Infrared evidence for gap anisotropy in YBa2Cu3O7

Abstract

Infrared reflectivity measurements of crystalline ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ (${\mathrm{T}}_{\mathrm{c}}$≃92 K) are performed with the electric field both parallel and perpendicular to the Cu-O planes. We find evidence for an unusually large in-plane energy gap (∼8${\mathrm{kT}}_{\mathrm{c}}$), while a much smaller energy scale (∼3${\mathrm{kT}}_{\mathrm{c}}$) is obtained with the field perpendicular to the planes suggesting substantial gap anisotropy. Temperature-dependent measurements near ${\mathrm{T}}_{\mathrm{c}}$ demonstrate a clear relationship between the development of the ∼8${\mathrm{kT}}_{\mathrm{c}}$ feature and the superconducting transition.