Multiple Andreev reflection in YBCO break-junctions

Abstract

In this paper we present current—voltage characteristics of HTSC point contacts prepared by the break junction technique with YBCO single crystals. The measured I( V), (d V/d I)( V) and (d 2 V/d I 2)( V) characteristics show features which clearly indicate the occurrence of multiple Andreev reflection in the junctions. We compare the measured characteristics with calculations carried out with the well-known OTBK theory [1]. In this way we show, that YBCO has an energy gap of 2 Δ = (42 ± 2) meV, which is broadened. Additionally, we find that the energy gap decreases with raising temperature already at low temperatures much faster than 2 Δ of a BCS superconductor. In the case of SNS-like junctions with very small or vanishing interface barriers at the two NS phase boundaries we obtain characteristics which cannot be understood with the OTBK theory. These characteristics can only be explained with the theory for SNS junctions published by Kümmel et al. [2], who take into account a contribution of bound states.