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

At T/sub c/-T/spl Lt//spl Lt/T/sub c/ (i.e., near T/sub c/), in order to demonstrate the conduction mechanism and temperature dependencies of the diamagnetic-shielding susceptibility and the penetration depth, we fabricated Ba/sub 1-x/K/sub x/BiO/sub 3/ (BKBO) thin films and measured the energy gap by tunnel effect and shielding susceptibilities which are compared with those measured for BKBO and YBCO single crystals. The shielding susceptibilities for BKBO and YBCO crystals well-fit /spl chi/(T)//spl chi/(0)=1-exp(-2/spl Delta/(T)/k/sub B/T), while that for the BKBO film follows /spl chi/(T)//spl chi/(0)=(1-T/T/sub c/) which may not be intrinsic. The exponential decrease of the susceptibilities near T/sub c/ indicates that the conduction mechanism is hopping. The energy gaps are observed as 2/spl Delta/(0)=(3.5/spl plusmn/0.1)k/sub B/T/sub c/ for the BKBO film by the tunnel effect, 2/spl Delta/(0)=(3.9/spl plusmn/0.1)k/sub B/T/sub c/ for the BKBO single crystal, and 2/spl Delta/(0)=(8/spl plusmn/0.2)k/sub B/T/sub c/ for the YBa/sub 2/Cu/sub 3/O/sub 7/ single crystal. Furthermore, for microwave device applications of superconductors, at T/sub c/-T/spl Lt/T/sub c/, the surface resistance R/sub s/(T)/spl ap//spl radic/(/spl omega//spl mu//sub 0//(2))/(/spl sigma//sub n/+(/spl sigma//sub s/(0)-/spl sigma//sub n/)f(T)) is derived from the surface impedance at /spl omega//spl tau//sub tr//spl Lt/, where /spl sigma//sub s/(0) and /spl sigma//sub n/ are the conductivities of the superconducting state and the normal state, respectively, and f(T)=/spl chi/(T)//spl chi/(0)=(1-exp(-2/spl Delta/(T)/k/sub B/T)).