Andreev reflection in point-contact tunneling spectroscopy on Bi 2Sr 1.94La 0.06CuO 6+ δ single crystals

Abstract

In this paper, we report tunneling spectroscopies on overdoped Bi 2Sr 1.94La 0.06CuO 6+ δ (Bi2201) single crystals taken by point-contact with gold tips. Below the superconducting transition temperature, temperature-dependent Andreev reflection, zero bias conductance peak (ZBCP), a dip feature at |eV|∼2 Δ and the asymmetrical conductance curve have been observed. Quantitative analysis using extended BTK theory results in that d x 2− y 2 and, in some cases, mixed order parameter symmetry can fit well to the experimental data. The fitted s-wave component is varied for different run experiments even with the same crystal, indicating that it is determined by the detailed contact condition between tip and crystal. The observed temperature ( T) dependence of the energy gap deviates from the BCS-type. A dip feature at |eV|∼2 Δ scales with Δ for T below T c, revealing it may relate to the pairing interaction in the high T c superconductor.