Hard x-ray photoemission study of the temperature-induced valence transition systemEuNi2(Si1−xGex)2

Abstract

We investigated the bulk-derived electronic structure of the temperature-induced valence transition system ${\mathrm{EuNi}}_{2}$(${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$)${}_{2}$ ($x=0.70$, 0.79, and 0.82) by means of hard x-ray photoemission spectroscopy (HAXPES). The HAXPES spectra clearly show distinct temperature dependencies in the spectral intensities of the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}\phantom{\rule{4pt}{0ex}}3d$ components. For $x=0.70$, the changes in the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}\phantom{\rule{4pt}{0ex}}3d$ spectral components with temperature reflect a continuous valence transition, whereas the sudden changes for $x=0.79$ and 0.82 reflect first-order valence transitions. The Eu $3d$ spectral shapes for all $x$ and particularly the drastic changes in the ${\mathrm{Eu}}^{3+}\phantom{\rule{4pt}{0ex}}3d$ feature with temperature are validated by a theoretical calculation based on the single-impurity Anderson model (SIAM). SIAM analysis reveals that the valence transition for each $x$ is controlled by the $c\ensuremath{-}f$ hybridization strength and the charge-transfer energy. Furthermore, the $c\ensuremath{-}f$ hybridization strength governs the valence transition of this system, which is either first order or continuous, consistent with Kondo volume collapse.