Superfluidity in the interior-gap states

TL;DR

This paper examines the stability of interior-gap superfluid states, finding that gapless states are unstable and only fully gapped states resemble conventional BCS superfluids.

Contribution

It demonstrates the instability of gapless interior-gap superfluid states at weak coupling, clarifying conditions under which such states can exist.

Findings

Gapless interior-gap states are unstable at weak coupling.

Superfluid density is negative in the gapless regime.

Fully gapped interior-gap states resemble BCS superfluids.

Abstract

We investigate superfluidity in the interior-gap states proposed by Liu and Wilczek. At weak coupling, we find the {\em gapless} interior-gap state unstable in physically accessible regimes of the parameter space, where the superfluid density is shown to be always negative. We therefore conclude that the spatially-uniform interior-gap phase is extremely unstable unless it is fully gapped; in this case, however, the state is rather similar to conventional BCS states.