Temperature dependences of the surface resistance and thediamagnetic shielding susceptibility at Tc-T≪Tcfor high-Tc superconductors

摘要

At Tc-T≪≪Tc (i.e., near Tc), in order to demonstrate the conduction mechanism and temperature dependencies of the diamagnetic-shielding susceptibility and the penetration depth, we fabricated Ba1-xKxBiO 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 χ(T)/χ(0)=1-exp(-2Δ(T)/kBT), while that for the BKBO film follows χ(T)/χ(0)=(1-T/Tc) which may not be intrinsic. The exponential decrease of the susceptibilities near Tc indicates that the conduction mechanism is hopping. The energy gaps are observed as 2Δ(0)=(3.5±0.1)kBTc for the BKBO film by the tunnel effect, 2Δ(0)=(3.9±0.1)kBTc for the BKBO single crystal, and 2Δ(0)=(8±0.2)kBTc for the YBa2Cu3O7 single crystal. Furthermore, for microwave device applications of superconductors, at T c-T≪Tc, the surface resistance Rs(T)≈√(ωμ0/(2))/(σn +(σs(0)-σn)f(T)) is derived from the surface impedance at ωτtr≪, where σ s(0) and σn are the conductivities of the superconducting state and the normal state, respectively, and f(T)=χ(T)/χ(0)=(1-exp(-2Δ(T)/kBT))