TUNNELING AND POINT-CONTACT SPECTROSCOPY OF HIGH-T-C SUPERCONDUCTING THIN-FILMS

Tunneling and point contact junctions between a normal metal and thin high temperature superconducting film were studied to obtain informati on on surface properties. Surface degradation and time development of contact resistance are investigated as well as peculiarities of the co nductance-voltage characteristics. The quasi-linear background is rela ted to carrier transport through degraded surface layer described by a model of inelastic scattering from a broad hat continuum of states in side the potential barrier. An asymmetry of the characteristics appear s as the result of low Fermi energy values in the high temperature sup erconductors in comparison to common metals. An extrinsic nature of th ese peculiarities is supported by effects of barrier formation by appl ied voltages in the 1 volt range. As an often observed anomaly a condu ctance peak at zero bias is observed which can be related to different mechanisms. In the case of high temperature superconductors this zero bias anomaly is related to the d-wave pairing symmetry of the pair po tential. Experimental results on YBCO and BSCCO films are compared to calculations taking into account the Andreev reflections for a junctio n between a normal metal and a d-wave superconductor.