Anomalously strong doping dependence of the maximum Josephson current in intrinsic Josephson junctions and inhomogeneous superconducting state in Bi2Sr2CaCu2O8+δ

Abstract

Using intrinsic Josephson junctions in Bi2Sr2CaCu2O8+δ of which the doping level p is varied in a wide range, we have measured the magnitude of the superconducting order parameter Δ, the normal tunneling resistance RN, and the maximum Josephson current density Jc. It is found that Jc decreases quite rapidly by more than two orders of magnitude as p decreases, while Δ and RN change only moderately. This implies that the superconducting state of Bi2Sr2CaCu2O8+δ is inhomogeneous and nonsuperconducting regions coexist on a fine scale when p is small. Based on the Ambegaokar–Baratoff theory, the volume fraction of the superconducting regions is estimated to be 10% or less when p∼0.1.