Quantum droplets in a resonant Bose-Fermi mixture

TL;DR

This paper demonstrates the existence of self-bound quantum droplets in resonant Bose-Fermi mixtures, revealing new quantum phases and phase transitions driven by boson-fermion interactions at zero temperature.

Contribution

It introduces a versatile variational ansatz capturing polaron limits and predicts quantum droplets and phase separation in Bose-Fermi mixtures with experimentally relevant parameters.

Findings

Quantum droplets can form in strongly interacting regimes.

Phase separation occurs at higher fermion densities.

First-order quantum phase transitions are significant in the phase diagram.

Abstract

We study the canonical problem of a Fermi gas interacting with a weakly repulsive Bose-Einstein condensate at zero temperature. To explore the quantum phases across the full range of boson-fermion interactions, we construct a versatile variational ansatz that incorporates pair correlations and correctly captures the different polaron limits. Remarkably, we find that self-bound quantum droplets can exist in the strongly interacting regime, preempting the formation of boson-fermion dimers, when the Fermi pressure is balanced by the resonant boson-fermion attraction. This scenario can be achieved in experimentally available Bose-Fermi mixtures for a range of boson-fermion mass ratios in the vicinity of equal masses. We furthermore show that a larger fermion density instead yields phase separation between a Bose-Fermi mixture and excess fermions, as well as behavior reminiscent of a liquid-gas critical point. Our results suggest that first-order quantum phase transitions play a crucial role in the phase diagram of Bose-Fermi mixtures.