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Superconductivity (SC) and charge-density wave (CDW) are two contrasting yetrelevant collective electronic states which have received sustained interestfor 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 at0.3 K, without any extrinsic doping. The Eu ions were found to exhibit ananomalously temperature-independent mixed valence of about +2.2, associatedwith the formation of CDW. The mixed valence of Eu gives rise to self electrondoping 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, theelectronic specific-heat coefficient is enhanced by ~ 50 times, owing to thesignificant hybridizations between Eu-4$f$ and Bi-6$p$ electrons, as verifiedby band-structure calculations. Thus, EuBiS$_2$F manifests itself as anunprecedented material that simultaneously accommodates SC, CDW and$f$-electron valence instability.

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