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 ($T_c$∼90 K) ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{9\mathrm{-}\mathrm{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_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.