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YBa 2 Cu 3 O 7−δ (YBCO) films were grown on CeO 2 -buffered r-cut sapphire (CbS) substrates using an off-axis dc magnetron sputtering method. Both as-grown and post-annealed CbS substrates were used for this purpose, with the post-annealed CbS prepared by annealing the as-grown CbS at 1000 • C for 1 h. The YBCO films on post-annealed CbS showed significantly improved surface morphology and structures than those on the as-grown CbS, with a peak-to-valley roughness of 3.2 nm and a full-width at half-maximum of (005) YBCO rocking curve of 0.47 • for a 300 nm thick YBCO. The surface resistance of the 300 nm thick YBCO film on post-annealed CbS appeared as small as about 110 µ at 60 K and about 230 µ at 77 K at about 8.6 GHz.

Proton irradiations on superconducting bulk materials and thin films have enabled to enhance the critical current density and the upper critical field in the mixed state due to increased vortex pinning. We investigated the microwave surface resistance ( ) of proton-irradiated YBa 2 Cu 3 O 7 (YBCO) films grown on LaAlO 3 (LAO) substrates. The intrinsic was measured at 8.5 GHz at temperatures of 7-90 K with the effects of the finite film thickness taken into account. Significant reduction in the of YBCO films were observed at temperatures of 40 K to 70 K when the films were irradiated with 20 MeV proton for 10 minutes to the fluence of 0 47 10 15 ions cm 2 with the value of 100 of the proton-irradiated one at 60 K appearing significantly lower than the corresponding one of 140 before irradiation. However, at temperatures below 20 K, the of the irradiated YBCO films was higher than that of the YBCO film before irradiation with the residual of the former appearing higher than that of the latter. No significant changes in the of YBCO films were observed when the films were irradiated with 20 MeV proton for 30-120 minutes to the fluences of 1 4 10 15 to 5 7 10 15 ions cm 2 . The reduction in the of proton-irradiated YBCO films at 40-70 K was attributed to enhanced impurity scattering rate of 2 10 12 s compared to a value of 7 6 10 11 s for unirradiated YBCO.

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