Observation of Andreev reflection in the c-axis transport of Bi_2Sr_2CaCu_2O_{8+x} single crystals near T_c and search for the preformed-pair state

We observed an enhancement of the $c$-axis differential conductance around the zero-bias in Au$/$Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ (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 non-superconducting 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 underdoped crystals, although recently proposed theoretical consideration suggests an even wider temperature range of enhanced zero-bias conductance. This seems to provide negative perspective to the existence of the phase-incoherent preformed pairs in the pseudogap state.Comment: 17 pages including 4 figure