arXiv subjects

Dragos-Victor Anghel

Publications and source records attributed to Dragos-Victor Anghel.

29 records · Page 2Linked to original sources

arXiv2012

The anisotropic glassy properties of quasicrystals within an amended tunneling model

We use an extended version of the standard tunneling model to explain the anisotropic sound absorption in decagonal quasicrystals. The glassy properties are determined by an ensemble of two level systems (TLS), arbitrarily oriented. The TLS is characterized by a $3\times3$ symmetric tensor, which couples to the strain field through a $3\times3\times3\times3$ tensor of coupling constants, $[[R]]$. The structure of $[[R]]$ reflects the symmetry of the quasicrystal. We also analyze the probability distributions of the elements of $[T]$ in this particular model for a better understanding of the characteristics of "isotropic" and "anisotropic" orientations distributions of the ensemble of TLSs. We observe that the distribution of the elements is neither simple, nor intuitive and therefore it is difficult to guess it a priory, using qualitative arguments based on the symmetry properties.

cond-mat.mtrl-sci↗

arXiv2012

Fractional exclusion statistics -- the method to describe interacting particle systems as ideal gases

I show that if the total energy of a system of interacting particles may be written as a sum of quasiparticle energies, then the system of quasiparticles can be viewed in general as an ideal gas with fractional exclusion statistics (FES). The general method for calculating the FES parameters is also provided. The interacting particle system cannot be described as an ideal gas of Bose and Fermi quasiparticles except in trivial situations.

cond-mat.quant-gas↗

arXiv2012

The Fermi liquid theory with fractional exclusion statistics

The Fermi liquid theory may provide a good description of the thermodynamic properties of an interacting particle system when the interaction between the particles contributes to the total energy of the system with a quantity which may depend on the total particle number, but does not depend on the temperature. In such a situation, the ideal part of the Hamiltonian, i.e. the energy of the system without the interaction energy, also provides a good description of the system's thermodynamics. If the total interaction energy of the system, being a complicated function of the particle populations, is temperature dependent, then the Landau's quasiparticle gas cannot describe accurately the thermodynamics of the system. A general solution to this problem is presented in this paper, in which the quasiparticle energies are redefined in such a way that the total energy of the system is identical to the sum of the energies of the quasiparticles. This implies also that the thermodynamic properties of the system and those of the quasiparticle gas are identical. By choosing a perspective in which the quasiparticle energies are fixed while the density of states along the quasiparticle axis vary, we transform our quasiparticle system into an ideal gas which obey fractional exclusion statistics.

cond-mat.quant-gas↗

arXiv2012

Fractional exclusion statistics applied to relativistic nuclear matter

The effect of statistics of the quasiparticles in the nuclear matter at extreme conditions of density and temperature is evaluated in the relativistic mean-field model generalized to the framework of the fractional exclusion statistics (FES). In the model, the nucleons are described as quasiparticles obeying FES and the model parameters were chosen to reproduce the ground state properties of the isospin-symmetric nuclear matter. In this case, the statistics of the quasiparticles is related to the strengths of the nucleon-nucleon interaction mediated by the neutral scalar and vector meson fields. The relevant thermodynamic quantities were calculated as functions of the nucleons density, temperature and fractional exclusion statistics parameter $α$. It has been shown that at high temperatures and densities the thermodynamics of the system has a strong dependence on the statistics of the particles. The scenario in which the nucleon-nucleon interaction strength is independent of the statistics of particles was also calculated, but it leads in general to unstable thermodynamics.

hep-ph↗

arXiv2010

Divergent specific heat at zero Kelvins: breakdown of the Third law of thermodynamics

Thermodynamics, the branch of physics concerned with the description of macroscopic bodies, heat exchange and the conversion of different forms of energy is based on four laws: the zeroth law, which states that bodies in thermal contact reach a state of thermal equilibrium, the first law, which postulates the energy conservation and establishes the way in which different forms of energy transform into each-other, the second law, which states that the entropy increases or stays constant in time in any isolated system, and, finally, the third law (or Nernst postulate), which states that the entropy of any system approaches a constant minimum when temperature approaches 0 K. One of the direct consequences of the third law is that the specific heat of any system converges to zero when its temperature decreases to zero Kelvins. Here we show that the third law is violated in a wide class of fermionic systems, by calculating the specific heat and showing that it diverges to infinity when the absolute zero temperature is approached. Until now, no exception to these four laws is know in physics. These findings ask for the revision of one of the most fundamental laws of nature.

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↗

arXiv2007

First Order Phase Transition in a Model for Generalized Statistics

A first order phase transition is found in a model which was introduced originally by Murthy and Shankar [Phys. Rev. B 60, 6517 (1999)] to describe systems of generalised exclusion statistics. I characterise the phase transition in the canonical and grandcanonical ensebles for the case when the statistical exclusion parameter is 1, which corresponds to the Fermi exclusion statistics. We observe that in the grandcanonical ensemble the phase transition has no latent heat, but it has a finite jump in the particle number. In canonical conditions--when the particle number is held fix--the internal energy is discontinuous at the transition.

cond-mat.stat-mech↗

arXiv2005

Fluctuations of the Fermi condensate in ideal gases

We calculate numerically and analytically the fluctuations of the fermionic condensate and of the number of particles above the condensate for systems of constant density of states. We compare the canonical fluctuations, obtained from the equivalent Bose condensate fluctuation, with the grandcanonical fermionic calculation. The fluctuations of the condensate are almost the same in the two ensembles, with a small correction comming from the total particle number fluctuation in the grandcanonical ensemble. On the other hand the number of particles above the condensate and its fluctuation is insensitive to the choice of ensemble.

cond-mat.stat-mech↗

arXiv2004

Canonical-grandcanonical ensemble in-equivalence in Fermi systems?

I discuss the effects of fermionic condensation in systems of constant density of states. I show that the condensation leads to a correction of the chemical potential and of the Fermi distribution in canonical Fermi systems at low temperatures. This implies that the canonical and grandcanonical ensembles are not equivalent even for Fermi systems.

cond-mat.stat-mech↗

arXiv2002

Capacitively coupled hot-electron nanobolometer as far-infrared photon counter

We show theoretically that hot-electron nanobolometers consisting of a small piece of normal metal, capacitively coupled to a superconducting antenna through a pair of normal metal--insulator--superconductor (NIS) tunnel junctions may be used as far-infrared photon counters. To make the device most effective at high counting rates, we suggest the use of the bolometer in the simplest configuration, when the NIS tunnel junctions are used as both an electron cooler and thermometer. The absorption of the photon in the normal metal produces a pulse in the electron temperature, which is measured by the NIS junctions. The counter may resolve photons up to 0.3--0.4 mm wavelength and has a typical re-equilibration time constant of about 20 ns.

cond-mat.mes-hall↗

arXiv2002

Universal behaviour of ideal and interacting quantum gases in two dimensions

I discuss ideal and interacting quantum gases obeying general fractional exclusion statistics. For systems with constant density of single-particle states, described in the mean field approximation, the entropy depends neither on the microscopic exclusion statistics, nor on the interaction. Such systems are called {\em thermodynamically equivalent} and I show that the microscopic reason for this equivalence is a one-to-one correspondence between the excited states of these systems. This provides a method, different from the bosonisation technique, to transform between systems of different exclusion statistics. In the last section the macroscopic aspects of this method are discussed. In Appendix A I calculate the fluctuation of the ground state population of a condensed Bose gas in grandcanonical ensemble and mean field approximation, while in Appendix B I show a situation where although the system exhibits fractional exclusion properties on microscopic energy intervals, a rigorous calculation of the population of single particle states reveals a condensation phenomenon. This also implies a malfunction of the usual and simplified calculation technique of the most probable statistical distributions.

cond-mat.stat-mech↗