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 ω = 2∆ 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.

Nonlinear electromagnetic response and Higgs-mode excitation in BCS superconductors with impurities

“…This result is in sharp contrast with the system without impurities where the Higgs mode generation is negligible and the resonant contribution comes from the other source 36,43 . It agrees qualitatively with the studies of linear Higgs mode generation in diffusive currentcarrying superconductor 44,45 .…”

“…where we denote as before s 1 = ω 2 1 + ∆ 2 and s 3 = ω 2 3 + ∆ 2 . Taking into account that ω (s −1 3 − s −1 3 ) = 0 the expression for polarization operator (45) can be transformed to…”

Classification and characterization of nonequilibrium Higgs modes in unconventional superconductors

“…It is important to note that the proposed experimental excitation of the Higgs mode is a Raman-like excitation and should not be confused with an infrared-active excitation 28 . In the latter case, a driven ac current would occur and thus the strength and polarization of the electric field is more important than the small momentum of the photon.…”

“…Other possibilities include momentum-dependent scattering processes as well as coupling to other finite-momentum modes. This is discussed for example for superconductors under external current 27,28 or as a possibility for phonon-coupled amplitudon dynamics in excitonic insulators 63,64 .…”

“…neglecting the small wave momentum q of the external field. There are other studies which show that a linear coupling of the vector potential to the condensate is possible if momentum transfer is involved, either by impurity scattering in dirty superconductors [22][23][24][25][26] or in current-carrying states 27,28 .…”