Anomalous 4$f$ fine structure in TmSe$_{1-x}$Te$_x$ across the metal-insulator transition

TL;DR

This study reveals complex 4$f$ electronic behavior in TmSe$_{1-x}$Te$_x$ across a metal-insulator transition, showing evidence of interactions beyond the traditional Anderson model through advanced photoemission spectroscopy.

Contribution

It provides experimental evidence of electronic interactions beyond the standard Anderson model in mixed-valence systems during a phase transition.

Findings

Subtle 4$f$ peak splitting observed near the Fermi level.

Peak separation increases with lattice parameter expansion.

Evidences interactions beyond the standard Anderson model.

Abstract

Hybridization between localized 4$f$ and itinerant 5$d$6$s$ states in heavy fermion compounds is a well-studied phenomenon and commonly captured by the paradigmatic Anderson model. However, the investigation of additional electronic interactions, beyond the standard Anderson model, has been limited, despite their predicted important role in the exotic quasiparticle formation in mixed-valence systems. We investigate the 4$f$ states in TmSe$_{1-x}$Te$_x$ throughout a semimetal-insulator phase transition, which drastically varies the interactions related to the 4$f$ states. Using synchrotron-based hard x-ray and extreme ultraviolet photoemission spectroscopy, we resolve subtle peak splitting in the 4$f$ peaks near the Fermi level in the mixed-valent semimetal phase. The separation is enhanced by several tens of meV by increasing the lattice parameter by a few percent. Our results elucidate the evolving nature of the 4$f$ state across the phase transition, and provide direct experimental evidence for electronic interactions beyond the standard Anderson model in mixed-valence systems.