Radiation-stimulated superconductivity

Abstract

Eliashberg has predicted that absorption of microwaves in a superconducting film leads to an increase of the energy gap by creating a nonequilibrium quasiparticle distribution. The frequency has to exceed the inverse relaxation time for inelastic scattering. In the present paper measurements are reported of the critical current of long, narrow, superconducting thin-film strips of aluminum subjected to high-frequency radiation (10 MHz–10 GHz). Above a critical frequency of about 200 MHz considerable enhancement of critical current and critical temperature is observed. Analysis of the results is performed by taking the critical current for a measure of the energy gap. The results are in reasonable agreement with Eliashberg's theory. As predicted, the transition between the superconducting and the normal states becomes of first order. The experimental results on critical current enhancement of micro-bridges (Dayem-Wyatt effect) can be explained consistently with gap enhancement.