Quantum Droplets in a Mixture of Bose–Fermi SuperfluidsSupported by the National Natural Science Foundation of China under Grant Nos. 11974331, 11421092 and 11534014, and the National Key Research and Development Program of China under Grant Nos. 2016YFA0301700, 2017YFA0304100, 2018YFA0307600 and 2016YFA0300603.

Abstract

We study the formation of quantum droplets in the mixture of a single-component Bose–Einstein condensate (BEC), and a two-species Fermi superfluid across a wide Feshbach resonance. With repulsive boson-boson and attractive boson-fermion interactions, we show that quantum droplets can be stabilized by attractive fermion-fermion interactions on the Bardeen–Cooper–Schrieffer (BCS) side of the resonance, and can also exist in the deep BEC regime under weak boson-fermion interactions. We map out the phase diagram for stable droplets with respect to the boson-boson and boson-fermion interactions, and discuss the role of different types of quantum fluctuations in the relevant regions of the BCS-BEC crossover. Our work reveals the impact of fermion pairing on the formation of quantum droplets in Bose–Fermi mixtures, and provides a useful guide for future experiments.