Microwave properties of highly oriented YBa sub 2 Cu sub 3 O sub 7 minus x thin films

We have performed intra- and extra-cavity microwave frequency (1--100 GHz) measurements on high quality Y{sub 1}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} superconducting thin films on (100) LaAlO{sub 3} substrates. The {similar to}0.3 {mu}m thin films fabricated by the pulsed laser deposition technique exhibit superconducting transition temperatures {gt}90 K, as determined by resistivity and ac susceptibility measurements, and critical current densities of 5{times}10{sup 6} A/cm{sup 2} at 77 K. Moreover, ion beam channeling minimum yields of {similar to}3% were measured, indicating the extremely high crystalline quality of films grown on the LaAlO{sub 3} substrate. Microwave surface resistance values at 77 K for these films are found to be more than one to two orders of magnitude lower than for copper at 77 K for almost the entire frequency range explored. We postulate that the reason we observe such low surface resistances in these films is the virtual absence of grain and phase boundaries coupled with the high degree of crystallinity. Furthermore, we believe that the residual resistance measured below {ital T}{sub {ital c}} is at present dominated by losses occurring in the substrate and the cavities rather than by losses intrinsic to the Y-Ba-Cu oxide superconductor.