Superconducting energy gap and normal-state reflectivity of single crystal Y-Ba-Cu-O

We report the first measurements of the normal-state reflectivity and superconducting energy gap of ${\mathrm{Y}}_1$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ single crystals. With the electric field in the metallic, a-b plane of our platelike crystals we observe a Drude normal-state reflectivity, and an enhancement of the reflectivity at temperatures below ${\mathrm{T}}_{\mathrm{c}}$(90 K) which follows the Mattis-Bardeen form. The superconducting energy gap obtained from these measurements is found to be temperature dependent, increasing to a value of 2Δ/${\mathrm{kT}}_{\mathrm{c}}$≃8 below ≃50 K. Within the Eliashberg-BCS framework this unusually large gap value suggests that ${\mathrm{Y}}_1$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{x}}$ is a very strongly coupled superconductor.