FIELD MODULATED MICROWAVE-ABSORPTION AND IRREVERSIBILITY LINE IN YBA2CU3O7 THIN-FILMS

Measurements of the field modulated microwave absorption of YBa2Cu3O7 thin films as a function of temperature and magnetic field are present ed. The Fourier spectrum of the reflected microwave power below T-c re veals a peak at the fundamental frequency of the modulation field and its even harmonics. The amplitude of the even harmonics decreases rapi dly when the de field superimposed on the modulation field is increase d, then shows a maximum and vanishes above a characteristic field B. We show that this maximum is related to the full penetration of the mo dulation field and that the B(T) curve can be viewed as an irreversib ility line which is consistent with that obtained from irreversible ma gnetization measurements.