Optical study of the electronic phase transition of strongly correlated YbInCu{sub 4}

Infrared, visible, and near-UV reflectivity measurements are used to obtain conductivity as a function of temperature and frequency in YbInCu{sub 4}, which exhibits an isostructural phase-transition into a mixed-valent phase below T{sub v}{approx_equal}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. (c) 2000 The American Physical Society.