  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.
