Zero-bias anomalies and current states in point-contacts of Bi 2Sr 2CaCu 2O 8+ Y

Abstract

We have studied the effect of transparency of the normal metal–high temperature superconductor boundary on the voltage–current characteristics, temperature dependence of the dynamic resistance of point-contact on the base of the fresh cleavages and, also, degraded surfaces of Bi 2Sr 2CaCu 2O 8+ Y single crystals. A normal metal-to-fresh single crystal cleavage contact may be a direct conduction channel free of tunnel interlayers. The temperature dependence of the dynamic resistance of the point-contact is a sensitive and informative characteristic reflecting the superconducting state near-surface layers of a single crystal and their degradation as the oxygen is being lost. It is shown that the current, flowing across the boundary, governs the relationship between the ` ab' and ` c' components, each of which is dominant in its own temperature and `voltaic' interval. Surface layers with degraded superconducting properties control the I– V characteristic features near zero voltage even at high boundary transparency as these features are related with resistivity arising at the Andreev reflection and with current redistribution between the ` c' and ` ab' directions in the boundary region. At large biases the boundary regions are normal state, the current flows in the λ ab layer i parallel with the surface. VAC exhibits a hysteresis in this voltaic region. Once a homogeneous degraded interlayer is formed at the boundary, the VAC form becomes quasitunnel with a zero-bias anomaly feature that weakly depends on a magnetic field and invariant in form (insplit). Such zero-bias anomaly may be regarded as the Andreev reflection at the diffuse normal metal–high temperature superconductor boundary.