Optical evidence for mass enhancement of quasiparticles in pyrochlore   Cd$_2$Re$_2$O$_7$
  We report on the results of optical studies of the newly discovered superconductor Cd$_2$Re$_2$O$_7$ in the normal state. We show that the compound has an exotic metallic state at low temperature. The optical conductivity spectrum exhibits two distinct features: a sharp renormalized resonance mode at zero frequency and a broad mid-infrared excitation band. Detailed analysis reveals a moderate enhancement of the effective mass at low temperature and low frequency. 
N. L. Wang, J. J. McGuire, T. Timusk, R. Jin, J. He, and D Mandrus