Possible charge-density wave, superconductivity, and f -electron valence instability in EuBiS2F

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, EuBiS2F, 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 a possible dynamic CDW. The mixed valence of Eu gives rise to self electron doping into the conduction bands mainly consisting of the in-plane Bi6p 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 Eu4f and Bi6p electrons, as verified by band-structure calculations. Thus EuBiS2F manifests itself as an unprecedented material that simultaneously accommodates SC, CDW, and f-electron valence instability.