Finite Momentum Pairing Instability of Band-Insulators With Multiple Bands

TL;DR

This paper demonstrates that multi-band band insulators with strong interband attraction are inherently unstable to forming a pair density wave superfluid state, with models applicable to cold atoms and solid-state systems.

Contribution

It introduces microscopic models showing that band insulators with multiple bands and interband attraction tend to develop nonzero momentum pairing, leading to a novel PDW superfluid phase.

Findings

Instability towards incommensurate PDW near Feshbach resonance

PDW instability analogous to exciton condensation in semiconductors

Experimental signatures discussed for detecting PDW state

Abstract

We show, based on microscopic models, that fermionic band insulators with multiple bands and strong interband attraction are generically unstable towards nonzero momentum Cooper pairing leading to a pair density wave (PDW) superfluid state. Our first model considers a band insulating state of fermionic atoms in a three-dimensional cubic optical lattice. We show that this insulator is unstable towards an incommensurate PDW in the vicinity of a Feshbach resonance. Our second model is a two-band tight binding model relevant to electrons in solids; we show that the insulating state of this model has a PDW instability analogous to the exciton condensation instability in indirect bandgap semiconductors. We discuss relevant experimental signatures of the PDW state.