Dissipation in a polariton superfluid beyond the Landau criterion

Polariton superfluids have a finite lifetime and are often created under a constant optical pumping in a semiconductor microcavity. The balance between the gain and loss of the particles in the superfluid generates a steady-state current. In this steady-state condition, the polariton superfluid experiences energy damping and is slowed down due to the collisions with the particles in a stationary reservoir that is comprised of free thermal electrons. When the reservoir is in relative motion with respect to the superfluid these collisions give rise to either an upward or a downward shift in the chemical potential of the superfluid depending on the direction of the relative motion. This phenomenon has been observed in recent experiments described in https://arxiv.org/abs/1808.07866.