Electronic structure changes across the valence transition inEuNi2(Si0.2Ge0.8)2

Abstract

We study the electronic structure of $\mathrm{Eu}{\mathrm{Ni}}_{2}{({\mathrm{Si}}_{0.2}{\mathrm{Ge}}_{0.8})}_{2}$, which exhibits a temperature dependent mixed valence transition, using $4d\text{\ensuremath{-}}4f$ resonant photoemission spectroscopy (RESPES), x-ray absorption spectroscopy (XAS) and temperature-dependent ultraviolet photoemission spectroscopy (UPS). The RESPES studies identify the divalent and trivalent Eu $4f$ character density of states (DOS) which participate in the valence transition. Using the photoionization cross section variation as a function of photon energy, we discuss the Eu, Ni, and $\mathrm{Ge}\ensuremath{-}\mathrm{Si}$ partial DOS in the valence band. The bulk divalent Eu $4f$ character states are centered at a binding energy of about $0.75\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, significantly away from the Fermi level. While the surface divalent feature is negligibly affected, the spectra obtained using He $II\ensuremath{\alpha}$ UPS exhibit temperature dependent bulk Eu $4f$ character states. The bulk divalent spectral weight is transferred to the high energy trivalent states, across the valence transition temperature, ${T}_{v}\ensuremath{\sim}80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The He $I\ensuremath{\alpha}$ UPS also exhibit spectral intensity changes across ${T}_{v}$. The non-$f$ character conduction band states at and near the Fermi level exhibit spectral weight changes up to $350\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ with a small energy $(\ensuremath{\sim}25\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ temperature dependent pseudogaplike feature. The results suggest an increase in effective hybridization strength between the conduction and $4f$ electrons in the low temperature nearly trivalent phase. While the $4f$ character changes across ${T}_{v}$ are qualitatively consistent with change in valence configurations, the temperature dependent spectral changes in the non-$f$ character DOS indicate direct participation in the valence transition in $\mathrm{Eu}{\mathrm{Ni}}_{2}{({\mathrm{Si}}_{0.2}{\mathrm{Ge}}_{0.8})}_{2}$.