Thermodynamics of dilute Bose gases: Beyond mean-field theory for binary mixtures of Bose-Einstein condensate

TL;DR

This paper develops an advanced theoretical framework to analyze the thermodynamics of binary Bose-Einstein condensate mixtures, emphasizing the effects of quantum and thermal fluctuations on phase behavior and superfluid properties.

Contribution

It introduces a beyond mean-field Popov theory for binary Bose mixtures, incorporating fluctuations in density and spin channels to better understand their thermodynamic and phase properties.

Findings

Thermal fluctuations influence the miscibility, promoting phase separation at finite temperature.

The theory predicts the temperature dependence of the superfluid behavior and the Andreev-Bashkin effect.

Asymmetry in interactions and masses affects the mixture's thermodynamic stability.

Abstract

We study the thermodynamic properties of binary Bose mixtures, by developing a beyond mean-field Popov theory which properly includes the effects of quantum and thermal fluctuations in both the density and spin channels. Results for key thermodynamic quantities, such as the isothermal compressibility and the magnetic susceptibility, are derived from a perturbative calculation of the grand-canonical potential. We find that thermal fluctuations can play a crucial role on the miscibility condition of a binary mixture, favoring phase separation at finite temperature even if the mixture is soluble at zero temperature, as already anticipated in a previous work [Ota et al., Phys. Rev. Lett. 123, 075301 (2019)]. We further investigate the miscibility condition for binary mixtures in the presence of asymmetry in the intra-species interactions, as well as in the masses of the two components. Furthermore, we discuss the superfluid behavior of the mixture and the temperature dependence of the Andreev-Bashkin effect.