Bloch modes corresponding to the irreducible representations E- and E+ are identified as the sole and non-interacting transmission channels for right- and left-circularly polarised light, respectively. For plane waves incident on a finite slab of the 8-srs, the reflection rates for both circular polarisations are identical for all frequencies and transmission rates are identical up to a critical frequency below which scattering in the far field is restricted to zero grating order. Simulations show the optical activity of the lossless dielectric 8-srs to be large, comparable to metallic metamaterials, demonstrating its potential as a nanofabricated photonic material.
EAM Import::2019-03-13

Circular Dichroism in Biological Photonic Crystals and Cubic Chiral Nets - Saba, Matthias; Thiel, M.; Turner, M. D.; Hyde, S. T.; Gu, Min; Grosse-Brauckmann, K.; Neshev, D. N.; Mecke, Klaus; Schröder-Turk, Gerd (Physical Review Letters, 2011)
Nature provides impressive examples of chiral photonic crystals, with the notable example of the cubic so-called srs network (the label for the chiral degree-three network modeled on SrSi2) or gyroid structure realized in wing scales of several butterfly species. By a circular polarization analysis of the band structure of such networks, we demonstrate strong circular dichroism effects: The butterfly srs microstructure, of cubic I4(1)32 symmetry, shows significant circular dichroism for blue to ultraviolet light, that warrants a search for biological receptors sensitive to circular polarization. A derived synthetic structure based on four like-handed silicon srs nets exhibits a large circular polarization stop band of a width exceeding 30%. These findings offer design principles for chiral photonic devices.

Classical phase synchronization in dissipative non-Hermitian coupled systems - Rohn, Jonas; Schmidt, Kai Phillip; Genes, Claudiu (Physical Review A, 2023)
We study the interplay between non-Hermitian dynamics and classical phase synchronization in a system of N bosonic modes commonly coupled to an auxiliary, driven mode. For any set of non-Hermitian bipartite interactions between the auxiliary and other modes, the system evolves towards a phase synchronized state. We provide analytical and numerical evidence of such classical phase synchronization for systems ranging from a few modes to the macroscopic limit of large N and analyze the effects of inhomogeneous frequency broadening and robustness under the action of external thermal noise.
CRIS-Team Scopus Importer:2023-09-08

Closed formula for the matrix elements of the volume operator in canonical quantum gravity - Thiemann, Thomas (Journal of Mathematical Physics, 1998)
We derive a closed formula for the matrix elements of the volume operator for canonical Lorentzian quantum gravity in four space-time dimensions in the continuum in a spin-network basis. We also display a new technique of regularization which is state dependent but we are forced to it in order to maintain diffeomorphism covariance and in that sense it is natural. We arrive naturally at the expression for the volume operator as defined by Ashtekar and Lewandowski up to a state-independent factor. (C) 1998 American Institute of Physics.

Coexistence of both gyroid chiralities in individual butterfly wing scales of Callophrys rubi - Winter, Benjamin; Butz, Benjamin; Dieker, Christel; Schroeder-Turk, Gerd E.; Mecke, Klaus; Spiecker, Erdmann (Proceedings of the National Academy of Sciences of the United States of America, 2015)
The wing scales of the Green Hairstreak butterfly Callophrys rubi consist of crystalline domains with sizes of a few micrometers, which exhibit a congenitally handed porous chitin microstructure identified as the chiral triply periodic single-gyroid structure. Here, the chirality and crystallographic texture of these domains are investigated by means of electron tomography. The tomograms unambiguously reveal the coexistence of the two enantiomeric forms of opposite handedness: the left- and right-handed gyroids. These two enantiomers appear with nonequal probabilities, implying that molecularly chiral constituents of the biological formation process presumably invoke a chiral symmetry break, resulting in a preferred enantiomeric form of the gyroid structure. Assuming validity of the formation model proposed by Ghiradella H (1989) J Morphol 202(1): 69-88 and Saranathan V, et al. (2010) Proc Natl Acad Sci USA 107(26): 11676-11681, where the two enantiomeric labyrinthine domains of the gyroid are connected to the extracellular and intra-SER spaces, our findings imply that the structural chirality of the single gyroid is, however, not caused by the molecular chirality of chitin. Furthermore, the wing scales are found to be highly textured, with a substantial fraction of domains exhibiting the < 001 > directions of the gyroid crystal aligned parallel to the scale surface normal. Both findings are needed to completely understand the photonic purpose of the single gyroid in gyroid-forming butterflies. More importantly, they show the level of control that morphogenesis exerts over secondary features of biological nanostructures, such as chirality or crystallographic texture, providing inspiration for biomimetic replication strategies for synthetic self-assembly mechanisms.
Keywords: electron tomography; chirality; crystallographic texture; photonic crystal; butterfly wing scales

Coexistence of dilute and densely packed domains of ligand-receptor bonds in membrane adhesion - Schmidt, Daniel; Bihr, Timo; Seifert, Udo; Smith, Ana-Suncana (EPL - Europhysics Letters, 2012)
We analyze the stability of micro-domains of ligand-receptor bonds that mediate the adhesion of biological model membranes. After evaluating the effects of membrane fluctuations on the binding affinity of a single bond, we characterize the organization of bonds within the domains by theoretical means. In a large range of parameters, we find the commonly suggested dense packing to be separated by a free energy barrier from a regime in which bonds are sparsely distributed. If bonds are mobile, a coexistence of the two regimes should emerge, which agrees with recent experimental observations. Copyright (C) EPLA, 2012
Keywords: CELL-ADHESION; FLUCTUATIONS; JUNCTIONS; VESICLES

Coherent states for canonical quantum general relativity and the infinite tensor product extension - Sahlmann, Hanno; Thiemann, Thomas; Winkler, Oliver (Nuclear Physics B, 2001)
We summarize a recently proposed concrete programme for investigating the (semi)classical limit of canonical, Lorentzian, continuum quantum general relativity in four spacetime dimensions. The analysis is based on a novel set of coherent states labelled by graphs. These fit neatly together with an Infinite Tensor Product (ITP) extension of the currently used Hilbert space. The ITP construction enables us to give rigorous meaning to the infinite volume (thermodynamic) limit of the theory which has been out of reach so far. (C) 2001 Elsevier Science B.V. All rights reserved.

Coherent states for fractional powers of the harmonic oscillator Hamiltonian - Giesel, Kristina; Vetter, Almut (Universe, 2021)
Inspired by special and general relativistic systems that can have Hamiltonians involving square roots or more general fractional powers, in this article, we address the question of how a suitable set of coherent states for such systems can be obtained. This becomes a relevant topic if the semiclassical sector of a given quantum theory is to be analysed. As a simple setup, we consider the toy model of a deparametrised system with one constraint that involves a fractional power of the harmonic oscillator Hamiltonian operator, and we discuss two approaches to finding suitable coherent states for this system. In the first approach, we consider Dirac quantisation and group averaging, as have been used by Ashtekar et al., but only for integer powers of operators. Our generalisation to fractional powers yields in the case of the toy model a suitable set of coherent states. The second approach is inspired by coherent states based on a fractional Poisson distribution introduced by Laskin, which however turn out not to satisfy all properties to yield good semiclassical results for the operators considered here and in particular do not satisfy a resolution of identity as claimed. Therefore, we present a generalisation of the standard harmonic oscillator coherent states to states involving fractional labels, which approximate the fractional operators in our toy model semiclassically more accurately and satisfy a resolution of identity. In addition, motivated by the way the proof of the resolution of identity is performed, we consider these kind of coherent states also for the polymerised harmonic oscillator and discuss their semiclassical properties.
Keywords: coherent states; constrained systems; operators involving fractional powers; semiclassical analysis

Coherent states on the circle: semiclassical matrix elements in the context of Kummer functions and the Zak transformation - Giesel, Kristina; Winnekens, David (Physica Scripta, 2023)
We extend former results for coherent states on the circle in the literature in two ways. On the one hand, we show that expectation values of fractional powers of momentum operators can be computed exactly analytically by means of Kummer’s confluent hypergeometric functions. Earlier, these expectation values have only been obtained by using suitable estimates. On the other hand, we consider the Zak transformation not only to map harmonic oscillator coherent states to coherent states on the circle as it has been discussed before, but we also use the properties of the Zak transformation to derive a relation between matrix elements with respect to coherent states in L 2 ( R ) and L 2(S 1). This provides an alternative way for computing semiclassical matrix elements for coherent states on the circle. In certain aspects, this method simplifies the semiclassical computations in particular if one is only interested in the classical limit, that is the zeroth order term in the semiclassical expansion.
CRIS-Team Scopus Importer:2023-08-25
Keywords: coherent states; Kummer functions; semiclassical analysis; Zak transformation

Coherent states, quantum gravity, and the Born-Oppenheimer approximation. I. General considerations - Stottmeister, Alexander; Thiemann, Thomas (Journal of Mathematical Physics, 2016)
This article, as the first of three, aims at establishing the (time-dependent) Born-Oppenheimer approximation, in the sense of space adiabatic perturbation theory, for quantum systems constructed by techniques of the loop quantum gravity framework, especially the canonical formulation of the latter. The analysis presented here fits into a rather general framework and offers a solution to the problem of applying the usual Born-Oppenheimer ansatz for molecular (or structurally analogous) systems to more general quantum systems (e.g., spin-orbit models) by means of space adiabatic perturbation theory. The proposed solution is applied to a simple, finite dimensional model of interacting spin systems, which serves as a non-trivial, minimal model of the aforesaid problem. Furthermore, it is explained how the content of this article and its companion affect the possible extraction of quantum field theory on curved spacetime from loop quantum gravity (including matter fields). Published by AIP Publishing.

Coherent states, quantum gravity, and the Born-Oppenheimer approximation. II. Compact Lie groups - Stottmeister, Alexander; Thiemann, Thomas (Journal of Mathematical Physics, 2016)
In this article, the second of three, we discuss and develop the basis of a Weyl quantisation for compact Lie groups aiming at loop quantum gravity-type models. This Weyl quantisation may serve as the main mathematical tool to implement the program of space adiabatic perturbation theory in such models. As we already argued in our first article, space adiabatic perturbation theory offers an ideal framework to overcome the obstacles that hinder the direct implementation of the conventional Born-Oppenheimer approach in the canonical formulation of loop quantum gravity. Additionally, we conjecture the existence of a new form of the Segal-Bargmann-Hall "coherent state" transform for compact Lie groups G, which we prove for G = U(1)(n) and support by numerical evidence for G = SU(2). The reason for conjoining this conjecture with the main topic of this article originates in the observation that the coherent state transform can be used as a basic building block of a coherent state quantisation (Berezin quantisation) for compact Lie groups G. But, as Weyl and Berezin quantisation for R-2d are intimately related by heat kernel evolution, it is natural to ask whether a similar connection exists for compact Lie groups as well. Moreover, since the formulation of space adiabatic perturbation theory requires a (deformation) quantisation as minimal input, we analyse the question to what extent the coherent state quantisation, defined by the Segal-Bargmann-Hall transform, can serve as basis of the former. Published by AIP Publishing.

Coherent states, quantum gravity, and the Born-Oppenheimer approximation. III.: Applications to loop quantum gravity - Stottmeister, Alexander; Thiemann, Thomas (Journal of Mathematical Physics, 2016)
In this article, the third of three, we analyse how the Weyl quantisation for compact Lie groups presented in the second article of this series fits with the projective-phase space structure of loop quantum gravity-type models. Thus, the proposed Weyl quantisation may serve as the main mathematical tool to implement the program of space adiabatic perturbation theory in such models. As we already argued in our first article, space adiabatic perturbation theory offers an ideal framework to overcome the obstacles that hinder the direct implementation of the conventional Born-Oppenheimer approach in the canonical formulation of loop quantum gravity. Published by AIP

Coherent state transforms for spaces of connections - Ashtekar, Abhay; Lewandowski, Jerzy; Marolf, Donald; Mourao, José Manuel; Thiemann, Thomas (Journal of Functional Analysis, 1996)
The Segal-Bargmann transform plays an important role in quantum theories of linear fields. Recently, Hall obtained a non-linear analog of this transform for quantum mechanics on Lie groups. Given a compact, connected Lie group G with its normalized Haar measure mu(H), the Hall transform is an isometric isomorphism hem L(2)(G, mu(H)) to H(G(C)) boolean AND L(2)(G(C), v), where G(C) the complexification of G, H(G(C)) the space of holomorphic functions on G(C), and v an appropriate heat-kernel measure on G(C). We extend the Hall transform to the infinite dimensional context of non-Abelian gauge theories by replacing the Lie group G by (a certain extension of) the space A/g of connections module gauge transformations. The resulting ''coherent state transform'' provides a holomorphic representation of the holonomy C* algebra of real gauge fields. This representation is expected to play a key role in a non-perturbative, canonical approach to quantum gravity in 4 dimensions. (C) 1996 Academic Press, Inc.

Collective excitations of the U (1)-symmetric exciton insulator in a cavity - Lenk, Katharina; Eckstein, Martin (Physical Review B, 2020)
We investigate the equilibrium state and the collective modes of an excitonic insulator (EI) in a Fabry-Pérot cavity. In an EI, two bands of a semiconductor or semimetal spontaneously hybridize due to the Coulomb interaction between electrons and holes, leading to the opening of a gap. The coupling to the electromagnetic field reduces the symmetry of the system with respect to phase rotations of the excitonic order parameter from U(1) to Z2. While the reduction to a discrete symmetry would, in general, lead to a gapped phase mode and enhance the stability of the ordered phase, the coupling to the cavity leaves the mean-field ground state unaffected. Its energy remains invariant under U(1) phase rotations, in spite of the lower Z2 symmetry imposed by the cavity. In a dipolar gauge, this can be traced back to the balancing of the linear light-matter coupling and the dipolar self-interaction at zero frequency. At nonzero frequency, however, the collective excitations do reflect the lower Z2 symmetry. While our model is studied using a mean-field decoupling of the light-matter approximation (which is exact in the single-mode limit), the results show that fluctuations beyond mean field could play a crucial role in finding the true phase at finite temperature.
CRIS-Team Scopus Importer:2020-12-11

Colloidal Ordering on a 2D Quasicrystalline Substrate - Schmiedeberg, Michael; Stark, Holger (Physical Review Letters, 2008)
By using Monte Carlo simulations, we study the complex phase behavior of charged-stabilized colloidal particles in a two-dimensional substrate potential with quasicrystalline decagonal symmetry. In the regime where the strengths of the substrate and colloidal pair potential are comparable, we identify a novel and unexpected quasicrystalline phase with pure 20-fold bond order and a disordered structure without any apparent rotational symmetry. Furthermore, we demonstrate how phasonic displacements in the substrate potential induce phasonic flips in the colloidal monolayer.

Colloidal Trajectories in Two-Dimensional Light-Induced Quasicrystals with 14-Fold Symmetry due to Phasonic Drifts - Martinsons, Miriam; Sandbrink, Matthias; Schmiedeberg, Michael (POLISH ACAD SCIENCES INST PHYSICS, 2014)
Quasicrystals are structures that are not periodic but possess a long range positional order. They can have any rotational symmetry including those that are forbidden in periodic crystals. The symmetry is classified by the point group and the rank D. In quasicrystals, phasons as additional hydrodynamic modes cause correlated rearrangements of the particles. The number of phasonic degrees of freedom depends on the rank. For colloidal quasicrystals that are induced by laser fields, specific phasonic displacements can be realized by changing the phases of the laser beams in a well-determined way. The arising trajectories of colloids in two-dimensional light-induced colloidal quasicrystals with rank D = 4 have already been analyzed in detail. Here, we analyze the colloidal trajectories in two-dimensional quasicrystals with 14-fold symmetry having rank D = 6. We observe complex paths of the colloids consisting of straight and winding lines as well as jumps.

Colloids in one dimensional random energy landscapes - Hanes, Richard; Dalle-Ferrier, Cecile; Schmiedeberg, Michael; Jenkins, Mathew; Egelhaaf, Stefan (Soft Matter, 2012)
Individual colloidal particles have been studied experimentally in a one dimensional random potential with energies that follow a Gaussian distribution. This rough, noise-like potential has been realised using a holographic optical set-up, which allows the width of the distribution to be varied. For different widths, the particle trajectories were followed and the particle dynamics characterised by, for example, the mean square displacement, non-Gaussian parameter, van Hove function, time-dependent diffusion coefficient and residence time distribution. The values obtained for these observables are consistent with the static properties of the system, in particular the barrier height distribution, which was obtained by a detailed characterisation of the tweezer-like set-up. The dynamics display three distinct behaviours: at short times normal diffusion, subsequently an extended regime of localisation within the different minima of the potential and finally a very slow approach towards long-time diffusive behaviour, for which diffusion coefficients consistent with theoretical predictions have been found.

Comment on "Communication: Is directed percolation in colloid-polymer mixtures linked to dynamic arrest?' - Schmiedeberg, Michael (Journal of Chemical Physics, 2022)
In their Communication [J. Chem. Phys. 148, 241101 (2018)], Richard et al. state that in the work of Kohl et al. [Nat. Commun. 7, 11817 (2016)], a mechanism for dynamical arrest in temporal networks has been proposed that actually has never been proposed (and would be obviously wrong) in this context. The actual findings of Kohl et al. are not tested nor affected by the communication. The work of Richard et al. rests on simulations in a regime of the phase diagram that significantly differs from the one that Kohl et al. consider. In this Comment, it is shown that both the effective density and the rescaled second virial coefficient indicate that the comparison presented by Richard et al. is invalid. Therefore, the implications that are based on this comparison are incorrect. There is no indication for a disagreement between the simulations of Richard et al. and those of Kohl et al., and I am confident that upon consistent comparison and interpretation of the results, both works can contribute to a more comprehensive picture of gel-forming system
Keywords: gel; percolation; network

Comment on "Quantum Quasicrystals of Spin-Orbit-Coupled Dipolar Bosons" - Sandbrink, Matthias; Roth, Johannes; Schmiedeberg, Michael (Physical Review Letters, 2014)

Communication: Fundamental measure theory for hard disks: Fluid and solid - Roth, Roland; Mecke, Klaus; Oettel, Martin (Journal of Chemical Physics, 2012)
Two-dimensional hard-particle systems are rather easy to simulate but surprisingly difficult to treat by theory. Despite their importance from both theoretical and experimental points of view, theoretical approaches are usually qualitative or at best semi-quantitative. Here, we present a density functional theory based on the ideas of fundamental measure theory for two-dimensional hard-disk mixtures, which allows for the first time an accurate description of the structure of the dense fluid and the equation of state for the solid phase within the framework of density functional theory. The properties of the solid phase are obtained by freely minimizing the functional. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3687921]
Keywords: density functional theory; equations of state

Communication: Mode-selective vibrational excitation induced by nonequilibrium transport processes in single-molecule junctions - Härtle, Rainer; Volkovich, Roie; Thoss, Michael; Peskin, Uri (Journal of Chemical Physics, 2010)
In a nanoscale molecular junction at finite bias voltage, the intramolecular distribution of vibrational energy can strongly deviate from the thermal equilibrium distribution and specific vibrational modes can be selectively excited in a controllable way, regardless of the corresponding mode frequency. This is demonstrated for generic models of asymmetric molecular junctions with localized electronic states, employing a master equation as well as a nonequilibrium Green's function approach. It is shown that the applied bias voltage controls the excitation of specific vibrational modes by tuning the efficiency of vibrational cooling processes due to energy exchange with the leads. © 2010 American Institute of Physics.
UnivIS-Import:2015-03-09:Pub.2010.nat.dphy.ITP.ppem.commun

Commuting simplicity and closure constraints for 4D spin-foam models - Han, Muxin; Thiemann, Thomas (Classical and Quantum Gravity, 2013)
Spin-foam models are supposed to be discretized path integrals for quantum gravity constructed from the Plebanski-Holst action. The reason for there being several models currently under consideration is that no consensus has been reached for how to implement the simplicity constraints. Indeed, none of these models strictly follows from the original path integral with commuting B fields, rather, by some nonstandardmanipulations one always ends up with non-commuting B fields and the simplicity constraints become in fact anomalous which is the source for there being several inequivalent strategies to circumvent the associated problems. In this paper, we construct a new Euclidian spin-foam model which is constructed by standard methods from the Plebanski-Holst path integral with commuting B fields discretized on a 4D simplicial complex. The resulting model differs from the current ones in several aspects, one of them being that the closure constraint needs special care. Only when dropping the closure constraint by hand and only in the large spin limit can the vertex amplitudes of this model be related to those of the FK gamma. model but even then the face and edge amplitude differ. Interestingly, a non-commutative deformation of the B-IJ variables leads from our new model to the Barrett-Crane model in the case of gamma =infinity.

Comparing light-induced colloidal quasicrystals with different rotational symmetries - Schmiedeberg, Michael; Stark, Holger (Journal of Physics: Condensed Matter, 2012)
Quasicrystals are aperiodic structures with long-range orientational order. Unlike crystals, quasicrystals can, in principle, possess any non-crystallographic rotational symmetry. However, only a few of these rotational symmetries have been observed. By using Monte Carlo simulations of colloidal particles in laser interference patterns with quasicrystalline symmetry, we compare the onset of quasicrystalline order for different rotational symmetries in two dimensions. We find that quasicrystals with 5-, 8-, 10-, and 12-fold rotational axes can be induced with lower laser intensities than quasicrystals with other non-crystallographic rotational symmetries. We relate this finding to the number of local symmetry centers in the respective interference patterns.

Comparing the generalized Kadanoff-Baym ansatz with the full Kadanoff-Baym equations for an excitonic insulator out of equilibrium - Tuovinen, Riku; Golez, Denis; Eckstein, Martin; Sentef, Michael A. (Physical Review B, 2020)
We investigate out-of-equilibrium dynamics in an excitonic insulator (EI) with a finite-momentum pairing perturbed by a laser-pulse excitation and a sudden coupling to fermionic baths. The transient dynamics of the excitonic order parameter is resolved using the full nonequilibrium Green's function approach and the generalized Kadanoff-Baym ansatz (GKBA) within the second-order Born approximation. The comparison between the two approaches after a laser-pulse excitation shows a good agreement in the weak and the intermediate photodoping regime. In contrast, the laser-pulse dynamics resolved by the GKBA does not show a complete melting of the excitonic order after a strong excitation. Instead we observe persistent oscillations of the excitonic order parameter with a predominant frequency given by the renormalized equilibrium band gap. This anomalous behavior can be overcome within the GKBA formalism by coupling to an external bath, which leads to a transition of the EI system toward the normal state. We analyze the long-time evolution of the system and distinguish decay timescales related to dephasing and thermalization.
CRIS-Team WoS Importer:2020-10-16

Comparison Between Dirac and Reduced Quantization in LQG-Models with Klein-Gordon Scalar Fields - Giesel, Kristina; Oelmann, Almut (Acta Physica Polonica B, 2017)

Compatibility waves drive crystal growth on patterned substrates - Neuhaus, Tim; Schmiedeberg, Michael; Löwen, Hartmut (New Journal of Physics, 2013)
We explore the crystallization in a colloidal monolayer on a structured template starting from a few-particle nucleus. The competition between the substrate structure and that of the growing crystal induces a new crystal growth scenario. Unlike with the crystal growth in the bulk where a well-defined and connected crystal-fluid interface grows into the fluid, we identify a mechanism where a 'compatibility wave' of the prescribed nucleus with the underlying substrate structure dictates the growth direction and efficiency. The growth process is strongly anisotropic and proceeds via transient island formation in front of an initial solid-fluid interface. We demonstrate the validity of this compatibility wave concept for a large class of substrate structures including a square lattice and a quasicrystalline pattern. Dynamical density functional theory that provides a microscopic approach to the crystallization process is employed for colloidal hard spheres. Our predictions can be verified in experiments on confined colloids and also bear consequences for molecular crystal growth on structured substrates.

Competing topological orders in three dimensions: X-cube versus toric code - Mühlhauser, Matthias; Schmidt, Kai Phillip; Vidal, Julien; Walther, Matthias (SciPost Physics, 2022)
We study the competition between two different topological orders in three dimensions by considering the X-cube model and the three-dimensional toric code. The corresponding Hamiltonian can be decomposed into two commuting parts, one of which displays a self-dual spectrum. To determine the phase diagram, we compute the high-order series expansions of the ground-state energy in all limiting cases. Apart from the topological order related to the toric code and the fractonic order related to the X-cube model, we found two new phases which are adiabatically connected to classical limits with nontrivial sub-extensive degeneracies. All phase transitions are found to be first order.
CRIS-Team Scopus Importer:2022-03-25

Competition between intermediate plaquette phases in SrCu2(BO3)(2) under pressure - Boos, Carolin; Crone, S. P. G.; Niesen, I. A.; Corboz, P.; Schmidt, Kai Phillip; Mila, F. (Physical Review B, 2019)
Building on the growing evidence based on NMR, magnetization, neutron scattering, electron spin resonance, and specific heat that, under pressure, SrCu2(BO3)(2) has an intermediate phase between the dimer and the Neel phase, we study the competition between two candidate phases in the context of a minimal model that includes two types of intra- and interdimer interactions without enlarging the unit cell. We show that the empty plaquette phase of the Shastry-Sutherland model is quickly replaced by a quasi-one-dimensional full plaquette phase when intra- and/or interdimer couplings take different values, and that this full plaquette phase is in much better agreement with available experimental data than the empty plaquette one.
CRIS-Team WoS Importer:2019-11-15

Complementary Molecular Dynamics and X-ray Reflectivity Study of an Imidazolium-Based Ionic Liquid at a Neutral Sapphire Interface - Brkljaca, Zlatko; Klimczak, Michael; Milicevic, Zoran; Weißer, Matthias; Taccardi, Nicola; Wasserscheid, Peter; Smith, David M.; Magerl, Andreas; Smith, Ana-Suncana (Journal of Physical Chemistry Letters, 2015)
Understanding the molecular-level behavior of ionic liquids (ILs) at IL–solid interfaces is of fundamental importance with respect to their application in, for example, electrochemical systems and electronic devices. Using a model system, consisting of an imidazolium-based IL ([C2Mim][NTf2]) in contact with a sapphire substrate, we have approached this problem using a complementary combination of high-resolution X-ray reflectivity measurements and atomistic molecular dynamics (MD) simulations. Our strategy enabled us to compare experimental and theoretically calculated reflectivities in a direct manner, thereby critically assessing the applicability of several force-field variants. On the other hand, using the best-matching MD description, we are able to describe the nature of the model IL–solid interface in appreciable detail. More specifically, we find that characteristic interactions between the surface hydroxyl groups and donor and acceptor sites on the IL constituents have a dominant role in inducing a multidimensional layering profile of the cations and anions.

Complete description of the magnetic ground state in spinel vanadates - Krishna, Jyoti; Singh, N.; Shallcross, Sam; Dewhurst, J. K.; Gross, E. K. U.; Maitra, T.; Sharma, S. (Physical Review B, 2019)
We explore a source-free local spin density approximation (LSDA)+U functional within density functional theory for its capabilities in describing noncollinear spin textures; capturing the noncollinear magnetic ground state of the spinel vanadates AV(2)O(4) (A = Mn, Fe, and Co) remains an outstanding challenge for state-of-the-art ab initio methods. We demonstrate that both the noncollinear spin texture, as well as the magnitude of local moments, are captured, provided the source term (i.e., magnetic monopole term) is removed from the exchange-correlation magnetic field B-XC. This suggests that for treatment of strongly correlated magnetic materials within the LSDA+U method the subtraction of the unphysical magnetic monopole term from the B-XC is essential.
CRIS-Team WoS Importer:2021-10-18

COMPLETE QUANTIZATION OF A DIFFEOMORPHISM INVARIANT FIELD-THEORY - Thiemann, Thomas (Classical and Quantum Gravity, 1995)
They turn out, as expected, to be non-local and form naturally a set of countable cardinality.

Complex critical points and curved geometries in four-dimensional Lorentzian spinfoam quantum gravity - Han, Muxin; Huang, Zichang; Liu, Hongguang; Qu, Dongxue (Physical Review D, 2022)
This paper focuses on the semiclassical behavior of the spinfoam quantum gravity in four dimensions. There has been long-standing confusion, known as the flatness problem, about whether the curved geometry exists in the semiclassical regime of the spinfoam amplitude. The confusion is resolved by the present work. By numerical computations, we explicitly find curved Regge geometries that contribute dominantly to the large-j Lorentzian Engle-Pereira-Rovelli-Livine (EPRL) spinfoam amplitudes on triangulations. These curved geometries are with small deficit angles and relate to the complex critical points of the amplitude. The dominant contribution from the curved geometry to the spinfoam amplitude is proportional to eiI, where I is the Regge action of the geometry plus corrections of higher order in curvature. As a result, in the semiclassical regime, the spinfoam amplitude reduces to an integral over Regge geometries weighted by eiI, where I is the Regge action plus corrections of higher order in curvature. As a by-product, our result also provides a mechanism to relax the cosine problem in the spinfoam model. Our results provide important evidence supporting the semiclassical consistency of the spinfoam quantum gravity.
CRIS-Team Scopus Importer:2022-09-02

Complex critical points in Lorentzian spinfoam quantum gravity: Four-simplex amplitude and effective dynamics on a double- Δ3 complex - Han, Muxin; Liu, Hongguang; Qu, Dongxue (Physical Review D, 2023)
The complex critical points are analyzed in the four-dimensional Lorentzian Engle-Pereira-Rovelli-Livine spinfoam model in the large-j regime. For the four-simplex amplitude, taking into account the complex critical point generalizes the large-j asymptotics to the situation with non-Regge boundary data and relates to the twisted geometry. For generic simplicial complexes, we present a general procedure to derive the effective theory of Regge geometries from the spinfoam amplitude in the large-j regime by using the complex critical points. The effective theory is analyzed in detail for the spinfoam amplitude on the double-Δ3 simplicial complex. We numerically compute the effective action and the solution of the effective equation of motion on the double-Δ3 complex. The effective theory reproduces the classical Regge gravity when the Barbero-Immirzi parameter γ is small.
CRIS-Team Scopus Importer:2023-08-25

Complex dewetting scenarios captured by thin-film models - Becker, Jürgen; Grün, Günther; Seemann, Ralf; Manz, Hubert; Jacobs, Karin; Mecke, Klaus; Blossey, Ralf (Nature Materials, 2003)
In the course of miniaturization of electronic and microfluidic devices, reliable predictions of the stability of ultrathin films have a strategic role for design purposes. Consequently, efficient computational techniques that allow for a direct comparison with experiment become increasingly important. Here we demonstrate, for the first time, that the full complex spatial and temporal evolution of the rupture of ultrathin films can be modelled in quantitative agreement with experiment. We accomplish this by combining highly controlled experiments on different film-rupture patterns with computer simulations using novel numerical schemes for thin-film equations. For the quantitative comparison of the pattern evolution in both experiment and simulation we introduce a novel pattern analysis method based on Minkowski measures. Our results are fundamental for the development of efficient tools capable of describing essential aspects of thin-film flow in technical systems.

Complexifier coherent states for quantum general relativity - Thiemann, Thomas (Classical and Quantum Gravity, 2006)
Recently, substantial amount of activity in quantum general relativity (QGR) has focused on the semiclassical analysis of the theory. In this paper, we want to comment on two such developments: (1) polymer-like states for Maxwell theory and linearized gravity constructed by Varadarajan which use much of the Hilbert space machinery that has proved useful in QGR, and (2) coherent states for QGR, based on the general complexifier method, with built-in semiclassical properties. We show the following. (A) Varadarajan's states are complexifier coherent states. This unifies all states constructed so far under the general complexifier principle. (B) Ashtekar and Lewandowski suggested a non-Abelian generalization of Varadarajan's states to QGR which, however, are no longer of the complexifier type. We construct a new class of non-Abelian complexifiers which come close to that underlying Varadarajan's construction. (C) Non-Abelian complexifiers close to Varadarajan's induce new types of Hilbert spaces which do not support the operator algebra of QGR. The analysis suggests that if one sticks to the present kinematical framework of QGR and if kinematical coherent states are at all useful, then normalizable, graph-dependent states must be used which are produced by the complexifier method as well. (D) Present proposals for states with mildened graph dependence, obtained by performing a graph average, do not approximate well coordinate-dependent observables. However, graph-dependent states, whether averaged or not, seem to be well suited for the semiclassical analysis of QGR with respect to coordinate-independent operators.

Complex magnetic order in nickelate slabs - Hepting, Matthias; Green, R. J.; Zhong, Zhicheng; Bluschke, M.; Suyolcu, Y. E.; Macke, S.; Frano, A.; Catalano, Sara; Gibert, Marta; Sutarto, R.; He, Feizhou; Cristiani, G.; et al. (Nature Physics, 2018)
Magnetic ordering phenomena have a profound influence on the macroscopic properties of correlated-electron materials, but their realistic prediction remains a formidable challenge. An archetypical example is the ternary nickel oxide system RNiO3 (R = rare earth), where the period-four magnetic order with proposals of collinear and non-collinear structures and the amplitude of magnetic moments on different Ni sublattices have been subjects of debate for decades(1-6). Here we introduce an elementary model system-NdNiO3 slabs embedded in a non-magnetic NdGaO3 matrix-and use polarized resonant X-ray scattering (RXS) to show that both collinear and non-collinear magnetic structures can be realized, depending on the slab thickness. The crossover between both spin structures is correctly predicted by density functional theory and can be qualitatively understood in a low-energy spin model. We further demonstrate that the amplitude ratio of magnetic moments in neighbouring NiO6 octahedra can be accurately determined by RXS in combination with a correlated double cluster model. Targeted synthesis of model systems with controlled thickness and synergistic application of polarized RXS and ab initio theory thus provide new perspectives for research on complex magnetism, in analogy to two-dimensional materials created by exfoliation(7).
CRIS-Team WoS Importer:2020-02-19

Configurational Forces in Bond Order Potentials - Birang Oskouei, Seyedehelmira; Smith, Ana-Suncana; Steinmann, Paul (Proceedings in Applied Mathematics and Mechanics, 2021)
In this contribution, a configurational mechanics framework is elaborated to assess the applicability of atomistic configurational forces in fracture of crystalline lattices. To this end, an analytical interatomic potential is reformulated in terms of the material positions occupied by the atoms participating in two- and three-body interactions. It is demonstrated that such a potential satisfies the requirements of invariances i.e., translational, rotational and parity. The focus of this work is developing the configurational setting for the bond order Tersoff potential. Two-dimensional pre-cracked mono-layer graphene modelled with the Tersoff potential is chosen to study the configurational force approach in determining energy release during crack propagation into the lattice.<br

Conical space-time defects and their phenomenological consequences - Roelcke, Carmen; Sahlmann, Hanno (2015)

Connecting the random organization transition and jamming within a unifying model system - Milz, Lars; Schmiedeberg, Michael (Physical Review E, 2013)
While the random organization transition describes the change from reversible to irreversible dynamics in a nonequilibrium system, the athermal jamming transition at zero shear rate occurs when particles can no longer avoid overlaps. Despite the obvious differences between these two transitions, we show that they both occur within the same model packing problem. In this unifying model system the particles are first randomly distributed and then displaced in each step if they overlap. For random displacements we obtain a random organization transition, while jamming occurs in the case of deterministic shifts. We also analyze the critical behavior of random organization. Our results show that random organization and jamming are opposite limits of random sphere packings, and we expect that various equilibrium and nonequilibrium transitions can be formulated as related intermediate packing problems.

Consistency check on volume and triad operator quantization in loop quantum gravity: I - Giesel, Kristina; Thiemann, Thomas (Classical and Quantum Gravity, 2006)
The volume operator plays a pivotal role for the quantum dynamics of loop quantum gravity (LQG). It is essential to construct triad operators that enter the Hamiltonian constraint and which become densely defined operators on the full Hilbert space, even though in the classical theory the triad becomes singular when classical GR breaks down. The expression for the volume and triad operators derives from the quantization of the fundamental electric flux operator of LQG by a complicated regularization procedure. In fact, there are two inequivalent volume operators available in the literature and, moreover, both operators are unique only up to a finite, multiplicative constant which should be viewed as a regularization ambiguity. Now on the one hand, classical volumes and triads can be expressed directly in terms of fluxes and this fact was used to construct the corresponding volume and triad operators. On the other hand, fluxes can be expressed in terms of triads and triads can be replaced by Poisson brackets between the holonomy and the volume operators. Therefore one can also view the holonomy operators and the volume operator as fundamental and consider the flux operator as a derived operator. In this paper we mathematically implement this second point of view and thus can examine whether the volume, triad and flux quantizations are consistent with each other. The results of this consistency analysis are rather surprising. Among other findings we show the following. ( 1) The regularization constant can be uniquely fixed. ( 2) One of the volume operators can be ruled out as inconsistent. ( 3) Factor ordering ambiguities in the definition of triad operators are immaterial for the classical limit of the derived flux operator. The results of this paper show that within full LQG triad operators are consistently quantized.

Consistency check on volume and triad operator quantization in loop quantum gravity: II - Giesel, Kristina; Thiemann, Thomas (Classical and Quantum Gravity, 2006)
In this paper, we provide the techniques and proofs for the results presented in our companion paper concerning the consistency check on volume and triad operator quantization in loop quantum gravity.

Consistent gauge-fixing conditions in polymerized gravitational systems - Giesel, Kristina; Li, Bao Fei; Singh, Parampreet; Weigl, Stefan (Physical Review D, 2022)
For classical gravitational systems the lapse function and the shift vector are usually determined by imposing appropriate gauge-fixing conditions and then demanding their preservation with respect to the dynamics generated by a canonical Hamiltonian. Effective descriptions encoding quantum geometric effects motivated by loop quantum gravity for symmetry-reduced models are often captured by polymerization of connection (or related) variables in gauge-fixing conditions as well as constraints. Usually, one chooses the same form of polymerization in both cases. A pertinent question is if the dynamical stability of the effective gauge-fixing conditions under the effective dynamics generated by the polymerized canonical Hamiltonian is provided by the lapse function and the shift vector obtained from the polymerization of their classical counterparts. If this is the case, then we say that gauge fixing and polymerization commute. In this manuscript we investigate these issues and obtain consistency conditions for the commutativity of gauge fixing and polymerization. Our analysis shows that such a commutativity occurs in rather special situations and reveals pitfalls in making seemingly well-motivated choices which turn out to be inconsistent with the effective dynamics. We illustrate these findings via examples of symmetry-reduced models in the loop quantization of the Schwarzschild interior and Lemaître-Tolman-Bondi spacetimes and report the noncommutativity of gauge fixing and polymerization and inherent limitations of some choices made in the literature with a consistent effective dynamics.
CRIS-Team Scopus Importer:2022-04-29

Consistent gauge-fixing conditions in polymerized gravitational systems - Giesel, Kristina; Li, Bao-Fei; Singh, Parampreet; Weigl, Stefan (Physical Review D - Particles, Fields, Gravitation and Cosmology, 2022)
For classical gravitational systems the lapse function and the shift vector are usually determined by imposing appropriate gauge-fixing conditions and then demanding their preservation with respect to the dynamics generated by a canonical Hamiltonian. Effective descriptions encoding quantum geometric effects motivated by loop quantum gravity for symmetry-reduced models are often captured by polymerization of connection (or related) variables in gauge-fixing conditions as well as constraints. Usually, one chooses the same form of polymerization in both cases. A pertinent question is if the dynamical stability of the effective gauge-fixing conditions under the effective dynamics generated by the polymerized canonical Hamiltonian is provided by the lapse function and the shift vector obtained from the polymerization of their classical counterparts. If this is the case, then we say that gauge fixing and polymerization commute. In this manuscript we investigate these issues and obtain consistency conditions for the commutativity of gauge fixing and polymerization. Our analysis shows that such a commutativity occurs in rather special situations and reveals pitfalls in making seemingly well-motivated choices which turn out to be inconsistent with the effective dynamics. We illustrate these findings via examples of symmetry-reduced models in the loop quantization of the Schwarzschild interior and Lemaître-Tolman-Bondi spacetimes and report the noncommutativity of gauge fixing and polymerization and inherent limitations of some choices made in the literature with a consistent effective dynamics.<br

Constraining F (R) bouncing cosmologies through primordial black holes - Banerjee, Shreya; Papanikolaou, Theodoros; Saridakis, Emmanuel N. (Physical Review D, 2022)
The phenomenology of primordial black hole (PBH) physics and the associated PBH abundance constraints can be used to probe the physics of the early universe. In this work, we investigate the PBH formation during the standard radiation-dominated era by studying the effect of an early F(R) modified gravity phase with a bouncing behavior which is introduced to avoid the initial spacetime singularity problem. In particular, we calculate the energy density power spectrum at horizon crossing time, and then we extract the PBH abundance in the context of peak theory as a function of the parameter α of our F(R) gravity bouncing model at hand. Interestingly, we find that to avoid gravitational-wave overproduction from an early PBH dominated era before big bang nucleosynthesis, α should lie within the range α≤10-19MPl2. This constraint can be translated to a constraint on the energy scale at the onset of the hot big bang phase, HRD∼α/2, which can be recast as HRD<10-10MPl.
CRIS-Team Scopus Importer:2022-12-23

Constraint Analysis of the D+1 dimensional Palatini action - Thurn, Andreas; Thiemann, Thomas (Friedrich-Alexander-Universität Erlangen-Nürnberg, 2009)

Constraints on Measurement-Based Quantum Computation in Effective Cluster States - Klagges, Daniel; Schmidt, Kai Phillip (Physical Review Letters, 2012)
The aim of this work is to study the physical properties of a one-way quantum computer in an effective low-energy cluster state. We calculate the optimal working conditions as a function of the temperature and of the system parameters. The central result of our work is that any effective cluster state implemented in a perturbative framework is fragile against special kinds of external perturbations. Qualitative aspects of our work are important for any implementation of effective low-energy models containing strong multisite interactions.

Constructing Hamiltonian quantum theories from path integrals in a diffeomorphism-invariant context - Ashtekar, Abhay; Marolf, Donald; Mourao, José Manuel; Thiemann, Thomas (Classical and Quantum Gravity, 2000)
Osterwalder and Schrader introduced a procedure to obtain a (Lorentzian) Hamiltonian quantum theory starting from a measure on the space of (Euclidean) histories of a scalar quantum field. In this paper, we extend that construction to more general theories which do not refer to any background, spacetime metric (and in which the space of histories does not admit a natural linear structure). Examples include certain gauge theories, topological field theories and relativistic gravitational theories. The treatment is self-contained in the sense that an a priori knowledge of the Osterwalder-Schrader theorem is not assumed.

Continuously varying critical exponents in long-range quantum spin ladders - Adelhardt, Patrick; Schmidt, Kai Phillip (SciPost Physics, 2023)
We investigate the quantum-critical behavior between the rung-singlet phase with hidden string order and the Néel phase with broken SU(2)-symmetry in quantum spin ladders with algebraically decaying unfrustrated long-range Heisenberg interactions. To this end, we determine high-order series expansions of energies and observables in the thermodynamic limit about the isolated rung-dimer limit. This is achieved by extending the method of perturbative continuous unitary transformations (pCUT) to long-range Heisenberg interactions and to the calculation of generic observables. The quantumcritical breakdown of the rung-singlet phase then allows us to determine the critical phase transition line and the entire set of critical exponents as a function of the decay exponent of the long-range interaction. We demonstrate long-range mean-field behavior as well as a non-trivial regime of continuously varying critical exponents implying the absence of deconfined criticality contrary to a recent suggestion in the literature.
CRIS-Team Scopus Importer:2023-10-20

Continuous similarity transformation for critical phenomena: Easy-axis antiferromagnetic XXZ model - Walther, Matthias; Hering, Dag-Bjoern; Uhrig, Goetz S.; Schmidt, Kai Phillip (Physical Review Research, 2023)
We apply continuous similarity transformations (CSTs) to the easy-axis antiferromagnetic XXZ model on the square lattice. The CST flow equations are truncated in momentum space by the scaling dimension d so that all contributions with d 2 are taken into account. The resulting quartic magnon-conserving effective Hamiltonian is analyzed in the zero-, one-, and two-magnon sector. In this way, a quantitative description of the ground-state energy, the one-magnon dispersion, and its gap as well as of two-magnon bound states is gained for anisotropies ranging from the gapped Ising model to the gapless Heisenberg model. We discuss the critical properties of the gap closing as well as the evolution of the one-magnon roton mininum. The excitation energies of two-magnon bound states are calculated, and their decay into the two-magnon continuum is determined via the inverse participation ratio.
CRIS-Team WoS Importer:2023-03-31

Controlling the conductance of molecular junctions using proton transfer reactions: A theoretical model study - Hofmeister, Chriszandro; Coto, Pedro B.; Thoss, Michael (Journal of Chemical Physics, 2017)
The influence of an intramolecular proton transfer reaction on the conductance of a molecular junction is investigated employing a generic model, which includes the effects of the electric field of the gate and leads electrodes and the coupling to a dissipative environment. Using a quantum master equation approach it is shown that, depending on the localization of the proton, the junction exhibits a high or low current state, which can be controlled by external electric fields. Considering different regimes, which range from weak to strong hydrogen bonds in the proton transfer complex and comprise situations with high and low barriers, necessary preconditions to achieve control are analyzed. The results show that systems with a weak hydrogen bond and a significant energy barrier for the proton transfer can be used as molecular transistors or diodes.
EAM Import::2019-03-13

Control of the metal-insulator transition in NdNiO3 thin films through the interplay between structural and electronic properties - Suyolcu, Y. E.; Fürsich, K.; Hepting, M.; Zhong, Z.; Lu, Y.; Wang, Y.; Christiani, G.; Logvenov, G.; Hansmann, Philipp; Minola, M.; Keimer, B.; Van Aken, P. A.; et al. (Physical Review Materials, 2021)
Heteroepitaxy offers a new type of control mechanism for the crystal structure, the electronic correlations, and thus the functional properties of transition-metal oxides. Here we combine electrical transport measurements, high-resolution scanning transmission electron microscopy (STEM), and density functional theory (DFT) to investigate the evolution of the metal-to-insulator transition (MIT) in NdNiO3 films as a function of film thickness and NdGaO3 substrate crystallographic orientation. We find that for two different substrate facets, orthorhombic (101) and (011), modifications of the NiO6 octahedral network are key for tuning the transition temperature TMIT over a wide temperature range. A comparison of films of identical thickness reveals that growth on [101]-oriented substrates generally results in a higher TMIT, which can be attributed to an enhanced bond disproportionation as revealed by the DFT+U calculations, and a tendency of [011]-oriented films to formation of structural defects and stabilization of nonequilibrium phases. Our results provide insights into the structure-property relationship of a correlated electron system and its evolution at microscopic length scales and give new perspectives for the epitaxial control of macroscopic phases in metal-oxide heterostructures.
CRIS-Team Scopus Importer:2021-05-14

Control of Yu-Shiba-Rusinov States through a Bosonic Mode - Müller, Hartmut; Eckstein, Martin; Viola Kusminskiy, Silvia (Physical Review Letters, 2023)
We investigate the impact of a bosonic degree of freedom on Yu-Shiba-Rusinov states emerging from a magnetic impurity in a conventional superconductor. Starting from the Anderson impurity model, we predict that an additional p-wave conduction band channel opens up if a bosonic mode is coupled to the tunneling between impurity and host, which implies an additional pair of odd-parity Yu-Shiba-Rusinov states. The bosonic mode can be a vibrational mode or the electromagnetic field in a cavity. The exchange couplings in the two channels depend sensitively on the state of the bosonic mode (ground state, few quanta, or classically driven Floquet state), which opens possibilities for phononics or photonics control of such systems, with a rich variety of ground and excited states.
CRIS-Team Scopus Importer:2023-04-07

Cooperative Quantum Phenomena in Light-Matter Platforms - Reitz, Michael; Sommer, Christian; Genes, Claudiu (PRX Quantum, 2022)
Quantum cooperativity is evident in light-matter platforms where quantum-emitter ensembles are interfaced with confined optical modes and are coupled via the ubiquitous electromagnetic quantum vacuum. Cooperative effects can find applications, among other areas, in topological quantum optics, in quantum metrology, or in quantum information. This tutorial provides a set of theoretical tools to tackle the behavior responsible for the onset of cooperativity by extending open quantum system dynamics methods, such as the master equation and quantum Langevin equations, to electron-photon interactions in strongly coupled and correlated quantum-emitter ensembles. The methods are illustrated on a wide range of current research topics such as the design of nanoscale coherent-light sources, highly reflective quantum metasurfaces, or low intracavity power superradiant lasers.
CRIS-Team WoS Importer:2022-02-04

Correction: Capillary assemblies in a rotating magnetic field (Soft Matter (2019) DOI: 10.1039/c9sm01414d) - Grosjean, Galien; Hubert, Maxime; Collard, Ylona; Sukhov, Alexander; Harting, Jens; Smith, Ana-Suncana; Vandewalle, Nicolas (Soft Matter, 2019)
In the original paper, the citations for references 28 and 38 are incorrect. The correct citations are: 28. A. Sukhov, S. Ziegler, Q. Xie, O. Trosman, J. Pande, G. Grosjean, M. Hubert, N. Vandewalle, A.-S. Smith and J. Harting, J. Chem. Phys., 2019, 151, 124707. 38. M. Hubert, G. Grosjean, Y.-E. Corbisier, G. Lumay, F. Weyer, N. Obara and N. Vandewalle, 2013, arXiv:1310.3094. The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.
CRIS-Team Scopus Importer:2019-12-10

Correction to: A new benchmark of soft X-ray transition energies of Ne, CO2, and SF6: paving a pathway towards ppm accuracy (The European Physical Journal D, (2022), 76, 3, (38), 10.1140/epjd/s10053-022-00355-0) - Stierhof, Jakob; Kuehn, S.; Winter, Michael; Micke, Peter; Steinbruegge, R.; Shah, C.; Hell, N.; Bissinger, Matthias; Hirsch, Maria; Ballhausen, Ralf; Lang, Melanie; Gräfe, Christina; et al. (European Physical Journal D, 2022)
CRIS-Team Scopus Importer:2022-11-11

Corrigendum: Temperature dependence of the transition packing fraction of thermal jamming in a harmonic soft sphere system (Journal of Physics Condensed Matter (2019) 31 (165101) DOI: 10.1088/1361-648X/ab01e9) - Maiti, Moumita; Schmiedeberg, Michael (Journal of Physics: Condensed Matter, 2019)
In the second to last paragraph of section 3 of our paper [1] there is a mistake in the formula of the rescaling. Instead of (Tμ/2 - bT)-1 it has to be (Tμ/2)-1 - b(T)-1. Note that in the figures 4 (e)–(g) we have employed the correct rescaling but the labels on the x-axes have to read |g,0 - |(Tμ/2)-1 - b(T)-1. We thank Monoj Adhikari for bringing this mistake to our attention.
CRIS-Team Scopus Importer:2019-10-08

Cosmic UHE neutrino signatures - Giesel, Kristina; Jureit, Jan-Hendrik; Reya, Ewald (Astroparticle Physics, 2003)
Utilizing the unique and reliable ultrasmall-x predictions of the dynamical (radiative) parton model, nominal event rates and their detailed energy dependence caused by a variety of cosmic UHE neutrino fluxes are calculated and analyzed. In addition, maximal Regge-model inspired small-x structure functions are employed for obtaining optimal rates which do not necessarily require 'new'physics interpretations. Upward mu(+) + mu(-) event rates are estimated by taking into account total and nadir-angle dependent regeneration effects due to neutral current interactions. For exploring extragalactic neutrino sources at highest energies (greater than or similar to 10(8) GeV) with modern (future) ground-level telescopes, we analyze horizontal air shower event rates and shower events caused by Earth-skimming tau-neutrinos, in particular their detailed shower- and cosmic neutrino-energy dependence. As an illustration of 'new physics' implications we estimate the relevant horizontal air shower event rates due to spin-2 Kaluza-Klein 'graviton' exchanges in neutral current neutrino-quark and neutrino-gluon interactions at low TeV-scales. (C) 2003 Elsevier B.V. All rights reserved.

Cosmological perturbation theory with Gaussian dust reference fields - Giesel, Kristina; Herold, Laura (Friedrich-Alexander-Universität Erlangen-Nürnberg, 2019)

Course of dislocation lines in templated three-dimensional colloidal quasicrystals - Sandbrink, Matthias; Schmiedeberg, Michael (Physical Review B, 2014)
We present an approach based on colloidal epitaxy to obtain three-dimensional colloidal quasicrystals, i.e., structures with long-range order but no periodicity. In contrast to other attempts to obtain soft-matter quasicrystals by design, no fine tuning of the colloid-colloid interactions is required. By using Monte Carlo simulations, we explore the development of multiple layers with the desired quasicrystalline symmetry when the colloidal particles sediment onto suitable quasicrystalline substrates. In addition, well-defined defect lines can be induced in the grown colloidal quasicrystal by constructing substrates with well-chosen defects that lead to induced defect lines. We present a study of the three-dimensional course of such dislocation lines by inducing and analyzing stable lines that bend, unstable ones that split up, or defect lines that merge with other defects. Dislocations are characterized by Burgers vectors that are composed of a phononic part as in a periodic crystal and an additional phasonic part that is unique to quasicrystals. As in a periodic crystal, the sum of all Burgers vectors is conserved along defect lines and at all forks. However, unlike in a periodic crystal, the course of a dislocation line in a quasicrystal depends on its position. Furthermore, although in a quasicrystal all Burgers vectors can always be written as sum of Burgers vectors with a smaller phononic part, we find dislocations that do not decompose into dislocations with smaller phononic Burgers vectors. For quasicrystals with eightfold and twelvefold rotational symmetry, we identify basis Burgers vectors that denote stable dislocations. Our results can be verified in experiments and are important for applications of colloidal quasicrystals, e.g., for the development of photonic quasicrystals.

Covariant origin of the U(1)(3) model for Euclidean quantum gravity - Bakhoda, S.; Thiemann, Thomas (Classical and Quantum Gravity, 2022)
If one replaces the constraints of the Ashtekar-Barbero SU(2) gauge theory formulation of Euclidean gravity by their U(1)(3) version, one arrives at a consistent model which captures significant structures of its SU(2) version. In particular, it displays a non-trivial realisation of the hypersurface deformation algebra which makes it an interesting testing ground for (Euclidean) quantum gravity as has been emphasised in a recent series of papers due to Varadarajan et al. The simplification from SU(2) to U(1)(3) can be performed simply by hand within the Hamiltonian formulation by dropping all non-abelian terms from the Gauss, spatial diffeomorphism, and Hamiltonian constraints respectively. However, one may ask from which Lagrangian formulation this theory descends. For the SU(2) theory it is known that one can choose the Palatini action, Holst action, or (anti-)selfdual action (Euclidean signature) as starting point all leading to equivalent Hamiltonian formulations. In this paper, we systematically analyse this question directly for the U(1)(3) theory. Surprisingly, it turns out that the abelian analog of the Palatini or Holst formulation is a consistent but topological theory without propagating degrees of freedom. On the other hand, a twisted abelian analog of the (anti-)selfdual formulation does lead to the desired Hamiltonian formulation. A new aspect of our derivation is that we work with (1) half-density valued tetrads which simplifies the analysis, (2) without the simplicity constraint (which admits one undesired solution that is usually neglected by hand) and (3) without imposing the time gauge from the beginning. As a byproduct, we show that also the non-abelian theory admits a twisted (anti-)selfdual formulation. Finally, we also derive a pure connection formulation of Euclidean GR including a cosmological constant by extending previous work due to Capovilla, Dell, Jacobson, and Peldan which may be an interesting starting point for path integral investigations and displays (Euclidean) GR as a Yang-Mills theory with non-polynomial Lagrangian.
CRIS-Team WoS Importer:2021-12-24

Creation and manipulation of anyons in the Kitaev model - Dusuel, Sebastien; Schmidt, Kai Phillip; Vidal, Julien (Physical Review Letters, 2008)
We analyze the effect of local spin operators in the Kitaev model on the honeycomb lattice. We show, in perturbation around the isolated-dimer limit, that they create Abelian anyons together with fermionic excitations which are likely to play a role in experiments. We derive the explicit form of the operators creating and moving Abelian anyons without creating fermions and show that it involves multispin operations. Finally, the important experimental constraints stemming from our results are discussed.