Temperature dependent Eu 3d-4f X-ray Absorption and Resonant Photoemission Study of the Valence Transition in $EuNi_2(Si_{0.2}Ge_{0.8})_2$

摘要

We study the mixed valence transition ($T$$_{v}$ $\sim$80 K) inEuNi$_{2}$(Si$_{0.2}$Ge$_{0.8}$)$_{2}$ using Eu 3$d-4f$ X-ray absorptionspectroscopy (XAS) and resonant photoemission spectroscopy (RESPES). TheEu$^{2+}$ and Eu$^{3+}$ main peaks show a giant resonance and the spectralfeatures match very well with atomic multiplet calculations. The spectra showdramatic temperature ($T$)-dependent changes over large energies ($\sim$10 eV)in RESPES and XAS. The observed non-integral mean valencies of $\sim$2.35 $\pm$0.03 ($T$ = 120 K) and $\sim$2.70 $\pm$ 0.03 ($T$ = 40 K) indicate homogeneousmixed valence above and below $T$$_{v}$. The redistribution betweenEu$^{2+}$$4f^7$+$[spd]^0$ and Eu$^{3+}$$4f^6$+$[spd]^1$ states is attributed toa hybridization change coupled to a Kondo-like volume collapse.