Infrared evidence for gap anisotropy in YBa2Cu3O7

Infrared reflectivity measurements of crystalline YBa2Cu3O7 (Tc92 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 (8kTc), while a much smaller energy scale (3kTc) is obtained with the field perpendicular to the planes suggesting substantial gap anisotropy. Temperature-dependent measurements near Tc demonstrate a clear relationship between the development of the 8kTc feature and the superconducting transition. © 1989 The American Physical Society.