ZERO-BIAS ANOMALIES IN THE CURRENT-VOLTAGE AND CONDUCTANCE-VOLTAGE CHARACTERISTICS OF HIGH-CRITICAL-TEMPERATURE SUPERCONDUCTOR JUNCTIONS

The occurrence of zero-bias anomalies (ZBAs) in the current-voltage and conductance-voltage characteristics of high-critical-temperature superconductors (HTS) is reviewed. Following a historical introduction, we summarize studies noting HTS ZBAs. Several explanations of the ZBAs in HTS tunneling characteristics have been proposed including a resistance in series with a Josephson junction, a conductive channel near the Fermi energy epsilon(F), localized magnetic states on film surfaces, proximity effects, a peak in the electron density of states near epsilon(F), an array of junctions in an HTS ceramic sample, coupling between a film's normal surface layer and the superconducting layer below the surface, quasiparticle tunneling, and phase diffusion. The plausibility and implications of these suggested explanations are discussed. Experiments to answer questions that remain regarding the source and significance of the ZBAs are proposed.