Self-bound Bose mixtures

TL;DR

This paper investigates the stability and properties of self-bound Bose mixtures beyond mean-field theory using a non-perturbative variational approach, revealing narrow stability conditions and deviations from universal behavior.

Contribution

It introduces a non-perturbative variational method to study dilute Bose mixtures, extending beyond previous mean-field and perturbative analyses.

Findings

Stable only in a narrow density ratio range

Liquid-gas phase separation occurs at spinodal instability

Deviations from universal scattering length dependence are significant

Abstract

Recent experiments confirmed that fluctuations beyond the mean-field approximation can lead to self-bound liquid droplets of ultra-dilute binary Bose mixtures. We proceed beyond the beyond-mean-field approximation, and study liquid Bose mixtures using the variational hypernetted-chain Euler Lagrange method, which accounts for correlations non-perturbatively. Focusing on the case of a mixture of uniform density, as realized inside large saturated droplets, we study the conditions for stability against evaporation of one of the components (both chemical potentials need to be negative) and against liquid-gas phase separation (spinodal instability), the latter being accompanied by a vanishing speed of sound. Dilute Bose mixtures are stable only in a narrow range near an optimal ratio $\rho_1/\rho_2$ and near the total energy minimum. Deviations from a universal dependence on the s-wave scattering lengths are significant despite the low density.