Perturbing a persistent current with an external potential

Abstract

We consider the changes in the spectrum of elementary excitations of a flowing superfluid that result when a weak external perturbing potential is added. As a specific application, we study the critical velocity of superfluid flow in a slightly distorted, tight, toroidal trap for atoms. The critical velocity is not accurately predicted by the argument that the flow turns dissipative when the local flow velocity exceeds the local speed of sound somewhere in the fluid. The unusual critical velocity probably reflects the essentially one-dimensional physics of the model.