Tetramer Bound States in Heteronuclear Systems
  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.
