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Bulletin of the Lebedev Physics Institute

On the thermodynamics of degenerate Bose gas with delta-shaped interaction potential

Viktor Borisovich Bobrov, Anatolij G. Zagorodny, S. A. Trigger

Published Dec 1, 2016• 0 citations
Publisher

Abstract

Within the self-consistent Hartree–Fock approximation, the equilibrium weakly nonideal Bose gas with a delta-shaped interaction potential in the presence of the Bose–Einstein condensate is considered without using quasi-averages. On this basis, using the virial theorem and diagram techniques of the perturbation theory for the equilibrium system in a macroscopic volume, the equation of state providing the isothermal compressibility finiteness, including the Bose–Einstein condensate domain of existence, is obtained.

DOI
10.3103/s106833561612006x

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Within the self-consistent Hartree–Fock approximation, the equilibrium weakly nonideal Bose gas with a delta-shaped interaction potential in the presence of the Bose–Einstein condensate is considered without using quasi-averages. On this basis, using the virial theorem and diagram techniques of the perturbation theory for the equilibrium system in a macroscopic volume, the equation of state providing the isothermal compressibility finiteness, including the Bose–Einstein condensate domain of existence, is obtained.
A formalism is proposed for describing an equilibrium Bose gas based on consideration of a macroscopic but finite volume. The presence of a Bose-Einstein condensate is taken into account through an off-diagonal long-range order in the transition to the thermodynamic limit. On this basis, degenerate Bose gases with delta-shaped interaction potentials are described in terms of a self-consistent Hartree-Fock approximation. Explicit expressions are obtained for the energy spectra of single-particle and collective excitations.
We investigate the thermodynamic properties of an interacting Bose gas with a condensate within the energy-functional formulation of the Hartree-Fock-Bogoliubov (HFB) approach. For a contact interaction, we derive a self-consistent solution to the HFB equations that intrinsically eliminates divergence. This solution characterizes the equilibrium state featuring a condensate of correlated pairs of particles. We analyze the temperature dependence of key thermodynamic quantities such as condensate density, chemical potential, entropy, pressure, specific heat capacity at constant volume, and isothermal compressibility and compare them with predictions from the Popov approximation (PA). We predict that the transition temperature shifts to higher values due interactions, with the HFB approach yielding a larger shift than the PA. Analysis of the compressibility indicates that a pure pair condensate is unstable, and the stable equilibrium corresponds to only a mixture of single-particle and pair condensates.
The ground-state energy of a finite inhomogeneous system of bosons located in a scalar external field was found within the framework of the self-consistent Hartree-Fock approximation, on the basis of the representation of the second quantization without using the formalism of anomalous averages. The ground-state wave function corresponds to the stationary Gross-Pitaevskii equation for the Bose-Einstein condensate wave function. It was shown that the ground-state energy can be found using the energy determined by the stationary Gross-Pitaevskii equation only for a system that satisfies the thermodynamic limit.
In this work, we build upon the theoretical framework developed in our previous study [N. V. Thu and D. T. Pham, Phys. Lett. A 523, 129787 (2024)], where the Cornwall–Jackiw–Tomboulis effective action formalism was applied to a dilute Bose gas, to investigate its thermodynamic properties in the condensed phase at sufficiently low temperatures within the improved Hartree–Fock approximation. We focus on the analyses of the chemical potential, pressure, energy density and specific heat at constant volume, which are derived within a unified, self-consistent and gapless framework. The results obtained show excellent agreement with those predicted by the Hartree–Fock–Bogoliubov theory in the appropriate limits. Remarkably, our results demonstrate universal behavior for the reduced thermodynamic quantities, and a comparison with available experimental data for the chemical potential is made.
A self-consistent field model for a mixture of Bose and Fermi particles is formulated. There is explored in detail the case of a delta-like interaction, for which the thermodynamic functions are obtained, and Bose–Einstein condensation of interacting particles in the presence of the admixture of fermions is studied. It is shown that at a fixed total density, the admixture of Fermi particles leads to reducing of the temperature of Bose–Einstein condensation and smoothing of features of thermodynamic quantities at the transition temperature. The regions of stability of the spatially uniform states of a mixture are found. As in the case of a pure Bose system, in the state of a mixture with condensate the dependence of the thermodynamic potential on the interaction constant between Bose particles has a nonanalytic character, so that it proves impossible to develop the perturbation theory in the magnitude of interaction of Bose particles.