High critical temperature superconductor tunneling spectroscopy using squeezable electron tunneling junctions

Tunneling spectroscopy measurements were performed on squeezable electron tunneling (SET) junctions using Bi-Sr-Ca-Cu-O, Y-Ba-Cu-O, and Nb electrodes in a variety of combinations. A zero-bias conductance peak has been seen repeatedly in the current-voltage (I(V)) and the conductance-voltage (G(V)) characteristics. A model is presented to explain this conductance peak in terms of quasi-particle tunneling, phase diffusion, and a supercurrent. Two additional structures have been seen repeatedly in I(V) and G(B). One of these structures has the characteristics of an energy gap feature. The other structure, which can mimic the gap feature, is explained in terms of the switching to the voltage state of a grain boundary junction that is in series with the SET junction. The dependence of these features upon temperature and the force applied to the junction are examined.