Superconductivity and magnetic ordering in chalcogen-intercalated graphene bilayers with charge compensation

This work introduces a new class of two-dimensional crystals with the structure AC$_8$XC$_8$, consisting of two layers of graphene, a chalcogen (X = O, S, Se, Te) intercalation layer, and an alkaline earth (A = Be, Ca, Mg, Sr, Ba) adlayer. The electronic band structure for the 20 compounds was studied using density functional theory. The chalcogen $p$ orbitals interact with the carbon $\pi$ orbitals to form weakly dispersing bands that give rise to complex Fermi surfaces featuring electron and hole pockets whose densities exactly compensate each other, and van Hove singularities that are very close to, or coincident with, the Fermi level in the majority of compounds studied. The resulting electron-electron interaction effects are studied using both the temperature-flow renormalisation group approach and a spin fluctuation model, which show a dominant ferromagnetic instability coexisting with $p$- or $f$-wave spin triplet superconductivity over a range of temperatures.