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

The microwave field dependent surface resistance R/sub s/(B/sub hf/) of unpatterned epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ films was measured at 19 GHz between 4.2 and 80 K with a Nb-shielded sapphire resonator. In addition, the dc field dependence R/sub s/(B/sub dc/) was investigated at 87 GHz and 4.2 K with a Cu host cavity. Maximum microwave field amplitudes B/sub cr//sup hf//spl ges/27, 23 and 15 mT were obtained at 4.2, 50 and 77 K with no detectable degradation of R/sub s/ at lower fields. Similarly, R/sub s/ stayed constant at 4.2 K up to maximum dc fields, B/sub cr//sup dc//spl les/40 mT. There is increasing evidence from both data sets that the maximum fields B/sub cr//sup hf/ and B/sub cr//sup dc/ are fundamentally limited by the lower critical field B/sub c1/(/spl lambda//sub L/) at a given, quality-dependent, penetration depth /spl lambda//sub L/. The R/sub s/(B/sub hf/) performance obtained for the best YBa/sub 2/Cu/sub 3/O/sub 7/ 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.