Correlation between hysteresis and time decay in the microwave second-harmonic emission of superconductors in the critical state

Abstract

We report experimental results of second-harmonic (SH) generation in superconductors in the critical state, exposed to intense pulsed microwave fields. The SH signal exhibits time evolution, within the pulse width, and magnetic hysteresis. We show that the two effects are strictly related to each other. The peculiarities of both the time evolution and the hysteresis depend on the nature of the investigated superconductor. However, in all the samples a correlation occurs between the hysteresis amplitude and the ratio of the decay rates of the SH signals, detected when the static magnetic field has been reached at increasing and decreasing values. The transient effects are ascribed to processes of flux redistribution, which come into play when the fluxon lattice is driven by high frequency em fields. We infer that the hysteretic behaviour is due to the fact that, when the static field is reached at decreasing or increasing values, the redistribution processes occur with different rates.