Observation of normal and superconducting state energy gap features from intrinsic c-axis interlayer tunnelling in Bi2Sr2CaCu2O8 crystals.

The non-linear, c-axis, inter-layer electrical properties of Bi2Sr2CaCu2O8 single crystals in the normal and superconducting states have been investigated. Measurements of the IV characteristics and dynamic conductivity across lithographically defined stacks of intrinsic junctions on the surface of single crystals of various doping are reported. We demonstrate that the increase in c-axis resistance at low temperatures lis related to the emergence of a normal state pseudo-gap. At T-c a zero bias Josephson tunnelling conductance peak emerges, along with additional voltage dependent features, consistent with an energy gap in the tunnelling density of states. The additional structure emerges at an already well-defined energy, implying the condensation of pre-formed pairs. Both the normal and superconducting state derived energy gaps are significantly smaller than obtained from ARPES and surface tunnelling measurements. The absence of significant thermal broadening suggests coherent c-axis tunnelling between adjacent CuO2 bi-layers, at variance with generally accepted views. Well into the superconducting state, the IV characteristics become strongly re-entrant with a functional form that suggests nonequilibrium tunnelling effects play an important role at large biases.