arXiv subjects

George Alexandru Nemnes

Publications and source records attributed to George Alexandru Nemnes.

23 records · Page 2Linked to original sources

arXiv2014

Ab initio investigation of optical properties in triangular graphene - boron nitride core-shell nanostructures

We calculate the optical properties of atomic-sized core-shell graphene - boron nitride nanoflakes with triangular shaped crossection using the density functional theory. The optical properties can be tuned by using different sizes and proportions of the core-shell materials. Anisotropic effects manifested in the absorption of unpolarized light with different orientations of the optical vector are pointed out.

cond-mat.mes-hall↗

arXiv2014

The BCS theory amended

In the standard theory of superconductivity a quasiparticle excitation changes the energy of the system by the quasiparticle energy. But the number of excitations determine also the gap energy which further determines the energy of the condensate and the energy spectrum of quasiparticles. Such extra contributions to the total energy of the system--which are not taken into account in the standard formalism--led us to a redefinition of the quasiparticle energies and of the quasiparticle populations by the usual method of the maximization of the superconductor partition function. The result of this correction is a critical temperature which is lower than the BCS critical temperature and a finite jump of the energy gap at the phase transition. The discontinuity of the energy gap at the critical temperature marks a first order phase transition which is also in disagreement with the standard BCS interpretation. The fact that the standard BCS formalism is theoretically inconsistent is revealed in the calculation of the heat capacity and of the internal energy. These inconsistencies are removed in our formulation. The purpose of our paper is not to adjust the theory to better describe the phenomenology of superconductivity, but to reevaluate and amend the BCS formalism.

cond-mat.supr-con↗

arXiv2014

Fractional exclusion statistics in disordered interacting particle systems

We develop a model based on the fractional exclusion statistics (FES) applicable to non-homogeneous interacting particle systems. Here the species represent elementary volumes in an (s+1)-dimensional space, formed by the direct product between the s-dimensional space of positions and the quasiparticle energy axis. The model is particularly suitable for systems with localized states. We prove the feasibility of our method by applying it to systems of different degrees of complexities. We first apply the formalism on simpler systems, formed of two sub-systems, and present numerical and analytical thermodynamic calculations, pointing out the quasiparticle population inversion and maxima in the heat capacity, in contrast to systems with only diagonal (direct) FES parameters. Further we investigate larger, non-homogeneous systems with repulsive screened Coulomb interactions, indicating accumulation and depletion effects at the interfaces. Finally, we consider systems with several degrees of disorder, which are prototypical for models with glassy behavior. We find that the disorder produces a spatial segregation of quasiparticles at low energies which significantly affects the heat capacity and the entropy of the system.

cond-mat.dis-nn↗

arXiv2013

Equivalence between fractional exclusion statistics and self-consistent mean-field theory in interacting particle systems in any number of dimensions

We describe a mean field interacting particle system in any number of dimensions and in a generic external potential as an ideal gas with fractional exclusion statistics (FES). We define the FES quasiparticle energies, we calculate the FES parameters of the system and we deduce the equations for the equilibrium particle populations. The FES gas is "ideal", in the sense that the quasiparticle energies do not depend on the other quasiparticle levels populations and the sum of the quasiparticle energies is equal to the total energy of the system. We prove that the FES formalism is equivalent to the semi-classical or Thomas Fermi limit of the self-consistent mean-field theory and the FES quasiparticle populations may be calculated from the Landau quasiparticle populations by making the correspondence between the FES and the Landau quasiparticle energies. The FES provides a natural semi-classical ideal gas description of the interacting particle gas.

cond-mat.stat-mech↗

arXiv2010

Stochastic simulations for the time evolution of systems which obey generalized statistics: Fractional exclusion statistics and Gentile's statistics

We present a stochastic method for the simulation of the time evolution in systems which obey generalized statistics, namely fractional exclusion statistics and Gentile's statistics. The transition rates are derived in the framework of canonical ensembles. This approach introduces a tool for describing interacting fermionic and bosonic systems in non-equilibrium as ideal FES systems, in a computationally efficient manner. The two types of statistics are analyzed comparatively, indicating their intrinsic thermodynamic differences and revealing key aspects related to the species size.

cond-mat.stat-mech↗