OPTICAL STUDY ON HEAVY-FERMION AND MIXED-VALENCE IN YB4AS3

Temperature dependence of the electronic structure of Yb4As3 was inves tigated by the observation of the reflectivity spectra in the energy r ange from 2 meV to 50 eV at several temperatures from 39 to 320 K. In the photon energy range below 10 meV, the temperature dependence of th e spectrum can be qualitatively explained by two-band model previously proposed. An absorption band with strong temperature dependence was o bserved at 0.4 eV. The origin is considered to be the mixing state bet ween unoccupied Yb3+-4f and 5d. The state is considered to be importan t to understand the anomalous temperature dependence of the physical p roperties. In the energy range above 1 eV, the spectrum does not chang e with the temperature. The structure of the occupied and unoccupied s tates near the Fermi level is clarified.