Spin-orbit coupling induced quantum droplet in ultracold Bose-Fermi mixtures

TL;DR

This paper introduces a novel mechanism for creating quantum droplets in heteronuclear ultracold atomic mixtures by applying synthetic spin-orbit coupling, which suppresses Fermi pressure and enables droplet formation even with weak attractions.

Contribution

The study demonstrates that spin-orbit coupling can induce quantum droplets in Bose-Fermi mixtures, revealing a new pathway for droplet formation through single-particle physics manipulation.

Findings

Spin-orbit coupling suppresses Fermi pressure enabling droplet formation.

Droplet density ratios depend universally on SOC strength.

Droplets occur in the mean-field collapsing regime with negative fluctuation energy.

Abstract

Quantum droplets have intrigued much attention recently in view of their successful observations in the ultracold homonuclear atoms. In this work, we demonstrate a new mechanism for the formation of quantum droplet in heteronuclear atomic systems, i.e., by applying the synthetic spin-orbit coupling(SOC). Take the Bose-Fermi mixture for example, we show that by imposing a Rashba SOC between the spin states of fermions, the greatly suppressed Fermi pressure can enable the formation of Bose-Fermi droplets even for very weak boson-fermion attractions, which are insufficient to bound a droplet if without SOC. In such SOC-induced quantum droplets, the boson/fermion density ratio universally depends on the SOC strength, and they occur in the mean-field collapsing regime but with a negative fluctuation energy, distinct from the interaction-induced droplets found in literature. The accessibility of these Bose-Fermi droplets in ultracold Cs-Li and Rb-K mixtures is also discussed. Our results shed light on the droplet formation in a vast class of heteronuclear atomic systems through the manipulation of single-particle physics.