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

We have measured the superconducting energy gap Δ of high-critical-temperature (T/sub c/--90 K)= YBa2Cu3O/sub 9-//sub y/ samples using tunneling and infrared techniques. From our tunneling data, we place limits on the reduced superconducting energy gap of 3.7<2Δ/k/sub B/T/sub 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