Tunneling studies in Bi2Sr2CaCu2O 8+x

Planar tunnel junctions have been realized on liquid-etched Bi2Sr2CaCu2O8+x (BSCCO) single crystals with natural barriers and Pb, Pb-Bi, Au counterelectrodes. At T=4.2 K and I-V characteristics of these junctions often show multiple switches that disappear at the Tc of the BSCCO compound. The dV/dI vs. V curves show gaplike structures at about ±22 mV and second features at about ±40 mV. These are very similar to the double structure observed in YBa2Cu 3O7 (YBCO) junctions. The background resistance above the gaplike features is flat in the majority of the junctions, while about 20% of the junctions show decreasing resistance with bias. For Pb and PbBi counterelectrodes, negligible leakage currents at low temperatures indicate good quality tunnel barriers, continuous and pin-hole free. The phonon structures of these materials indicate a single-step tunneling process. The behavior of the zero-bias resistance dip observed in some junctions is discusse