Temperature dependences of the surface resistance and the diamagnetic shielding susceptibility at T/sub c/-T≪T/sub c/ for high-T/sub c/ superconductors

Abstract

At T_c-T<<T_c(i.e., near T_c), in order to demonstrate the condudction mechanism and temperature dependencies of the diamagnetic-shielding susceptibility and the penetration depth, we fabricated Ba_1_xBiO_3(BKBO) thin films and measured the energy gap by tunnel effect and shielding susceptibilities which are compared with those measureed for BKBO and YBCO single crystals. The shielding susceptibilities for BKBO and YBCO single crystals well-fit \chi(T)/\chi(0)=1-exp(-2\triangle(T)/k_BT), while that for the BKBO film follows \chi(T)/\chi(0)=1-T/T_c) which may not be intrinsic. The exponential decrease of the susceptibilities near T_c indicates that the conduction mechanism is hopping. The energy gaps are observed as 2\triangle(0)=(3.5+-0.1)k_BT_c for the BKBO film by the tunnel effect, 2\triangle(0)=(3.9+-0.1)k_BT_c for the BKBO single crystal, and 2\triangle(0)=(8+-0.2)k_BT_c for the YBCO single crystal. Furthermore, for microwave device applications of superconductors, at T_c-T<<T_c, the surface resistance R_s(T) is derived from the surface impedance at \omega\tau_(tr)<<1, where \sigma_s(0) and \sigma_n are the conductivities of the superconducting state and the normal state, respectively, and f(T)=\chi(T)/\chi(0)=1-exp(-2\tringle(T)/k_BT).