MICROWAVE POWER-INDUCED FLUX-PENETRATION AND LOSS IN HIGH-TEMPERATURE SUPERCONDUCTORS

Extensive studies have been performed in a number of laboratories of the microwave properties of granular and partially granular superconductors at elevated levels of microwave power. The power-induced surface impedance initially increases at a rate that is quadratic in the microwave field. At intermediate power levels the surface resistance increases linearly with the microwave field, finally saturating in microwave fields between 10 and 100 Oe. The induced surface resistance is generally found to increase at low power with the square of the frequency and at high power directly with the frequency. A transmission-line analysis of the penetration of intergranular microwave vortices, pinned by defects, leads to results in qualitative agreement with these observations.