Optical study of archetypical valence-fluctuating Eu-systems
  We have investigated the optical conductivity of the prominent valence fluctuating compounds EuIr2Si2 and EuNi2P2 in the infrared energy range to get new insights into the electronic properties of valence fluctuating systems. For both compounds we observe upon cooling the formation of a renormalized Drude response, a partial suppression of the optical conductivity below 100 meV and the appearance of a mid-infrared peak at 0.15 eV for EuIr2Si2 and at 0.13 eV for EuNi2P2. Most remarkably, our results show a strong similarity with the optical spectra reported for many Ce- or Yb-based heavy fermion metals and intermediate valence systems, although the phase diagrams and the temperature dependence of the valence differ strongly between Eu- and Ce-/Yb-systems. This suggests that the hybridization between 4f- and conduction electrons, which is responsible for the properties of Ce- and Yb-systems, plays an important role in valence fluctuating Eu-systems. 
V. Guritanu, S. Seiro, J. Sichelschmidt, N. Caroca-Canales, T. Iizuka,   S. Kimura, C. Geibel, and F. Steglich
cond-mat.str-el