We develop a full microscopic theory for the optical conductivity, σ(ω), of a dirty current-carrying superconductor. Within the Keldysh sigma model formalism, we obtain the general analytical expression for σ(ω), applicable for arbitrary frequency ω, temperature T, and dc supercurrent I. In addition to altering the usual Mattis-Bardeen conductivity, a finite supercurrent introduces two new contributions: σ^qp(ω) from quasiparticle redistribution and σ^SH(ω) from the amplitude (Schmid-Higgs) mode excitation by the ac field. We investigate, both analytically and numerically, the main features of the optical conductivity in the presence of a dc supercurrent. They include a peak in Re σ(ω) above the optical gap and a sign change of Im σ(ω), with both effects becoming more pronounced at higher I and lower T. We also elucidate the role of inelastic relaxation, which governs the low-frequency response, leading to a giant microwave absorption and a suppression of the apparent superfluid density at the critical current. The optical conductivity measurement of a superconductor biased by a finite dc supercurrent enables the direct observation of the Schmid-Higgs mode via transport measurements.

Biography

Mikhail Skvortsov graduated from the Moscow Institute of Physics and Technology in 1995 and got his PhD in 1998. Since then, he has been continuously employed at the Landau Institute for Theoretical Physics. From 2014 to 2021, he served as an Associate Professor at the Skolkovo Institute of Science and Technology. M. A. Skvortsov is a recognized specialist in the physics of disordered and superconducting systems. His key scientific contributions include explaining the giant fluctuation Nernst effect in superconductors, investigating ergodicity and localization on random regular graphs, developing the Keldysh action approach for disordered superconductors, describing the inhomogeneous state in dirty superconductors, constructing the theory of dynamical localization in quantum dots under periodic driving, characterizing the statistics of the never-falling trajectory in the random Whitney problem.

Inviter: Chu-Shun Tian