Valence-transition-induced changes of the electronic structure in EuPd$_2$Si$_2$

TL;DR

This study investigates how the electronic structure of EuPd₂Si₂ changes across a valence transition using advanced spectroscopy and theoretical calculations, revealing significant band shifts and partial charge transfer.

Contribution

It provides the first detailed experimental and theoretical analysis of valence-induced electronic structure changes in EuPd₂Si₂, linking valence state variations to band structure modifications.

Findings

Eu valence decreases from 2.5 to 2.1 across the transition

Eu 5d bands shift to lower binding energies with increasing Eu 4f occupancy

Partial charge transfer occurs between Eu and Pd/Si sites

Abstract

We present results of hard X-ray angle-resolved photoemission spectroscopy and photoemission diffraction measurements performed on high-quality single crystals of the valence transition compound EuPd$_2$Si$_2$ for temperatures 25~K $\leq$ T $\leq$ 300~K. At low temperatures we observe a Eu $4f$ valence $v=2.5$, % occupation number $n=6.5$, which decreases to $v=2.1$ for temperatures above the valence transition around $T_V \approx 160$~K. The experimental valence numbers resulting from an evaluation of the Eu(III)/Eu(II) $3d$ core levels, are used for calculating band structures using density functional theory. The valence transition significantly changes the band structure as determined by angle-resolved photoemission spectroscopy. In particular, the Eu $5d$ valence bands are shifted to lower binding energies with increasing Eu $4f$ occupancy. To a lesser extent, bands derived from the Si $3p$ and Pd $4d$ orbitals are also affected. This observation suggests a partial charge transfer between Eu and Pd/Si sites. Comparison with {\it ab-initio} theory shows a good agreement with experiment, in particular concerning the unequal band shift with increasing Eu $4f$ occupancy.