Direct observation of heterogeneous valence state in Yb-based quasicrystalline approximants

We study the electronic structure of Tsai-type cluster-based quasicrystalline approximants, Au${}_{64}$Ge${}_{22}$Yb${}_{14}$ (AGY-I), Au${}_{63.5}$Ge${}_{20.5}$Yb${}_{16}$ (AGY-II), and Zn${}_{85.4}$Yb${}_{14.6}$ (Zn-Yb), by means of photoemission spectroscopy. In the valence band hard x-ray photoemission spectra of AGY-II and Zn-Yb, we separately observe a fully occupied Yb 4$f$ state and a valence fluctuation derived Kondo resonance peak, reflecting two inequivalent Yb sites, a single Yb atom in the cluster center and its surrounding Yb icosahedron, respectively. The fully occupied 4$f$ signal is absent in AGY-I containing no Yb atom in the cluster center. The results provide direct evidence for a heterogeneous valence state in AGY-II and Zn-Yb.