Coexistence of extended and point-like van Hove singularity in heavily n-doped graphene

Tuning the Fermi level to a van Hove singularity (VHS) can lead to new quantum phases such as superconductivity, charge, or spin density waves due to the instability of the Fermi surface emerging from the divergence of the density of states. Here, we present an angle-resolved photoemission spectroscopy study of Yb-intercalated graphene on a 4H-SiC(0001) substate doped to the VHS. We observe different topologies of the two VHS below (point-like) and above (extended) the Dirac point of graphene. The appearance of an extended VHS suggests a renormalization of the electronic structure due to the enhanced many-body interactions in the system. Strong hybridization of Yb states with the π bands of graphene is observed that open up an energy gap of ∼ 300 meV in the graphene Dirac states. Our results also show that the majority of the intercalated Yb atoms stay in the Yb²⁺ charge state. The mixing of states of intercalated atoms with the graphene states ensures the possibility of realizing spin-polarized Dirac states when doped with the magnetic atoms.