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

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.

arXiv comment5 pages, 4 figures

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• Terahertz third harmonic generation in c-axis La${}_{1.85}$Sr${}_{0.15}$CuO${}_4$
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• Electronic Raman response of a superconductor across a time reversal symmetry breaking phase transition
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• Nonequilibrium nonlinear response theory of amplitude-dependent dissipative conductivity in disordered superconductors
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• Transverse Photoresistivity from Photothermal Current Deflection in Metal Films