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Canadian Journal of Physics

Bose-Einstein condensates and thermal field theory

E Kovalchuk, R. Kobes

Published Jun 1, 2007• 0 citations
Publisher

Abstract

We consider a homogeneous nonideal Bose gas in equilibrium at nonzero temperature below the critical temperature Tc in the framework of thermal field theory. Quantum corrections up to second order are calculated, which are shown to be crucial in obtaining the correct behavior in thermodynamic quantities such as the specific heat. PACS Nos.: 67.40.–w, 11.10.Wx

DOI
10.1139/p07-058

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We consider a homogeneous nonideal Bose gas in equilibrium at nonzero temperature below the critical temperature Tc in the framework of thermal field theory. Quantum corrections up to second order are calculated, which are shown to be crucial in obtaining the correct behavior in thermodynamic quantities such as the specific heat. PACS Nos.: 67.40.–w, 11.10.Wx
We introduce a time-dependent projected Gross-Pitaevskii equation to describe a partially condensed homogeneous Bose gas, and find that this equation will evolve randomized initial wave functions to equilibrium. We compare our numerical data to the predictions of a gapless, second order theory of Bose-Einstein condensation [S. A. Morgan, J. Phys. B 33, 3847 (2000)], and find that we can determine a temperature when the theory is valid. As the Gross-Pitaevskii equation is nonperturbative, we expect that it can describe the correct thermal behavior of a Bose gas as long as all relevant modes are highly occupied. Our method could be applied to other boson fields.
We consider a homogeneous non-ideal Bose gas at nonzero temperature in equilibrium below the critical temperature TCT_CTC​ in the framework of finite temperature field theory. An algorithm is described in which a manageable subset of diagrams can be considered which, coupled with a self--consistent condition related to the minimization of the free energy, can be used to obtain physically reasonable results.
For the relativistic Bose gas in the gravitational field,the Bose-Einstein condensation(BEC) and the thermodynamic properties are investigated using the local density approximation(LDA).The thermodynamic quantities are calculated in the non-relativistic(NR) and ultra-relativistic(UR) limits,including the transition temperature,the ground state fraction,the energy and the heat capacity of the system.We also discussed the continuity of the heat capacity at the critical temperature.It is found that the effect of the relativistic results in the increase of the gap of the heat capacity.
Condensation of the weakly nonideal gas in an anisotropic parabolic trap was considered. The first order (with respect to the interaction parameter) correction Δ 0 = 0 ( T )— 0 0 ( T ) to the temperature dependence of the ground state population 0 0 ( T ) of the ideal gas atoms was found using thermodynamic perturbation theory. The correction found 0 proved to be much larger than the one calculated within the mean field theory [see Giorgini S, Pitaevskii L P, Stringari S Phys. Rev. A 54 R4633 (1996)] at temperatures below critical. This correction tends to zero as the temperature approaches the critical one; i.e., the interaction of the atoms does not lead to variation in the critical temperature. A simple analytic expression was found that describes the smooth variation of 0 0 ( T ) as it approaches zero in the vicinity of the critical temperature. A new criterion for applicability of the thermodynamic limit is given.
This paper presents a study of the grand canonical Bose-Einstein (BE) statistics for a finite number of particles in an arbitrary quantum system. The thermodynamical quantities that identify BE condensation---namely, the fraction of particles in the ground state and the specific heat---are calculated here exactly in terms of temperature and fugacity. These calculations are complemented by a numerical calculation of fugacity in terms of the number of particles, without taking the thermodynamic limit. The main advantage of this approach is that it does not rely on approximations made in the vicinity of the usually defined critical temperature, rather it makes calculations with arbitrary precision possible, irrespective of temperature. Graphs for the calculated thermodynamical quantities are presented in comparison to the results previously obtained in the thermodynamic limit. In particular, it is observed that for the gas trapped in a three-dimensional box, the derivative of specific heat reaches smaller values than what was expected in the thermodynamic limit---here, this result is also verified with analytical calculations. This is an important result for understanding the role of the thermodynamic limit in phase transitions and makes possible to further study BE statistics without relying neither on the thermodynamic limit nor on approximations near critical temperature.