Interlayer coherence in Superconductor Bilayers

TL;DR

This paper explores the coexistence of superconducting and excitonic orders in bilayer systems, revealing conditions for interlayer coherence and proposing experimental signatures through current-voltage measurements.

Contribution

It demonstrates, using mean-field theory, that strong interlayer repulsion can lead to simultaneous superconducting and excitonic orders with observable interlayer coherence effects.

Findings

Coexistence of superconducting and excitonic orders in bilayers.

Presence of non-zero interlayer pair amplitudes despite repulsion.

Predicted unique current-voltage signatures for experimental detection.

Abstract

We investigate the possibility and implications of interlayer coherence in electrically isolated superconducting bilayers. We find that in a mean-field approximation bilayers can have superconducting and excitonic order simultaneously if repulsive interactions between layers are sufficiently strong. The excitonic order implies interlayer phase coherence, and can be studied in a representation of symmetric and antisymmetric bilayer states. When both orders are present we find several solutions of the mean-field equations with different values of the the symmetric and antisymmetric state pair amplitudes. The mixed state necessarily has non-zero pair amplitudes for electrons in different layers in spite of the repulsive interlayer interactions, and these are responsible for spatially indirect Andreev reflection processes in which an incoming electron in one layer can be reflected as a hole in the opposite layer. We evaluate layer diagonal and off-diagonal current-voltage relationships that can be used to identify this state experimentally.