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

We study the electronic structure of Tsai-type cluster-based quasicrystalline approximants, ${\mathrm{Au}}_{64}{\mathrm{Ge}}_{22}{\mathrm{Yb}}_{14}$ (AGY-I), ${\mathrm{Au}}_{63.5}{\mathrm{Ge}}_{20.5}{\mathrm{Yb}}_{16}$ (AGY-II), and ${\mathrm{Zn}}_{85.4}{\mathrm{Yb}}_{14.6}$ (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 $\mathrm{Yb} 4f$ 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 $4f$ 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.

Figure 1

Two types of successive shell structures of the Tsai clusters. For Yb-based quasicrystalline approximants in this study, the pink spheres correspond to Yb atoms and the blue spheres are non-Yb atoms such as Au, Ge, or Zn atoms [12, 13, 14, 15]. The difference is in the innermost component; (a) non-Yb tetrahedron or (b) single Yb atom. The former is orientationally disordered in most cases. Successively surrounding non-Yb dodecahedron, Yb icosahedron, and non-Yb icosidodecahedron are common to both clusters. The inequivalent Yb atoms are labeled as Yb1 for the vertex of the icosahedron and Yb2 for the center of the cluster.

Figure 2

(a) $\mathrm{Yb} 3d_{5/2}$ core-level HAXPES spectra of AGY-I, AGY-II, and Zn-Yb. The inset shows the overall $3d$ spectrum of Zn-Yb. (b) The fitting results (black line), using the atomic multiplet model for free ${\mathrm{Yb}}^{2+}$ (pink shading) and ${\mathrm{Yb}}^{3+}$ (cyan shading) ions, and the integral-type background (orange dotted line). The inset shows the $\mathrm{Zn} 2p$ core-level spectrum of Zn-Yb in a logarithmic scale plot. Arrows represent the plasmon energy, which is reflected as the yellow line in fitting of the $\mathrm{Yb} 3d_{5/2}$ spectrum of Zn-Yb.

Figure 3

Valence band HAXPES spectra of AGY-I, AGY-II, and Zn-Yb. The spectra are normalized to an integrated intensity up to $E_{\mathrm{B}}=0.5$ eV. Arrows indicate the additional feature discussed in the text.

Figure 4

Valence band spectra of (a) AGY-I, (b) AGY-II, and (c) Zn-Yb, measured with $h\nu$'s of hard x-ray (7940 eV), SX (600 eV), and VUV (20 eV). The spectra are normalized to an integrated intensity up to $E_{\mathrm{B}}=0.5$ eV.