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

Abstract

We report the first measurements of the normal-state reflectivity and superconducting energy gap of ${\mathrm{Y}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$ single crystals. With the electric field in the metallic, a-b plane of our platelike crystals we observe a Drude normal-state reflectivity, and an enhancement of the reflectivity at temperatures below ${\mathrm{T}}_{\mathrm{c}}$(90 K) which follows the Mattis-Bardeen form. The superconducting energy gap obtained from these measurements is found to be temperature dependent, increasing to a value of 2\ensuremath{\Delta}/${\mathrm{kT}}_{\mathrm{c}}$\ensuremath{\simeq}8 below \ensuremath{\simeq}50 K. Within the Eliashberg-BCS framework this unusually large gap value suggests that ${\mathrm{Y}}_{1}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$ is a very strongly coupled superconductor.