The discovery, disappearance and re-emergence of radiation-stimulated superconductivity

Abstract

We trace the historical fate of experiment and theory of microwave-stimulated superconductivity as originally reported for constriction-type superconducting weak links. It is shown that the observed effect disappeared by improving weak links to obtain the desired Josephson-properties. Separate experiments were carried out to evaluate the validity of the proposed theory of Eliash'berg for $\textit{energy-gap-enhancement}$ in superconducting films in a microwave field, without reaching a full quantitatively reliable measurement of the stimulated energy gap in a microwave-field, but convincing enough to understand the earlier deviations from the Josephson-effect. Over the same time-period microwave-stimulated superconductivity continued to be present in superconductor-normal metal-superconductor Josephson weak links. This experimental body of work was left unexplained for several decades and could only be understood properly after the microscopic theory of the proximity-effect had matured enough, including its non-equilibrium aspects. It implies that the increase in critical current in weak-link Josephson-junctions is due to an $\textit{enhancement of the phase-coherence}$ rather than to an $\textit{enhancement of the energy-gap}$ as proposed by Eliash'berg. The complex interplay between proximity-effect and the occupation of states continuous to be, in a variety of ways, at the core of the ongoing research on hybrid Josephson-junctions. The subject of radiation-enhanced superconductivity has re-emerged in the study of the power-dependence of superconducting microwave resonators, but also in the light-induced emergence of superconductivity in complex materials.