Reproducible Tunneling Data on Etched Single Crystals of YBa sub 2 Cu sub 3 O sub x

We have developed a procedure for preparation of reproducible tunnel junctions of the type S sub 1 -I-S sub 2 type, where S sub 1 is a chemically etched single crystal of YBa sub 2 Cu sub 3 O sub x (YBCO), S sub 2 is a film of Pb, and I is an insulating barrier of unknown nature which is formed at the surface of YBCO at 100C in air. Some two dozen samples etched for different lengths of time, and in different etchants, show essentially identical tunneling dV/di vs V curves, with the same strength of all the features. The current through the junction is of tunneling nature, as evidenced by the absence of leakage at bases below the lead gap at T = 2K. The structure in the dV/di (V) first appears at T ~ 90K and sharpens as T is decreased. There are two gap-like structures in dV/di at ~10 and ~20mV, and an additional bump at ~45mV. If we are observing the intrinsic behavior, then the density of states in YBCO is of non-BCS type. Most of this information is lost when we tunnel into non-etched (as-prepared) crystals.