  Optical reflectivity R(w) of YbInCu4 single crystals has been measured across
its first-order valence transition at T_v ~ 42 K, using both polished and
cleaved surfaces. R(w) measured on cleaved surfaces Rc(w) was found much lower
than that on polished surface Rp(w) over the entire infrared region. Upon
cooling through T_v, Rc(w) showed a rapid change over a temperature range of
less than 2 K, and showed only minor changes with further cooling. In contrast,
Rp(w) showed much more gradual and continuous changes across T_v, similarly to
previously reported data on polished surfaces. The present result on cleaved
surfaces demonstrates that the microscopic electronic structures of YbInCu4
observed with infrared spectroscopy indeed undergo a sudden change upon the
valence transition. The gradual temperature-evolution of Rp(w) is most likely
due to the compositional and/or Yb-In site disorders caused by polishing.
