Observation of the Andreev reflection in the c-axis transport of Bi2Sr2CaCu2O8+x single crystals near T-c and the search for the preformed-pair state

Abstract

We observed an enhancement of the c-axis differential conductance around the zero bias. in Au/Bi2Sr2CaCu2O8+x (Bi2212) junctions near the superconducting transition temperature T-c. We attribute the conductance enhancement to the Andreev reflection between the surface Cu-O bilayer with suppressed superconductivity and the neighboring superconducting inner bilayer. The continuous evolution from depression to an enhancement of the zero-bias differential conductance, as the temperature approaches T-c from below, points to weakening of the barrier strength of the nonsuperconducting layer between adjacent Cu-O bilayers. We observed that the conductance enhancement persisted up to a few degrees above T-c in junctions prepared on slightly overdoped Bi2212 crystals. However, no conductance enhancement was observed above T-c in under-doped crystals, although a recently proposed theoretical consideration suggests an even wider temperature range of enhanced zero-bias conductance. This seems to provide a negative perspective to the existence of the phase-incoherent preformed pairs in the pseudogap state.