Ordered states coexisting with superconductivity in BICH₂ materials

BiCh₂-based superconductors (Ch = S, Se) exhibit novel properties due to a layered crystal structure alternately stacked with conducting BiCh₂ layers and electron-supply layers. One of the novel properties is an ordered state coexisting with superconductivity. This ordered state has so far been classified into two groups. The first is a ferromagnetic-like ordered state that emerges in materials with electron-supply layers including Ce ions. The magnetic susceptibility of these materials shows a ferromagnetic-like jump. The magnetic order is considered to originate from the Ce ions in the electron-supply layers. This result suggests that the superconductivity and the magnetic order are realized in different layers. The second is a charge-density-wave (CDW)-like ordered state. CDW instability has been predicted in BiCh₂ materials by theoretical studies. Real-space observation by scanning tunneling microscopy visualized a CDW-like supermodulation. However, the signatures of this structure are inconsistent with those of the predicted CDW order. Therefore, the origin of this CDW-like supermodulation is still an open question. In this paper, characteristics of BiCh₂ materials with the above ordered states will be reviewed.