Exponentially Suppressed Interlayer Josephson Current in Underdoped Bi2Sr2CaCu2O8+y. Bulk Evidence for Inhomogeneous Superconductivity

Josephson plasma resonance is used to determine the interlayer Josephson critical current density jc in Bi2(Sr,La)2CaCu2O8+y. We find that jc shows an exponential doping dependence in a wide doping range. In the underdoped regime, our experimental jc values are much smaller than expected in the d‐wave coherent tunneling model. This discrepancy can be naturally understood if each CuO2 planes in the bulk have a large portion of non‐superconducting regions where Josephson current cannot flow. A simple analysis provides that the fraction of superconducting region gradually decreases at lower dopings, which points to the importance of inhomogeneity on the suppression of Tc in underdoped cuprates.