Persistent Magnetism and Tunable Doping of Monolayer Graphene via Europium Density Modulation

Reaching the van Hove singularity (VHS) in a material enables the emergence of exotic electronic and magnetic phases, such as superconductivity and the quantum anomalous Hall effect. This is demonstrated in cuprates, magic-angle bilayer graphene, and more recently, monolayer graphene interfaced with alkali and rare earth elements. Here, the europium density at the graphene/rhenium interface is modulated to tune the electron doping level in monolayer graphene across the VHS point, forming either a dense or diluted europium phase. The dense phase enables flat bands at the Fermi level, while graphene remains decoupled from the Re(0001) substrate in both cases. The Dirac point is shifted over 1.5 eV below the Fermi level, and europium lifts the degeneracy of the Dirac cones: one branch hybridizes with Eu 4f states, the other retains Dirac-like dispersion, as corroborated by density functional theory. X-ray absorption spectroscopy reveals a mixed Eu(II)/Eu(III) valence state in the dense phase and the persistence of Eu magnetic response up to room temperature in both. The intercalated phases exhibit exceptional thermal stability, with the diluted phase stable up to 960 K. These results highlight the potential of rare-earth-doped graphene for engineering flat bands, tunable Dirac-cone splitting, and robust interfacial magnetism.