Relaxation and Electromagnetic Response of a Superconductor in a State of Supercurrent Flow

The situation is investigated in which a superconductor in a state of supercurrent flow and near the transition temperature is exposed to an alternating electric field. The induced alternating current is shown to exhibit a frequency dependence (1-iΩτR)^-1, which is due to a relaxation process in the Cooper-pair density. The relaxation time τR is determined by electron-phonon collisions (collision time τc), in which case τRτc[Tc(Tc-T)]1/2; impurity scattering is important only under certain conditions, in which case τR10^-11[Tc(Tc-T)]^-5 sec.