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 Tc. We suggest that the fact can be conveniently used to investigate the possible existence of preformed pairs in the pseudogap state above Tc