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 Y1Ba2Cu3O7-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 Tc(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/kTc8 below 50 K. Within the Eliashberg-BCS framework this unusually large gap value suggests that Y1Ba2Cu3O7-x is a very strongly coupled superconductor. © 1987 The American Physical Society.