ORIGIN OF THE ZERO-BIAS CONDUCTANCE PEAKS OBSERVED UBIQUITOUSLY IN HIGH-T-C SUPERCONDUCTORS

The midgap surface states predicted previously to exist on non-{n0m} s urfaces of d(xa2-xb2-) superconductors (SC's) can be extended to midga p interface states (MIS's), which exist at almost all interface betwee n grains of such SC's of different principal axes orientations. They c an Five rise to a zero-bias conductance peak (ZBCP) in quasiparticle t unneling along any axis as shown in our model calculation. When the co unterelectrode is a low-T-c SC, its gap is shown to appear as a dip at the center of the (broadened) ZBCP. These and other results support t he proposal that such MIS's are responsible for most if not all of the ZBCP's observed ubiquitously in tunneling experiments performed on hi gh-T-c SC's.