Infrared activation of the Higgs mode by supercurrent injection in superconducting NbN

TL;DR

This paper demonstrates that applying a dc supercurrent to NbN superconductors makes the Higgs mode infrared active, allowing direct observation via optical conductivity measurements, which challenges previous assumptions about its electromagnetic coupling.

Contribution

The study reveals that supercurrent injection activates the Higgs mode in superconductors, enabling its detection through linear optical spectroscopy, a novel approach in superconductor research.

Findings

Resonant peak at ω=2Δ observed with supercurrent

Infrared activity of Higgs mode confirmed experimentally

Method applicable to various superconductors

Abstract

Higgs mode in superconductors, i.e. the collective amplitude mode of the order parameter does not associate with charge nor spin fluctuations, therefore it does not couple to the electromagnetic field in the linear response regime. On the contrary to this common understanding, here, we demonstrate that, if the dc supercurrent is introduced into the superconductor, the Higgs mode becomes infrared active and is directly observed in the linear optical conductivity measurement. We observed a sharp resonant peak at $\omega=2\Delta$ in the optical conductivity spectrum of a thin-film NbN in the presence of dc supercurrent, showing a reasonable agreement with the recent theoretical prediction. The method as proven by this work opens a new pathway to study the Higgs mode in a wide variety of superconductors.