FLUX-QUANTIZATION IN SINGLE-CRYSTAL YBA2CU3O7-DELTA AS A FUNCTION OF DC AND MICROWAVE MAGNETIC-FIELDS

Using the modulated-microwave-absorption technique, we have studied the flux quantization in single-crystal samples of YBa2Cu307-delta as a function of an externally applied dc magnetic field (dc scan) at constant amplitude of the microwave magnetic field, and as a function of the microwave-field amplitude (microwave scan) at nominally zero dc field. Periodic oscillations in the modulated microwave absorption due to flux quantization are observed in both scans. In the dc scan an increasing number of junctions are activated as the temperature is increased at a fixed microwave power level or as the power level is increased at a fixed temperature. Surprisingly, only a single junction is activated in the microwave scan in the temperature range 0.1 < t less-than-or-equal-to 0.92, where t = T/T(c) is the reduced temperature. It is shown that different junctions are activated in the two scans. From the temperature dependence of the microwave scan spectra, the superconducting penetration depth lambda-L is shown to obey the empirical two-fluid temperature dependence lambda-L(T)/lambda-L(0) = (1-t4)-1/2 in the temperature range 10-85 K, which is consistent with the s-wave-pairing mechanism. In the same temperature range the temperature dependence of the junction critical current can be described by J(c)(T)/J(c)(0) = (1-t)1.9 +/- 0.2.