Symmetry and Temperature dependence of the Order parameter in MgB2 from point contact measurements

We have performed differential conductance versus voltage measurements of Au/MgB2 point contacts. We find that the dominant component in the conductance is due to Andreev reflection. The results are fitted to the theoretical model of BTK for an s-wave symmetry from which we extract the value of the order parameter (Delta) and its temperature dependence. From our results we also obtain a lower experimental bound on the Fermi velocity in MgB2.Comment: 7 pages (Including figure captions) and 4 figure

Observation of Bound Surface States in Grain Boundary Junctions of High Temperature Superconductors

We have performed a detailed study of the tunneling spectra of bicrystal grain boundary junctions (GBJs) fabricated from the HTS YBCO, BSCCO, LSCO, and NCCO. In all experiments the tunneling direction was along the CuO planes. With the exception of NCCO, for all materials a pronounced zero bias conductance peak was observed which decreases with increasing temperature and disappears at the critical temperature. These results can be explained by the presence of a dominating d-wave symmetry of the order parameter resulting in the formation of zero energy Andreev bound states at surfaces and interfaces of HTS. The absence of a ZBCP for NCCO is consistent with a dominating s-wave symmetry of the pair potential in this material. The observed nonlinear shift of spectral weight to finite energies by applying a magnetic field is in qualitative agreement with recent theoretical predictions.Comment: 4 pages, 4 figures, to be published in Phys. Rev.