Point contact tunnelling studies on ceramic YBCO with scanning tunnelling microscope tips

A detailed study of point contact tunnelling into ceramic YBCO with electrocliemically etched tips of Pt, Nb and W is reported. The superconducting gap parameter (\Delta) has been extracted from I - V and dI/dV- V curves using various procedures. Our results indicate a gap value of about 20meV. We observe that the zero bias conductance is strongly dependent on the junction resistance. The normal state conductance varies linearly with bias voltage and the conductance curves are asymmetric with respect to polarity of the bias voltage. With contacts of very high junction resistance, we observe G(O)/G(l00mV) has a value as low as 1/6. This may be the lowest value reported so far