Critical velocities for a superfluid in a periodic potential

TL;DR

This paper identifies two distinct critical velocities in a superfluid within a periodic potential, expanding understanding beyond the single critical velocity in homogeneous superfluids, with implications for BECs and supersolids.

Contribution

It introduces and characterizes two different critical velocities for superfluids in periodic potentials, a novel concept not previously detailed.

Findings

Two critical velocities are identified: inside and trawler velocities.

The inside critical velocity pertains to impurity motion without friction.

The trawler critical velocity relates to lattice movement maintaining superfluidity.

Abstract

In contrast to the homogeneous superfluid which has only one critical velocity, there exist two critical velocities for a superfluid in a periodic potential. The first one, which we call inside critical velocity, is for a macroscopic impurity to move frictionlessly in the periodic superfluid system; the second, which is called trawler critical velocity, is the largest velocity of the lattice relative to the lab frame for the superfluidity to maintain. The results are relevant to the superfluidity observed in the Bose-Einstein condensate in an optical lattice and supersolid helium.