Superconductivity in Europium Bismuth Sulfofluorides

The layered bismuth sulfofluorides EuBiS₂F and Eu₃Bi₂S₄F₄ show unique properties among all the BiS₂-based superconductors. Both compounds exhibit a possible charge-density-wave (CDW) instability around 280 K and undergo superconducting transitions at 0.3 and 1.5 K, respectively, without extrinsic doping. The self-doping effect due to the mixed valence state of Eu ions, which allows charge transfer from Eu to BiS₂ layers, accounts for the metallic behavior and superconductivity. Here we present a brief review of the research progress in these Eu-containing bismuth sulfofluorides, including the structure and electronic properties, external pressure effects, chemical pressure effects and doping effects. The results can provide important information towards a deeper understanding of BiS₂-based superconductors. ©2019 The Physical Society of Japan