arXiv subjects

A. I. Bugrij

Publications and source records attributed to A. I. Bugrij.

14 recordsLinked to original sources

arXiv2020

On the theory of ideal Bose-gas at a finite particle number

The ideal Bose-gas with finite number $N$ of particles is investigated. The exact expressions for the partition functions and occupation numbers in the grand canonical, canonical and microcanonical ensembles are found. The asymp\-totic expressions (in the case $N\gg1$) for the partition functions and occupation numbers in the canonical and microcanonical ensembles are evaluated. It is shown that the chemical potential $μ$ of the ideal Bose-gas can lie in the range $-\infty<μ<\infty$ oppositely to the widely adopted opinion that the value of this potential is negative.

cond-mat.stat-mech↗

arXiv2014

On the theory of inhomogeneous Bose-Einstein condensation of magnons in yttrium garnet

The Bose-Einstein condensation (BEC) of magnons created by a strong pumping in ferromagnetic thin films of yttrium iron garnet used as systems of finite size is considered analytically. Such a peculiarity, typical for this magnetic material, as the presence of a minimum in the spectrum of spin waves at a finite value of the wave vector is taken into account. The definition of hightemperature BEC is introduced and its characteristics are discussed. A role of boundary conditions for spin variables is analyzed, and it is shown that in the case of free spins on the boundary the magnon lattice can form in the system. The factors responsible for its appearance are discussed.

cond-mat.quant-gas↗

arXiv2009

On the theory of Bose-condensate fluctuations in finite size systems

An asymptotic expansions for the grand partition function of ideal Bose gas in the canonical ensemble with arbitrary number of particles is obtained. It is shown that the expressions found are valid in the whole temperature region, the critical temperature included. A comparison between the asymptotic formulas for Bose-condensate fluctuations and the exact ones is carried out and their quantitative agreement is established.

cond-mat.supr-con↗

arXiv2009

The peculiarities of Bose-condensation of quasiparticles

An attempt is made to consider the difference between the proccesses of the Bose-Einstein condensation of particles and quasiparticles. An equation for particle number of the Bose-condensate as a function of the total number of particles in the system is obtained. This equation is also written for the caseof quasiparticles with taking into account their creation by pumping and the existence ofequilibriumthermal excitations in the system. From the analysis of both these equations the chemical potential of pumped quasiparticles and their number in the condensate as a function of pumping intensity is found. It is shown that for low energy quasiparticles the proccess of Bose-condensation begins and proceeds at arbitrary temperatures high enough included.

cond-mat.stat-mech↗

arXiv2007

Correlation function of the two-dimensional Ising model on a finite lattice. II

We calculate the two-point correlation function and magnetic susceptibility in the anisotropic 2D Ising model on a lattice with one infinite and the other finite dimension, along which periodic boundary conditions are imposed. Using exact expressions for a part of lattice form factors, we propose the formulas for arbitrary spin matrix elements, thus providing a possibility to compute all multipoint correlation functions in the anisotropic Ising model on cylindrical and toroidal lattices. The scaling limit of the corresponding expressions is also analyzed.

nlin.SI↗

arXiv2007

Spin matrix elements in 2D Ising model on the finite lattice

We present explicit formulas for all spin matrix elements in the 2D Ising model with the nearest neighbor interaction on the finite periodic square lattice. These expressions generalize the known results [Phys. Rev. D19, (1979), 2477--2479; hep-th/0107117; hep-th/0112167] (coincide with them in the appropriate limits) and fulfill the test of straightforward transfer matrix calculations for finite $N$.

nlin.SI↗

arXiv2007

Magnetic susceptibility of the 2D Ising model on a finite lattice

Form factor representation of the correlation function of the 2D Ising model on a cylinder is generalized to the case of arbitrary disposition of correlating spins. The magnetic susceptibility on a lattice, one of whose dimensions ($N$) is finite, is calculated in both para- and ferromagnetic regions of parameters of the model. The singularity structure of the susceptibility in the complex temperature plane at finite values of $N$ and the thermodynamic limit $N\to\infty$ are discussed.

hep-th↗

arXiv2007

On the theory of Bose-Einstein condensation of quasiparticles: on the possibility of condensation of magnons at high temperatures

Bose condensation of quasiparticles in physical systems of finite size iz considered for the case of ferromagnetic thin films. It is shown that in accordance with present-day experimental capabilities which permit one to achieve densities of long-wave spin excitation of 1018-1019 cm-3, in such films the formation of condensate of such quasiparticles begins at temperatures T~100 K (room one including). It is found that Bose condensation is accompanied by scaling phenomenon according to which the main thermodynamic variable is not the number N of particles but the ratio N/T. This indicates that condensation of magnons can be observed at relatively low magnon densities. The roles played by the shape of the spectrum and film thickness are analyzed.

cond-mat.stat-mech↗

arXiv2001

Form factor representation of the correlation function of the two dimensional Ising model on a cylinder

The correlation function of the two dimensional Ising model with the nearest neighbours interaction on the finite size lattice with the periodical boundary conditions is derived. The expressions similar to the form factor expansion are obtained both for the paramagnetic and ferromagnetic regions of coupling parameter. The peculiarities caused by finite size are analyzed. The scaling limit of the lattice form factor expansion is evaluated.

hep-th↗

arXiv2001

The correlation function in two dimensional Ising model on the finite size lattice. I

The correlation function of two dimensional Ising model with the nearest neighbours interaction on the finite size lattice with the periodical boundary conditions is derived. The expressions similar to the form factor representation are obtained both for the ferromagnetic and paramagnetic regions of coupling constant. The peculiarities caused by the finite size of the system are discussed. The asymptotic behaviour of the corresponding quantities as functions of size is calculated. Some generalization of the obtained result is conjectured.

hep-th↗

arXiv1997

Duality in 2D Spin Models on Torus

Method of derivation of the duality relations for two-dimensional Z(N)-symmetric spin models on finite square lattice wrapped on the torus is proposed. As example, exact duality relations for the nonhomogeneous Ising model (N=2) and the Z(N)-Berezinsky-Villain model are obtained.

hep-th↗

arXiv1995

Three-loop contributions to the free energy of $λφ^4$ QFT

The massive scalar field with $λφ^4$ interaction placed in $(3+1)$ dimensional box is considered. The sizes of the box are $V\times β$ $(V=L^3$ is the volume, $T=1/β$ is the temperature). The free energy is evaluated up to the 2nd order of $PT$. The averaging on the vacuum fluctuations is separated from the averaging on the thermal fluctuations explicitly. As result the free-energy is expressed through the scattering amplitudes. We find that in 3-loop approximation the expression for free energy coincides with the ansatz of Bernstein, Dashen, Ma suggested on the base of $S$-matrix formulation of statistical mechanics. The obtained expressions are generalized for higher order of $PT$.

hep-th↗

arXiv1995

$(l,q)$-Deformed Grassmann Field and the Two-dimensional Ising Model

In this paper we construct the exact representation of the Ising partition function in the form of the $ SL_q(2,R)$-invariant functional integral for the lattice free $(l,q)$-fermion field theory ($l=q=-1$). It is shown that the $(l,q)$-fermionization allows one to re-express the partition function of the eight-vertex model in external field through functional integral with four-fermion interaction. To construct these representations, we define a lattice $(l,q,s)$-deformed Grassmann bispinor field and extend the Berezin integration rules to this field. At $l=q=-1, s=1$ we obtain the lattice $(l,q)$-fermion field which allows us to fermionize the two-dimensional Ising model. We show that the Gaussian integral over $(q,s)$-Grassmann variables is expressed through the $(q,s)$-deformed Pfaffian which is equal to square root of the determinant of some matrix at $q=\pm 1, s=\pm 1$.

hep-th↗

arXiv1995

$S$-matrix representation of the finite temperature propagator in $λϕ^4$-QFT

The two-point Green function of the massive scalar $(3+1)$-quantum field theory with $λϕ^4$ interaction at finite temperature is evaluated up to the 2nd order of perturbation theory. The averaging on the vacuum fluctuations is separated from the averaging on the thermal fluctuations explicitly. As a result, the temperature dependent part of the propagator is expressed through the scattering amplitudes. The obtained expression is generalized for higher orders of perturbation theory.

hep-th↗