The third-harmonic response of a superconductor near its transition temperature to a weak electromagnetic field is calculated, both in the gap and gapless regimes, including the effect of inelastic scattering of electrons by the phonons. The response is related to the relaxation time of the order parameter in the perturbed superconductor; it is shown that the temperature dependence of the relaxation time, depending on the depairing strength, is changed as $T_c$ is approached from below.

• Received 10 November 1977

DOI:https://doi.org/10.1103/PhysRevB.18.4762