Electronic structure changes across the valence transition in EuNi ₂(Si_0.2Ge_0.8)₂

We study the electronic structure of EuNi₂(Si _0.2Ge_0.8)₂, which exhibits a temperature dependent mixed valence transition, using 4d-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 Ge-Si partial DOS in the valence band. The bulk divalent Eu 4f character states are centered at a binding energy of about 0.75 eV, significantly away from the Fermi level. While the surface divalent feature is negligibly affected, the spectra obtained using He IIα 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 _υ ∼ 80 K. The He Iα UPS also exhibit spectral intensity changes across T_υ. The non-f character conduction band states at and near the Fermi level exhibit spectral weight changes up to 350 meV with a small energy (∼25 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_υ 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 EuNi₂(Si _0.2Ge_0.8)₂.