Multiple Andreev-reflection in YBCO break-junctions

We present current-voltage characteristics of HTSC point-contacts prepared by the break-junction technique with YBCO single crystals. We compare the measured I(V)-, $frac{{dV}} {{dI}}(V)$ (V)- and $frac{{d^2 V}} {{dI^2 }}(V)$ (V)-characteristics to calculated characteristics of the OTBK-theory [1], which describes SINIS-junctions in which multiple Andreevreflection (AR) occurs. Taking into account systematic deviations between characteristics measured with conventional weak-links and characteristics calculated with the OTBK-theory, discussed in [2] and [3], we can demonstrate, that the characteristics of the YBCO breakjunctions clearly show all features, which indicate the occurrence of multiple AR. Thus we determine the energy gap of YBCO with a high precision to 2Δ = (42 ± 2) meV. In addition we find, that the structures in the characteristics at eV = 2Δ always show a broadening, whereas the structures at higher order subharmonic gap voltages $left( {eV = frac{{2Delta }} {n}} right)$ are not broadened. The reason for this behavior is subject for discussion. The investigation of the temperature dependence of the energy gap shows that 2Δ decreases substantially with rising temperature already at low temperatures. We confirm this result with two other independent methods. Break-junctions with very small or vanishing interface barriers at the two NIS-phase boundaries show characteristics which indicate also the occurrence of multiple Andreev-reflection, which cannot be described with the OTBK-theory. We demonstrate, that these characteristics can be explained with the relaxation time model for SNS-junctions published by Kümmel et al. [4].