Ultradilute self-bound quantum droplets in Bose-Bose mixtures at finite temperature

TL;DR

This paper explores how finite temperature influences the structure and excitations of self-bound quantum droplets in Bose-Bose mixtures, revealing significant effects on stability, phase transitions, and excitation spectra.

Contribution

It provides a theoretical analysis of temperature effects on quantum droplets, including phase diagrams and excitation spectrum modifications, extending zero-temperature models.

Findings

Critical particle number increases with temperature.

Self-evaporation region shrinks and disappears at high temperatures.

All collective modes soften at the droplet-to-gas transition.

Abstract

We theoretically investigate the finite-temperature structure and collective excitations of a self-bound ultradilute Bose droplet in a flat space realized in a binary Bose mixture with attractive inter-species interactions on the verge of mean-field collapse. As the droplet formation relies critically on the repulsive force provided by Lee-Huang-Yang quantum fluctuations, which can be easily compensated by thermal fluctuations, we find a significant temperature effect in the density distribution and collective excitation spectrum of the Bose droplet. A finite-temperature phase diagram as a function of the number of particles is determined. We show that the critical number of particles at the droplet-to-gas transition increases dramatically with increasing temperature. Towards the bulk threshold temperature for thermally destabilizing an infinitely large droplet, we find that the excitation-forbidden, self-evaporation region in the excitation spectrum, predicted earlier by Petrov using a zero-temperature theory, shrinks and eventually disappears. All the collective excitations, including both surface modes and compressional bulk modes, become softened at the droplet-to-gas transition. The predicted temperature effects of a self-bound Bose droplet in this work could be difficult to measure experimentally due to the lack of efficient thermometry at low temperatures. However, these effects may already present in the current cold-atom experiments.