High-resolution photoemission study of the valence transition in YbInCu4

We have performed a photoemission study of YbInCu4, which undergoes a first-order phase transition at Tv≃40 K between the low-temperature intermediate-valence state and the high-temperature local-moment state. The Yb 4f photoemission peak (Kondo peak) just below the Fermi level, which probably originates from the subsurface region of the YbInCu4 samples but tracks the valence transition in the bulk YbInCu4, has been studied above and below Tv. It has been found that the 4f photoemission peak is shifted from ∼− 35 meV at 75 K to −40 meV at 7 K.By analyzing the 4f peak position and the Yb valence in the two phases using the Anderson impurity model, we find that the bare 4f level, which is located close to the Fermi level, is shifted downward and the hybridization strength decreases in going from the high-temperature to low-temperature phases.