No abstract

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.

A depth profile of the surface resistance (RS) is obtained at 8.6 GHz for a thick MgB2 film both in the superconducting state and in the normal state, for which 1 µm thick MgB2 films are grown in situ on a c-plane sapphire substrate using the hybrid physical–chemical vapor deposition (HPCVD). A critical temperature TC of 40.4 K, a value higher than that of 39 K for MgB2 single crystals, is observed for the pristine 1 µm thick MgB2 film due to the thermal strain caused by its epitaxial nature. The depth profile of the effective RS for the 1 μm thick MgB2 film shows a drastic increase in the effective RS at low temperatures for films having thicknesses of 600 nm or less. The results for the x-ray diffraction and the normal-state resistivity provide evidence for the existence of Mg-rich phases in the lower part of the pristine MgB2 film. Our results show that thick MgB2 films grown using HPCVD could have very low RS despite the existence of Mg-rich phases in the lower part of the MgB2 films and that microwave technique could provide a sensitive method for investigating the homogeneity of thick MgB2 films.