Far-Infrared Conductivity of the High-$T_c$ Superconductor YB${\mathrm{a}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

We present reflectance data for the high-temperature superconductor Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, and the Kramers-Kronig-transformed conductivity from 50 to 900 ${\mathrm{cm}}^{-1\mathrm{}}$. There is a rich phonon structure with peaks at 151, 191, 279, 310, 548, and 609 ${\mathrm{cm}}^{-1\mathrm{}}$. 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 ${\mathrm{cm}}^{-1\mathrm{}}$.