Small Josephson current and product deduced by means of measurement for an inhomogeneous superconductor: Extension of the Ambegaokar - Baratoff theory

Abstract

For an inhomogeneous high-T_c superconductor, band-filling dependence of Josephson current and Josephson product is deduced at T=0 K by means of measurement; this is an extension of Ambegaokar-Baratoff Josephson current and product. The observed Josephson current, J_{obs}, is given by J_{obs}=ρJ_i, where 0<ρ<=1 is band filling (or local density). When ρ=1, J_{obs} = J_i is the intrinsic supercurrent occurring by Cooper pair. When 0<ρ<1, J_{obs} is an average of J_i over the measurement region and is the effect of measurement. The observed Ambegaokar-Baratoff Josephson product, based on the s-wave theory, is given by J_{obs}R_n =(π/{2})(ρ\triangle_i), where \triangle_i is the intrinsic superconducting gap and small, which results in small Josephson products observed by the experiments. The intrinsic gap, 4<{\triangle_i}<10 meV, is analyzed from the Josephson-product data of BSCCO. In addition, the triple-π-junction experiments, observing the half-flux quantum ({Φ_0}/2) as evidence of d-wave symmetry, are discussed by using means of measurement. Comments on a flux trap of the magnetic modulation experiment using Pb-YBCO dc SQUIDs are also given. Author's thought is given on the angle dependence of $J_c$ measured in whisker.