Optical study of the electronic phase transition of strongly correlated YbInCu4

Infrared, visible, and near-UV reflectivity measurements are used to obtain conductivity as a function of temperature and frequency in YbInCu4, which exhibits an isostructural phase-transition into a mixed-valent phase below T-upsilon similar or equal to 42 K. In addition to a gradual loss of spectr al weight with decreasing temperature extending up to 1.5 eV, a sharp reson ance appears at 0.25 eV in the mixed-valent phase. This feature can be desc ribed in terms of excitations into the Rondo (Abrikosov-Suhl) resonance, an d, like the sudden reduction of resistivity, provides a direct reflection o f the onset of coherence in this strongly correlated electron system.