  Infrared measurements are used to obtain conductivity as a function of
temperature and frequency in YbInCu_4, which exhibits an isostructural
transition to a mixed-valent state at T_v \simeq 42 K. In addition to a gradual
loss of spectral weight with decreasing temperature extending up to 1.5 eV,
sharp resonances appear in the mixed-valent state at 0 and 0.25 eV . These
features may be key to understanding both YbInCu_4 and the nature of the
mixed-valent Kondo state.

Strongly Correlated Electrons