Supercurrent Growth in Nonequilibrium Superconductors

In ultrafast experiments on superconductors, a pump laser pulse often heats up the electronic system and suppresses the density of superfluid electrons. Subsequently, the electrons undergo a cooling process because of electron-phonon thermalization so that the superfluid density recovers in time. We study the nonequilibrium electromagnetic response of the system in this cooling process. We show that if a supercurrent is initiated by a probe electric field pulse, an intriguing phenomenon of `supercurrent growth' occurs, meaning that the net current grows in time with the increasing superfluid density. Using the Boltzmann kinetic equation, we uncover its microscopic origin as the momentum-relaxing scattering of Bogoliubov quasiparticles by impurities and phonons, in stark contrast to the widely accepted intuition that impurities always attenuate currents. We further show that supercurrent growth has important experimental manifestations, including the ultrafast Meissner effect and an optical reflectivity exceeding unity.