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ζ-function regularization and the interacting Bose gas at low temperature

David J. Toms

Published Dec 31, 2002• 0 citations
Publisher

Abstract

Thermodynamic properties of a single and two species Bose gas were obtained using ζ-function techniques. Ground-state energy and pressure results were obtained at different temperatures. Results indicated that it was possible to obtain finite-temperature corrections to the zero-temperature results by the examination of thermodynamic potential.

DOI
10.25405/ncl.34605483

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Thermodynamic properties of a single and two species Bose gas were obtained using ζ-function techniques. Ground-state energy and pressure results were obtained at different temperatures. Results indicated that it was possible to obtain finite-temperature corrections to the zero-temperature results by the examination of thermodynamic potential.
$\ensuremath{\zeta}$-function methods are used to study the properties of the nonrelativistic interacting Bose gas at finite temperature and density. Results for the ground-state energy and pressure are obtained at both zero and finite temperatures. The method used does not restrict the form of the interaction, which can be completely general. A similar procedure is then applied to evaluate the ground-state energy of a binary mixture consisting of different bosons. Analytical results are obtained in the case where the two species have the same mass.
zeta-function methods are used to study the properties of the nonrelativistic interacting Bose gas at finite temperature and density. Results for the ground-state energy and pressure are obtained at both zero and finite temperatures. The method used does not restrict the form of the interaction, which can be completely general. A similar procedure is then applied to evaluate the ground-state energy of a binary mixture consisting of different bosons. Analytical results are obtained in the case where the two species have the same mass.
By using exact path-integral Monte Carlo methods we calculate the equation of state of an interacting Bose gas as a function of temperature both below and above the superfluid transition. The universal character of the equation of state for dilute systems and low temperatures is investigated by modeling the interatomic interactions using different repulsive potentials corresponding to the same sss-wave scattering length. The results obtained for the energy and the pressure are compared to the virial expansion for temperatures larger than the critical temperature. At very low temperatures we find agreement with the ground-state energy calculated using the diffusion Monte Carlo method.
Abstract: The one-loop effective potential for non-relativistic bosons with a delta function repulsive potential is calculated for a given chemical potential using functional methods. After renormalization and at zero temperature it reproduces the standard ground state energy and pressure as function of the particle density. At finite temperatures it is found necessary to include ring corrections to the one-loop result in order to satisfy the Goldstone theorem. It is natural to introduce an effective chemical potential directly related to the order parameter and which uniformly decreases with increasing temperatures. This is in contrast to the the ordinary chemical potential which peaks at the critical temperature. The resulting thermodynamics in the condensed phase at very low temperatures is found to be the same as in the Bogoliubov approximation where the degrees of freedom are given by the Goldstone bosons. At higher temperatures the ring corrections dominate and result in a critical temperature unaffected by the interaction. 1
The low-density expansions for the energy, chemical potential, and condensate depletion of the homogeneous dilute dipolar Bose gas are obtained by regularizing the dipole-dipole interaction at long distances. It is shown that the leading term, proportional to the density, allows a simple physical interpretation and consistently describes the thermodynamic stability of the system. The long-range asymptotics are obtained analytically for the normal and anomalous one-particle correlation functions and the pair distribution function. We discuss the properties of the two-body scattering with zero relative momentum for the dipole-dipole interaction; in particular, we derive the asymptotics of the wave function and a correction to the scattering length for small values of the dipolar range. We show how the density expansions can be derived within the Bogoliubov model of weakly interacting particles without any divergence from the assumption of universality of the expansions at low densities.
Now that the low temperature properties of quantum-mechanical many-body systems (bosons) at low density, ρρρ, can be examined experimentally it is appropriate to revisit some of the formulas deduced by many authors 4-5 decades ago. For systems with repulsive (i.e. positive) interaction potentials the experimental low temperature state and the ground state are effectively synonymous -- and this fact is used in all modeling. In such cases, the leading term in the energy/particle is 2πℏ2aρ/m2π\hbar^2 a ρ/m2πℏ2aρ/m where aaa is the scattering length of the two-body potential. Owing to the delicate and peculiar nature of bosonic correlations (such as the strange N7/5N^{7/5}N7/5 law for charged bosons), four decades of research failed to establish this plausible formula rigorously. The only previous lower bound for the energy was found by Dyson in 1957, but it was 14 times too small. The correct asymptotic formula has recently been obtained by us and this work will be presented. The reason behind the mathematical difficulties will be emphasized. A different formula, postulated as late as 1971 by Schick, holds in two-dimensions and this, too, will be shown to be correct. With the aid of the methodology developed to prove the lower bound for the homogeneous gas, two other problems have been successfully addressed. One is the proof by us that the Gross-Pitaevskii equation correctly describes the ground state in the `traps' actually used in the experiments. For this system it is also possible to prove complete Bose condensation, as we have shown. Another topic is a proof that Foldy's 1961 theory of a high density Bose gas of charged particles correctly describes its ground state energy.