Influence of the electron phase transition on the lattice dynamics of YbInCu4

Measurements of the acoustic phonon branches along the [110] direction of YbInCu4 at temperatures from 10 to 295K have been performed in order to study the effects of the transition to an intermediate-valence state on the lattice dynamics. At all measured wave vectors, the phonon energies are higher below the transition temperature (Tc=40K) than at room temperature. The strongest effects (40%) are observed near the zone boundary. The unit-cell volume increases below the transition. Hence, the observed hardening of the acoustic vibrations should undoubtedly be related to the electronic transition into the intermediate valence state.