Charge-density wave, superconductivity and $f$-electron valence instability in EuBiS$_2$F

TL;DR

EuBiS₂F exhibits a rare coexistence of charge-density wave and superconductivity driven by Eu valence fluctuations, with strong Eu-4f and Bi-6p hybridization enhancing electronic correlations.

Contribution

This study reveals EuBiS₂F as the first material to simultaneously host CDW, SC, and f-electron valence instability without doping, linked to Eu valence fluctuations and hybridization.

Findings

CDW transition at 280 K

Superconductivity at 0.3 K

Enhanced electronic specific heat due to Eu-Bi hybridization

Abstract

Superconductivity (SC) and charge-density wave (CDW) are two contrasting yet relevant collective electronic states which have received sustained interest for decades. Here we report that, in a layered europium bismuth sulfofluoride, EuBiS$_2$F, a CDW-like transition occurs at 280 K, below which SC emerges at 0.3 K, without any extrinsic doping. The Eu ions were found to exhibit an anomalously temperature-independent mixed valence of about +2.2, associated with the formation of CDW. The mixed valence of Eu gives rise to self electron doping into the conduction bands mainly consisting of the in-plane Bi-6$p$ states, which in turn brings about the CDW and SC. In particular, the electronic specific-heat coefficient is enhanced by ~ 50 times, owing to the significant hybridizations between Eu-4$f$ and Bi-6$p$ electrons, as verified by band-structure calculations. Thus, EuBiS$_2$F manifests itself as an unprecedented material that simultaneously accommodates SC, CDW and $f$-electron valence instability.