Maximum Josephson current and inhomogeneous superconductivity in Bi2Sr2CaCu2O8+δ

The maximum Josephson current (MJC) is measured for a small thin stack of approximately 10 intrinsic Josephson junctions in Bi2Sr2CaCu2O8+δ (BSCCO) single crystals with various carrier doping levels. By interlayer tunneling spectroscopy, tunneling spectra are also measured to obtain values for the superconducting energy gap 2Δ/e. It is found that the MJC decreases with decreasing doping by more than two orders of magnitude from almost 104 A/cm2 to less than 102 A/cm2. The anomalous decrease of two orders of magnitude in the MJC is inexplicable in terms of the Ambegaokar-Baratoff theory. It is explained only when the superconducting state is inhomogeneous. This leads to an astonishing implication that the superconducting state in BSCCO is inhomogeneous and phase-separated on a fine scale