Tunneling and Point Contact Spectroscopy of High-T_cSuperconducting Thin Films

Abstract

Tunneling and point contact junctions between a normal metal and thin high temperature superconducting film were studied to obtain information on snrface properties. Surface degradation and time development of contact resistance are investigated as well as peculiarities of the conductance-voltage characteristics. The quasi-linear background is related to carrier transport through degraded surface layer described by a model of inelastic scattering from a broad flat continuum of states inside the potential barrier. An asymmetry of the characteristics appears as the result of low Fermi energy values in the high temperature superconductors in comparison to common metals. An extrinsic nature of these peculiarities is supported by effects of barrier formation by applied voltages in the 1 volt range. As an often observed anomaly a conductance 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 potential. Experimental results on YBCO and BSCCO films are compared to calculations taking into account the Andreev reflections for a junction between a normal metal and a d-wave superconductor.