Theory of nonequilibrium dynamics of multiband superconductors

¹ Max-Planck Institute for the Chemical Physics of Solids - D-01187 Dresden, Germany, EU
² Theoretische Physik III, Ruhr-Universität Bochum - D-44780, Bochum, Germany, EU
³ Max-Planck-Institut für Festkörperforschung - D-70569 Stuttgart, Germany, EU

Received: 10 December 2012
Accepted: 18 December 2012

Abstract

We study the nonequilibrium dynamics of multiband BCS superconductors subjected to ultrashort pump pulses. Using density-matrix theory, the time evolution of the Bogoliubov quasiparticle densities and the superconducting order parameters are computed as a function of pump pulse frequency, duration, and intensity. Focusing on two-band superconductors, we consider two different model systems. The first one, relevant for iron-based superconductors, describes two-band superconductors with a repulsive interband interaction V ₁₂ which is much larger than the intraband pairing terms. The second model, relevant for MgB₂, deals with the opposite limit where the intraband interactions are dominant and the interband pair scattering V ₁₂ is weak but attractive. For ultrashort pump pulses, both of these models exhibit a nonadiabatic behavior which is characterized by oscillations of the superconducting order parameters. We find that for nonvanishing V ₁₂, the superconducting gap on each band exhibits two oscillatory frequencies which are determined by the long-time asymptotic values of the gaps. The relative strength of these two frequency components depends sensitively on the magnitude of the interband interaction V ₁₂.