Optical study of the electronic phase transition of strongly correlated   YbInCu_4
  Infrared, visible and near-UV reflectivity measurements are used to obtain conductivity as a function of temperature and frequency in YbInCu_4, which exhibits an isostructural phase-transition into a mixed-valent phase below T_v=42 K. In addition to a gradual loss of spectral weight with decreasing temperature extending up to 1.5 eV, a sharp resonance appears at 0.25 eV in the mixed-valent phase. This feature can be described in terms of excitations into the Kondo (Abrikosov-Suhl) resonance, and, like the sudden reduction of resistivity, provides a direct reflection of the onset of coherence in this strongly correlated electron system. 
Sean R. Garner, Jason N. Hancock, Yvonne W. Rodriguez, Zack   Schlesinger (UCSC), Benno Bucher (ITR), Zach Fisk (NHMFL/FSU),...