LOW-FIELD MICROWAVE MAGNETOABSORPTION IN THE CUPRATE SUPERCONDUCTORS

We review results on the low-field microwave magnetoabsorption (LFMMA) in granular samples of YBCO and BSCCO. First, we show that the virgin response (increase in absorption on first application of H) obeys a universal form (Delta P/alpha) = (1 + H-0/H)(-1). Interpreting this in conjunction with de magnetization isotherms on the same samples, we demonstrate that the data support a model in which the LFMMA arises from field dependent reduction in the critical currents of the weak links in the sample surface. Next, we present examples of the variety of hysteresis loops obtained on subsequent field cycling and interpret them in terms of the universal form provided that H is replaced by an effective value H-eff. Attempts are made to delineate the various terms appearing in H-eff.