Infrared study of the valence transition compound YbInCu4 using cleaved surfaces

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