Significantly doping-dependent Josephson critical current — inhomogeneity in real space or heterogeneity in k-space

Abstract

The Josephson critical current density Jc has been measured for intrinsic Josephson junctions (IJJs) in Bi2Sr2>CaCu2O7+δ using mesas 5 ∼10 /μm on a side and 7∼15 nm in thickness at various doping levels. The superconducting gap and the normal tunneling resistance are measured at the same time by short-pulse tunneling spectroscopy. It is found that Jc decreases significantly by approximately one order of magnitude when the doping level p is decreased from p = 0.14 for the optimal doping to p = 0.09 in the underdoped region. This significant decrease in Jc, reflecting the bulk properties, can be interpreted in two different ways. If there occurs a phase separation in which superconducting (S) and normal regions (N) coexist on a fine scale, the significant decrease in Jc is explained as a decrease in the S region area. If a decrease in the doping level causes a decrease in the carrier density in the antinodal directions, it is expected that the superconducting pair density decreases due to the d-wave symmetry.