Tunneling Measurements of the Energy Gap in High Tc Oxide Superconductors

Abstract

We have measured the current-voltage (I-V) characteristics for tunneling into the high critical temperature superconducting materials La-Sr-Cu-O and Y-Ba-Cu-O. We find superconducting energy gaps that are consistent with strong coupling superconductivity in both materials. We find no significant differences in the measured gaps when the tunneling tip approaches a single crystal of Y-Ba-Cu-O from a direction normal to the CuO planes, as opposed to parallel to them. There are structures in the conductance-voltage characteristics above the gap energy that can occasionally have very large amplitudes. The physical origin of this structure is not certain. However, its presence makes it difficult to obtain quantitative information on the electron-phonon coupling strength from such tunneling measurements.