Transport of Spin and Mass at Normal-Superfluid Interfaces in the Unitary Fermi Gas

Transport in strongly interacting Fermi gases provides a window into the non-equilibrium behavior of strongly correlated fermions. In particular, the interface between a strongly polarized normal gas and a weakly polarized superfluid at finite temperature presents a model for understanding transport at normal-superfluid and normal-superconductor interfaces. An excess of polarization in the normal phase or a deficit of polarization in the superfluid brings the system out of equilibrium, leading to transport currents across the interface. We implement a phenomenological mean-field model of the unitary Fermi gas, and investigate the transport of mass and spin across the interface under non-equilibrium conditions. We calculate the spin current and show how it can be understood in terms of the threshold for creating excitations in the superfluid. We find that a large net (mass) current acts to dilute excess polarization in the normal region, and show that the net current results primarily from reverse Andreev reflection.