Far-Infrared Conductivity of the High-Tc Superconductor YBa2Cu3O7

We present reflectance data for the high-temperature superconductor YBa2Cu3O7, and the Kramers-Kronig-transformed conductivity from 50 to 900 cm-1. There is a rich phonon structure with peaks at 151, 191, 279, 310, 548, and 609 cm-1. The continuous background is Drude type at high temperature but below the superconducting transition there is a region of suppressed conductivity consistent with a superconducting gap with the weak-coupling BCS value. Also associated with superconductivity are changes to the low-lying phonons and a plasma-type edge at 60 cm-1.