Heavy-Light Fermion Mixtures at Unitarity

We investigate fermion pairing in the unitary regime for a mass ratio corresponding to a ⁶Li-⁴⁰K mixture using quantum Monte Carlo methods. The ground-state energy and the average light- and heavy-particle excitation spectrum for the unpolarized superfluid state are nearly independent of the mass ratio. In the majority light system, the polarized superfluid is close to the energy of a phase separated mixture of nearly fully polarized normal and unpolarized superfluid. For a majority of heavy particles, we find an energy minimum for a normal state with a ratio of ∼3∶1 heavy to light particles. A slight increase in attraction to k_Fa≈2.5 yields a ground state energy of nearly zero for this ratio. A cold unpolarized system in a harmonic trap at unitarity should phase separate into three regions, with a shell of unpolarized superfluid in the middle.