bluuebunny/crossref_metadata_2025_split huggingface dataset:
Interacting bosons in an optical lattice
Authors: Moseley C. , Fialko O. , Ziegler K. 
Published: June 2022

AbstractA strongly interacting Bose gas in an optical lattice is studied using a hard‐core interaction. Two different approaches are introduced, one is based on a spin‐1/2 Fermi gas with attractive interaction, the other one on a functional integral with an additional constraint (slave‐boson approach). The relation between fermions and hard‐core bosons is briefly discussed for the case of a one‐dimensional Bose gas. For a three‐dimensional gas we identify the order parameter of the Bose‐Einstein condensate through a Hubbard‐Stratonovich transformation and treat the corresponding theories within a mean‐field approximation and with Gaussian fluctuations. This allows us to evaluate the phase diagram, including the Bose‐Einstein condensate and the Mott insulator, the density‐density correlation function, the static structure factor, and the quasiparticle excitation spectrum. The role of quantum and thermal fluctuations are studied in detail for both approaches, where we find good agreement with the Gross‐Pitaevskii equation and with the Bogoliubov approach in the dilute regime. In the dense regime, which is characterized by the phase transition between the Bose‐Einstein condensate and the Mott insulator, we discuss a renormalized Gross‐Pitaevskii equation. This equation can describe the macroscopic wave function of the Bose‐Einstein condensate in the dilute regime as well as close to the transition to the Mott insulator. Finally, we compare the results of the attractive spin‐1/2 Fermi gas and those of the slave‐boson approach and find good agreement for all physical quantities.
What is missing from Minkowski's “Raum und Zeit” lecture
Authors: Damour T. 
Published: June 2022

AbstractThis contribution tries to highlight the importance of Minkowski's “Raum und Zeit” lecture in a “negative” way, where negative is taken in the photographic sense of reversing lights and shades. Indeed, we focus on the “shades” of Minkowski's text, i.e. what is missing, or misunderstood. In particular, we focus on two issues: (i) why are Poincaré's pioneering contributions to four‐dimensional geometry not quoted by Minkowski (while he abundantly quoted them a few months before the Cologne lecture)?, and (ii) did Minkowski fully grasp the physical (and existential) meaning of “time” within spacetime? We think that this “negative” approach (and the contrast between Poincaré's and Minkowski's attitudes towards physics) allows one to better grasp the boldness of the revolutionary step taken by Minkowski in his Cologne lecture.
Nonlocal special relativity
Authors: Mashhoon B. 
Published: June 2022

AbstractIn the special theory of relativity, Lorentz invariance is extended in Minkowski spacetime from ideal inertial observers to actual observers by means of the hypothesis of locality, which postulates that accelerated observers are always pointwise inertial. A critical examination of the locality assumption reveals its domain of validity: it is true for pointwise coincidences, but is in conflict with wave‐particle duality. To remedy this situation, a nonlocal theory of accelerated systems is presented that reduces to the standard theory in the limit of small accelerations. Some of the main consequences of nonlocal special relativity are briefly outlined.
Electromagnetic energy and momentum in moving media
Authors: Obukhov Y.N. 
Published: June 2022

AbstractThe problem of the electromagnetic energy‐momentum tensor is among the oldest and the most controversial in macroscopic electrodynamics. In the center of the issue is a dispute about the Minkowski and the Abraham tensors for moving media. An overview of the current situation is presented. After putting the discussion into a general Lagrange‐Noether framework, the Minkowski tensor is recovered as a canonical energy‐momentum. It is shown that the balance equations of energy, momentum, and angular momentum are always satisfied for an open electromagnetic system despite the lack of the symmetry of the canonical tensor. On the other hand, although the Abraham tensor is not defined from first principles, one can formulate a general symmetrization prescription provided a timelike vector is available. We analyze in detail the variational model of a relativistic ideal fluid with isotropic electric and magnetic properties interacting with the electromagnetic field. The relation between the Minkowski energy‐momentum tensor, the canonical energy‐momentum of the medium and the Abraham tensor is clarified. It is demonstrated that the Abraham energy‐momentum is relevant when the 4‐velocity of matter is the only covariant variable that enters the constitutive tensor.
Resonance‐like Goos‐Hänchen shift induced by nano‐metal films
Authors: Gruschinski R. , Nimtz G. , Stahlhofen A.A. 
Published: June 2022

AbstractThe influence of nano‐metal films on the Goos‐Hänchen shift (GHS) is investigated. The films were deposited at the total reflecting surface of a perspex prism and had a sheet resistance varying between Z□ = 25 and 3 000 Ω. A resonance‐like enhancement of the shift and of the absorption is found for TE polarized waves, when the sheet resistance approached the value of the vacuum impedance. For TM waves the influence of the metal films on the GHS was marginal. The experiments were carried out with microwaves.
Matter waves in terms of the unitary representations of the Lorentz group
Authors: Frick R.A. 
Published: June 2022

AbstractIn a generalized Heisenberg/Schrödinger picture, the unitary representations of the Lorentz group may, for a massive relativistic particle, be used to attribute to waves an extra wavelength that is longer than the de Broglie wavelength. Propagators are defined as spacetime transitions between states with different eigenvalues of the first or the second Casimir operator of the Lorentz algebra.
Prediction of spinodal wavelength in continuously cooled metallic liquid
Authors: Borchers C. , Schroers J. , Busch R. 
Published: June 2022

AbstractThe spinodal decomposition of a deeply undercooled metallic liquid Zr41.2 Ti13.8Cu12.5Ni10Be22.5 between 800 and 700 K is analysed in the framework of the theory of Cahn and Hilliard for continuous cooling, and the wavelength with maximum amplification is predicted, using as input parameters thermodynamic values gained in experiments. Electron microscopical studies show the microstructure of glass forming alloys Zr41.2Ti13.8Cu12.5Ni10Be22.5. The as‐cast material exhibits a two phase mixture of amorphous regions with different compositions. Evidence for spinodal decomposition is given, and the computed maximum‐amplitude wavelength corresponds well with the one found in the experiments.
Thermoballistic spin‐polarized electron transport in paramagnetic semiconductors
Authors: Lipperheide R. , Wille U. 
Published: June 2022

AbstractSpin‐polarized electron transport in diluted magnetic semiconductors (DMS) in the paramagnetic phase is described within the thermoballistic transport model. In this (semiclassical) model, the ballistic and diffusive transport mechanisms are unified in terms of a thermoballistic current in which electrons move ballistically across intervals enclosed between arbitrarily distributed points of local thermal equilibrium. The contribution of each interval to the current is governed by the momentum relaxation length. Spin relaxation is assumed to take place during the ballistic electron motion. In paramagnetic DMS exposed to an external magnetic field, the conduction band is spin‐split due to the giant Zeeman effect. In order to deal with this situation, we extend our previous formulation of thermoballistic spin‐polarized transport so as to take into account an arbitrary (position‐dependent) spin splitting of the conduction band. The current and density spin polarizations as well as the magnetoresistance are each obtained as the sum of an equilibrium term determined by the spin‐relaxed chemical potential, and an off‐equilibrium contribution expressed in terms of a spin transport function that is related to the splitting of the spin‐resolved chemical potentials. The procedures for the calculation of the spin‐relaxed chemical potential and of the spin transport function are outlined. As an illustrative example, we apply the thermoballistic description to spin‐polarized transport in DMS/NMS/DMS heterostructures formed of a nonmagnetic semiconducting sample (NMS) sandwiched between two DMS layers. We evaluate the current spin polarization and the magnetoresistance for this case and, in the limit of small momentum relaxation length, find our results to agree with those of the standard drift‐diffusion approach to electron transport.
On the crystalline states of the dilute jellium model
Authors: Ciccariello S. 
Published: June 2022

AbstractStrongly localized quantum crystalline states (SLQCS) are determinantal wave‐functions made up of the single‐particle wave‐functions that are obtained from all the distinct crystalline translations of a particular wave‐function different from zero only within a primitive cell of the considered crystal and dependent on some parameters α1, … , αn. The expectation value of the Hamiltonian of the jellium model is numerically evaluated over a class of SLQCS depending on a single parameter α. The minimization of this value with respect to α yields an energy smaller than that of the polarized jellium fluid. The results, compared with the more accurate results obtained by Hartree‐Fock and quantum Monte‐Carlo procedures, further clarify the importance of the electron localization for the transition to a crystalline phase as well as the localization increase with decreasing density.
Evaluation of the scientific impact, productivity and biological age based upon the h‐index in three Latin American countries: the materials science case
Authors: Romero A.H. , García A. , Kiwi M. 
Published: June 2022

AbstractWe discuss the scientific impact of Latin American scientists in the field of materials science. The analysis is based on the h‐index as the scientometric index used to quantify the scientific productivity of an individual. In particular, we focus our analysis in México, Chile and Colombia. We compare the level of productivity between all these countries. We also analyzed the h‐index as function of the biological age, by using the first year of publication of a given scientists as a reference and discussed the general distribution of its quantification. We do not find a clear relationship between these two quantities. Based in our results we propose some political measures that these countries could implement to improve productivity as well as scientific development in this field.
Thermal photon dispersion law and modified black‐body spectra
Authors: Ludescher J. , Hofmann R. 
Published: June 2022

AbstractBased on the postulate that photon propagation is governed by an SU(2) gauge principle we numerically compute the one‐loop dispersion for thermalized photon propagation on the radiatively induced mass shell. Formerly, the dispersion was addressed by assuming p2 = 0. While this approximation turns out to be excellent for temperatures ≤ 2 TCMB the exact result exhibits a much faster (power‐like) shrinking of the gap in the black‐body spectral intensity with rising temperature. Our previous statements on anomalous large‐angle CMB temperature‐temperature correlations, obtained in the approximation p2 = 0, remain valid.
Mixtures of correlated bosons and fermions: Dynamical mean‐field theory for normal and condensed phases
Authors: Byczuk K. , Vollhardt D. 
Published: June 2022

AbstractWe derive a dynamical mean‐field theory for mixtures of interacting bosons and fermions on a lattice (BF‐DMFT). The BF‐DMFT is a comprehensive, thermodynamically consistent framework for the theoretical investigation of Bose‐Fermi mixtures and is applicable for arbitrary values of the coupling parameters and temperatures. It becomes exact in the limit of high spatial dimensions d or coordination number Z of the lattice. In particular, the BF‐DMFT treats normal and condensed bosons on equal footing and thus includes the effects caused by their dynamic coupling. Using the BF‐DMFT we investigate two different interaction models of correlated lattice bosons and fermions, one where all particles are spinless (model I) and one where fermions carry a spin one‐half (model II). In model I the local, repulsive interaction between bosons and fermions can give rise to an attractive effective interaction between the bosons. In model II it can also lead to an attraction between the fermions.
On the variational formulation of systems with non‐holonomic constraints
Authors: Delphenich D.H. 
Published: June 2022

AbstractIn a previous article by the author, it was shown that one could effectively give a variational formulation to non‐conservative mechanical systems by starting with the first variation functional instead of an action functional. In this article, it is shown that this same approach will also allow one to give a variational formulation to systems with non‐holonomic constraints. The key is to use an adapted anholonomic local frame field in the formulation, which then implies the replacement of ordinary derivatives with covariant ones. The method is then applied to the case of a vertical disc rolling without slipping or friction on a plane.
Approximate analytical solutions of the pseudospin symmetric Dirac equation for exponential‐type potentials
Authors: Arda A. , Sever R. , Tezcan C. 
Published: June 2022

AbstractThe solvability of The Dirac equation is studied for the exponential‐type potentials with the pseudospin symmetry by using the parametric generalization of the Nikiforov–Uvarov method. The energy eigenvalue equation, and the corresponding Dirac spinors for Morse, Hulthen, and q‐deformed Rosen–Morse potentials are obtained within the framework of an approximation to the spin‐orbit coupling term, so the solutions are given for any value of the spin‐orbit quantum number κ = 0, or κ ≠ 0.
Network modules help the identification of key transport routes, signaling pathways in cellular and other networks
Authors: Palotai R. , Csermely P. 
Published: June 2022

AbstractComplex systems are successfully reduced to interacting elements via the network concept. Transport plays a key role in the survival of networks – for example the specialized signaling cascades of cellular networks filter noise and efficiently adapt the network structure to new stimuli. However, our general understanding of transport mechanisms and signaling pathways in complex systems is yet limited. Here we summarize the key network structures involved in transport, list the solutions available to overloaded systems for relaxing their load and outline a possible method for the computational determination of signaling pathways. We highlight that in addition to hubs, bridges and the network skeleton, the overlapping modular structure is also essential in network transport. Path‐lenghts in the module‐space of the yeast protein‐protein interaction network indicated that module‐based paths may cross fewer modular boundaries than shortest paths. Moreover, by locating network elements in the space of overlapping network modules and evaluating their distance in this ‘module space’, it may be possible to approximate signaling pathways computationally, which, in turn could serve the identification of signaling pathways of complex systems. Our model may be applicable in a wide range of fields including traffic control or drug design.
Purely electronic transport and localization in the Bose glass
Authors: Müller M. 
Published: June 2022

AbstractWe discuss transport and localization properties on the insulating side of the disorder dominated superconductor‐insulator transition, described in terms of the dirty boson model. Analyzing the spectral properties of the interacting bosons in the absence of phonons, we argue that the Bose glass phase admits three distinct regimes. For strongest disorder the boson system is a fully localized, perfect insulator at any temperature. At smaller disorder, only the low temperature phase exhibits perfect insulation while delocalization takes place above a finite temperature. We argue that a third phase must intervene between these perfect insulators and the superconductor. This conducting Bose glass phase is characterized by a mobility edge in the many body spectrum, located at finite energy above the ground state. In this insulating regime purely electronically activated transport occurs, with a conductivity following an Arrhenius law at asymptotically low temperatures, while a tendency to superactivation is predicted at higher T. These predictions are in good agreement with recent transport experiments in highly disordered films of superconducting materials.
Optically driven Mott‐Hubbard systems out of thermodynamic equilibrium
Authors: Lubatsch A. , Kroha J. 
Published: June 2022

AbstractWe consider the Hubbard model at half filling, driven by an external, stationary laser field. This stationary, but periodic in time, electromagnetic field couples to the charge current, i.e. it induces an extra contribution to the hopping amplitude in the Hubbard Hamiltonian (photo‐induced hopping). We generalize the dynamical mean‐field theory (DMFT) for nonequilibrium with periodic‐in‐time external fields, using a Floquet mode representation and the Keldysh formalism. We calculate the non‐equilibrium electron distribution function, the density of states and the optical DC conductivity in the presence of the external laser field for laser frequencies above and below the Mott‐Hubbard gap. The results demonstrate that the system exhibits an insulator‐metal transition as the frequency of the external field is increased and exceeds the Mott‐Hubbard gap. This corresponds to photo‐induced excitations into the upper Hubbard band.
Non‐linear conductivity in Coulomb glasses
Authors: Caravaca M. , Voje A. , Bergli J. , Ortuño M. , Somoza A.M. 
Published: June 2022

AbstractWe have studied the nonlinear conductivity of two‐dimensional Coulomb glasses. We have used a Monte Carlo algorithm to simulate the dynamic of the system under an applied electric field E. We have compared results for two different models: a regular square lattice with only diagonal disorder and a random array of sites with diagonal and off‐diagonal disorder. We have found that for moderate fields the logarithm of the conductivity is proportional to

reproducing experimental results. We have also found that in the nonlinear regime the site occupancy in the Coulomb gap follows a Fermi‐Dirac distribution with an effective temperature Teff higher than the phonon bath temperature T.
Light transport and correlation length in a random laser
Authors: Frank R. , Lubatsch A. , Kroha J. 
Published: June 2022

AbstractA random laser is a strongly disordered, laser‐active optical medium. The coherent laser feedback, which has been demonstrated experimentally to be present in these systems beyond doubt, requires the existence of spatially localized photonic quasimodes. However, the origin of these quasimodes has remained controversial. We develop an analytical theory for diffusive random lasers by coupling the transport theory of the disordered medium to the semiclassical laser rate equations, accounting for (coherent) stimulated and (incoherent) spontaneous emission. From the causality of wave propagation in an amplifying, diffusive medium we derive a novel length scale which we identify with the average mode radius of the lasing quasi‐modes. We show that truly localized modes do not exist in the system without photon number conservation. However, we find that causality in the amplifying medium implies the existence of a novel, finite intensity correlation length which we identify with the average mode volume of the lasing quasimodes. We show further that the surface of the laser‐active medium is crucial in order to stabilize a stationary lasing state. We solve the laser transport theory with appropriate surface boundary conditions to obtain the spatial distributions of the light intensity and of the occupation inversion. The dependence of the intensity correlation length on the pump rate agrees with experimental findings.
Conductance distribution at criticality: one‐dimensional Anderson model with random long‐range hopping
Authors: Méndez A. , Gopar V. , Varga I. 
Published: June 2022

AbstractWe study numerically the conductance distribution function w(T) for the one‐dimensional Anderson model with random long‐range hopping described by the Power‐law Banded Random Matrix model at criticality. We concentrate on the case of two single‐channel leads attached to the system. We observe a smooth transition from localized to delocalized behavior in the conductance distribution by increasing b, the effective bandwidth of the model. Also, for b < 1 we show that w(ln T/Ttyp) is scale invariant, where Ttyp = exp 〈 ln T 〉 is the typical value of T. Moreover, we find that for T < Ttyp, w(ln T/Ttyp) shows a universal behavior proportional to (T/Ttyp)‐1/2.
Broken time‐reversal symmetry scattering at the Anderson transition
Authors: Alcazar A. , Méndez A. , Varga I. 
Published: June 2022

AbstractWe study numerically the statistical properties of some scattering quantities for the Power‐law Banded Random Matrix model at criticality in the absence of time‐reversal symmetry, with a small number of single‐channel leads attached to it. We focus on the average scattering matrix elements, the conductance probability distribution, and the shot noise power as a function of the effective bandwidth b of the model. We find a smooth transition from insulating‐ to metallic‐like behavior in the scattering properties of the model by increasing b. We contrast our results with existing random matrix theory predictions.
Organic glasses: cluster structure of the random energy landscape
Authors: Novikov S.V. 
Published: June 2022

AbstractAn appropriate model for the random energy landscape in organic glasses is a spatially correlated Gaussian field. We calculated the distribution of the average value of a Gaussian random field in a finite domain. The results of the calculation demonstrate a strong dependence of the width of the distribution on the spatial correlations of the field. Comparison with the simulation results for the distribution of the size of the cluster indicates that the distribution of an average field could serve as a useful tool for the estimation of the asymptotic behavior of the distribution of the size of the clusters for “deep” clusters where value of the field on each site is much greater than the rms disorder. We also demonstrate significant modification of the properties of energetic disorder in organic glasses at the vicinity of the electrode.
A noncommutative Friedman cosmological model
Authors: Heller M. 
Published: June 2022

AbstractThe closed Friedman cosmological model, based on noncommutative geometry, is presented. Two global effects exhibited by the model are discussed. The first effect is the “generation of matter out of geometry”. Gravitational field equation in this model has the form of the eigenvalue equation for the Einstein operator. It turns out that the eigenvalues of this operator reproduce components of the energy‐momentum tensor. The second effect concerns the existence of the initial and final singularities. Because of the strongly probabilistic character of the noncommutative dynamics on the fundamental level, although singularities do exist, they are probabilistically irrelevant.
Can singularities be avoided in quantum cosmology?
Authors: Kiefer C. 
Published: June 2022

AbstractMany cosmological models based on general relativity contain singularities. In this contribution I address the question whether consistent models without singularities can exist in quantum cosmology. The discussion is based on the Wheeler–DeWitt equation of quantum geometrodynamics. The models under consideration are motivated by recent discussions of dark energy. Employing some natural criteria of singularity avoidance in the quantum theory, I show that this can indeed happen in these models.