We consider the ground state of a single “spin-down” impurity atom interacting attractively with a “spin-up” atomic Fermi gas. By constructing variational wave functions for polarons, molecules, and trimers, we perform a detailed study of the transitions between these dressed bound states as a function of mass ratio $r=m_{\uparrow }/m_{\downarrow }$ and interaction strength. Crucially, we find that the presence of a Fermi sea enhances the stability of the $p$-wave trimer, which can be viewed as a Fulde-Ferrell-Larkin-Ovchinnikov molecule that has bound an additional majority atom. For sufficiently large $r$, we find that the transitions lie outside the region of phase separation of the imbalanced Fermi gas and should thus be observable in experiment, unlike the well-studied equal-mass case.

• Received 3 May 2010

DOI:https://doi.org/10.1103/PhysRevLett.106.166404