Interplay of Valence and Semimetal-to-Insulator Transitions in SmS

TL;DR

This paper models the valence and phase transitions in SmS under pressure, revealing a quantum critical point and exciton condensates, which explain experimental anomalies and suggest a broader relevance of valence fluctuation phenomena.

Contribution

It introduces a model for SmS that links Coulomb interactions to simultaneous valence and semimetal-insulator transitions, identifying a quantum critical point and exciton condensates.

Findings

Identification of a quantum critical point in SmS

Explanation of specific heat and compressibility anomalies

Prediction of exciton condensates in phase diagram

Abstract

Recent discoveries of a new type of quantum criticality arising from Yb-valence fluctuations in Yb-based metal in periodic crystal and quasicrystal have opened a new class of quantum critical phenomena in correlated electron systems. To clarify whether this new concept can be generalized to other rare-earth-based semimetal and insulator, we study SmS which exhibits golden-black phase transition under pressure. By constructing the model for SmS, we show that Coulomb repulsion between 4f and 5d orbitals at Sm drives first-order valence transition (FOVT) and semimetal-to-insulator transition (MIT) simultaneously, which explains the golden-black phase transition. We clarify the ground-state phase diagram for the FOVT and MIT by identifying the quantum critical point of the FOVT. We find that exciton condensates in both semimetal and insulator phases. Our result explains measured peak anomalies in the specific heat and compressibility in pressurized golden SmS and provides a cue to clarify recently-observed anomalies in black SmS.