Tunneling and infrared measurements of the energy gap in the high-critical-temperature superconductor Y-Ba-Cu-O

Abstract

We have measured the superconducting energy gap \ensuremath{\Delta} of high-critical-temperature (${T}_{c}$\ensuremath{\sim}90 K) ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{9\mathrm{\ensuremath{-}}\mathrm{y}}$ samples using tunneling and infrared techniques. From our tunneling data, we place limits on the reduced superconducting energy gap of 3.72\ensuremath{\Delta}/${k}_{B}$${T}_{c}$5.6. The tunneling gaps for a single-crystal sample are essentially the same if the tunneling tip is oriented either parallel or perpendicular to the CuO planes. The infrared measurements give a smaller apparent gap. We discuss the origins of the differences between the infrared and tunneling results.