Fundamental limits of the linear microwave power response of epitaxial Y-Ba-Cu-O films

The microwave field dependent surface resistance R-s(B-hf) of unpatterned epitaxial YBa2Cu3O7 films was measured at 19 GHz between 4.2 and 80 K with a Nb-shielded sapphire resonator. In addition, the de field dependence R-s(B-dc) was investigated at 87 GHz and 4.2 K with a Cu host cavity. Maximum microwave field amplitudes B-cr(hf) greater than or equal to 27, 23 and 15 mT were obtained at 4.2, 50 and 77 K with no detectable degradation of R-s at lower fields. Similarly, R-s stayed constant at 4.2 K up to maximum dc fields, B-cr(dc) less than or equal to 40 mT. There is increasing evidence from both data sets that the maximum fields B-cr(hf) and B-cr(dc) are fundamentally limited by the lower critical field B-cl(lambda(L)) at a given, quality-dependent, penetration depth lambda(L). The R-s(B-hf) performance obtained for the best YBa2Cu3O7 films comes already close to this limit. Microwave field limitation in less optimized films is expected to be caused by local heating at non-superconducting defects.