Observation of normal and superconducting state energy gap features from intrinsic c-axis interlayer tunneling in Bi 2 Sr 2 CaCu 2 O 8 crystals

Abstract

The non-linear, c-axis, inter-layer electrical properties of Bi<SUB>2</SUB>Sr<SUB>2</SUB>CaCu<SUB>2</SUB>O<SUB>8</SUB> 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 is related to the emergence of a normal state pseudo-gap. At T<SUB>c</SUB> a zero bias Josephson tunneling conductance peak emerges, along with additional voltage dependent features, consistent with an energy gap in the tunneling 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 tunneling measurements. The absence of significant thermal broadening suggests coherent c-axis tunneling between adjacent CuO<SUB>2</SUB> bi-layers, at variance with generally accepted views. Well into the superconducting state, the IV characteristics become strongly re-entrant with a functional from that suggests non-equilibrium tunneling effects play an important role at large biases.