Andreev-reflection in YBCO “break-junctions”

We present measurements at break-junctions prepared of polycrystalline and textured YBCO-pellets with different resistances. The higher-ohmic junctions show the well known behaviour of HTSC-tunnel contacts with gap-like structures in their dI/dV-characteristics. Lower-ohmic junctions show the behaviour of point contacts. TheirI(VV)-and dI/dV-characteristics indicate all properties of the Andreev-reflection corresponding to the theory of Kümmel et al. [1] for BCS-superconductors. This could be an alternative method to study the energy gap in high temperature superconductors. Additionally we will show, that sharp minima in the dI/dV-characteristics could also be explained by the process of Andreev-reflection. For correcting the measured characteristics we substract the typical background to compare the characteristics directly with theoretical calculations. We studied the influence of magnetic fields up to 8 T.