Inhomogeneous superconductivity in deeply underdoped Bi2Sr2CaCu2O8+δ

To study electronic states in deeply underdoped cuprates, we have measured the interlayer tunneling spectra of Bi2Sr2CaCu2O8+δ (Tc = 44 K) realized by partially substituting Sr with La. A mesa structure is manufactured which is composed of 10 or less stacked intrinsic tunnel Josephson junctions. The tunneling characteristics exhibits a totally unknown behavior that the differential conductance increases significantly with decreasing temperature below Tc. This anomalous behavior observed only for deeply underdoped samples strongly suggest that inhomogeneous superconductivity or phase separation in CuO2 planes is essential bulk property of deeply underdoped cuprates.