Far-infrared optical properties of pyrochlore heavy fermion superconductor Cd$_2$Re$_2$O$_7$ in the normal and superconducting states*

Cd$_2$Re$_2$O$_7$ is a pyrochlore oxide which exhibits superconductivity with a transition temperature \textit{T$_C$} near 1 K. The far-infrared optical properties of Cd$_2$Re$_2$O$_7$ will be presented at temperatures above and below \textit{T$_C$}. Superconductivity induced changes in the phonon structure are observed. Thermal reflectance spectra show two absorption features, near 9.6 and 19.3 cm$^{-1}$ which arise in the superconducting state. Optical conductivity spectra reveal a softening ($\sim$ 1 cm$^{-1}$) of the phonon mode at 35 cm$^{-1}$ in the superconducting state. Analysis of the frequency dependent optical effective mass and scattering rate support the classification of this material as a modest heavy electron system at low temperatures.