Supercurrent induced resonant optical response

TL;DR

This paper develops a theory for how a supercurrent affects the optical conductivity in superconductors, revealing a current-dependent peak at the gap edge due to lattice effects, and explains recent experimental observations.

Contribution

It introduces a novel theoretical framework for supercurrent-induced optical responses, highlighting the role of lattice effects and explaining experimental findings in NbN.

Findings

Current induces a peak in optical conductivity at the gap edge.

Lattice effects enable current-dependent optical responses in superconductors.

The theory explains recent experimental observations in NbN.

Abstract

The optical conductivity encodes the current response to a time dependent electric field. We develop a theory of the optical conductivity $\sigma(\omega)$ in presence of a dc supercurrent. Current induced optical response is prohibited by Galilean invariance from occurring in systems with a single parabolic band. However, we show that lattice effects give rise to a pronounced current dependent peak in $\sigma(\omega)$ at the gap edge $\omega = 2 \Delta$, which diverges in the clean limit. We demonstrate this in a model of a multi-band superconductor. Our theory explains the recent observation of a current induced peak in the optical conductivity in NbN by Nakamura et al. (2019), and provides a new mechanism for direct activation of the Higgs mode with light.