Progressive disappearance of the c-axis tunneling barrier in Bi2Sr2CaCu2O8+x single crystals near Tc

We observed an enhancement of the differential conductance dI/dV around the zero bias in junctions of Au/Bi2Sr2CaCu2O8+x(Bi2212) single crystals near the superconducting transition temperature of Bi2212 crystals with the tunneling current along the c axis. We attribute such an enhancement to an Andreev reflection (AR) between the surface Cu-O bilayer, which is in the normal state, and the next superconducting bilayer below the crystal surface. The continuous evolution from a gap-like depression to the AR peak structure of the dI/dV curves around the zero bias indicates weakening of the barrier strength of the non-superconducting layers between adjacent Cu-O bilayers as the temperature is raised up to T-c. We suggest that the fact can be conveniently used to investigate the possible existence of preformed pairs in the pseudogap state above T-c.