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

Prof. Mikhail A. Skvortsov is a Chief Researcher at the Landau Institute for Theoretical Physics, Russian Academy of Sciences. He earned his academic degrees from the Moscow Institute of Physics and Technology (MIPT), where he also holds a professorial appointment. His research focuses on condensed matter theory, particularly the physics of disordered superconductors, mesoscopic physics, and quantum transport.

Inviter: Chu-Shun Tian