TUNNELING AND PROXIMITY EFFECT STUDIES OF THE HIGH-TEMPERATURE SUPERCONDUCTORS

Tunneling and proximity effect studies of the high-temperature superconductors are reviewed. The data from various studies are compared in order to discern their common features. A considerable degree of consistency is found. The existence of very large gaps as reported in the early experiments is confirmed, and evidence for additional, smaller gaps is presented. The anomalous linear differential conductance observed at high voltages in the early experiments is found to be, perhaps, a property of tunneling in the c-axis direction. Proximity effect studies demonstrate that Josephson proximity coupling appears to exist only into the a-b plane of these materials, and not along the c-axis. It is also found to be apparently very sensitive to any boundary resistance (likely of intrinsic origins) at the superconducting/normal metal interface.