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The rf surface resistance of a single bulk polycrystalline YBa2Cu307 -sample was measured with TEM resonant cavities as a function of temperature from 4.2 to 100 K, rf field amplitude from 0 to 640 G, and frequency from 175 to 1050 MHz. The surface resistance increased monotonically with rf field amplitude, saturating at a value approximately 5% of the normal-state surface resistance. The surface resistance is strongly frequency and temperature dependent below the saturation region and weakly frequency and temperature dependent above. Low-field surface resistances as small as~1.1 pQ (at 175 MHz) were observed at T=4.2 K. The superconducting state did not break down, even at the highest field achieved ( -640 G).

The increase in the microwave surface resistance Rs of high Tc superconductors at elevated microwave power levels is reported for both oriented and unoriented Tl-based films as a function of rf magnetic field at 820 MHz and 18 GHz. The application of dc magnetic fields produces qualitatively similar increases in Rs and in the surface reactance Xs. The increase in Rs with dc field is shown to arise from simple decoupling of grains by intergranular magnetic flux. The increase in Rs with microwave power, on the other hand, is a consequence of hysteretic intergranular processes.

with a formatting error in Eq. (15). It has been corrected as of 16 July 2004.

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