Title: Tetramer Bound States in Heteronuclear Systems,
Abstract: We calculate the universal spectrum of trimer and tetramer states in
heteronuclear mixtures of ultracold atoms with different masses in the vicinity
of the heavy-light dimer threshold. To extract the energies, we solve the
three- and four-body problem for simple two- and three-body potentials tuned to
the universal region using the Gaussian expansion method. We focus on the case
of one light particle of mass $m$ and two or three heavy bosons of mass $M$
with resonant heavy-light interactions. We find that trimer and tetramer cross
into the heavy-light dimer threshold at almost the same point and that as the
mass ratio $M/m$ decreases, the distance between the thresholds for trimer and
tetramer states becomes smaller. We also comment on the possibility of
observing exotic three-body states consisting of a dimer and two atoms in this
region and compare with previous work.