Unified understanding of the valence transition in the rare-earth monochalcogenides under pressure

TL;DR

This paper presents a unified model explaining pressure-induced valence transitions in rare-earth monochalcogenides, highlighting the roles of 4f electron promotion and band hybridization in these complex materials.

Contribution

It introduces a comprehensive picture of valence transitions driven by 4f electron promotion into the 5d band, linking the process to density of states and hybridization effects.

Findings

Valence transition driven by 4f electron promotion into 5d band.

Transition nature depends on density of states at low pressure.

High pressure transition governed by hybridization strength.

Abstract

Valence instability is a key ingredient of the unusual properties of f electron materials, yet a clear understanding is lacking as it involves a complex interplay between f electrons and conduc- tion states. Here we propose a unified picture of pressure-induced valence transition in Sm and Yb monochalcogenides, considered as model system for mixed valent 4f-electron materials. Using high-resolution x-ray absorption spectroscopy, we show that the valence transition is driven by the promotion of a 4f electron specifically into the lowest unoccupied (LU) 5d t2g band. We demonstrate with a promotional model that the nature of the transition at low pressures is intimately related to the density of states of the LU band, while at high pressures it is governed by the hybridization strength. These results set a new standard for the generic understanding of valence fluctuations in f-electron materials.