Superfluidity in the interior-gap states

Abstract
We investigate superfluidity in the interior-gap states proposed by Liu and Wilczek. At weak coupling, we find the 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.
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