Two-dimensional elemental Ca sd2 Dirac honeycomb

The discovery of graphene inaugurated research on the novel relativistic and topological electronic properties of 2D honeycomb quantum materials. One wonders whether other elemental graphene analogues beyond the group 4a elements exist. Motivated by revolutionary exploration of the synthesis and electronic characterization in Xenes, we address whether metallic elements can also embody similar molecular and electronic structures. We find that alkaline earth Ca monolayer (calcene) on Cu(111) indeed forms a new 2D honeycomb monolayer with relativistic and topological electronic properties. Structural characterization reveals that an epitaxial monoatomic calcene layer with a hexagonal  lattice with respect to Cu(111), leading to  atomic corrugation. Further evaluation by electronic spectroscopy and density functional theory reveals calcene overlayer on Cu(111) to possesses Dirac energy bands with an ultrahigh Fermi velocity (~  m/s), which is attributed to a theoretically anticipated s/d orbital hybridization (sd2 graphene) in an electronic Kagome lattice. Further theoretical investigations predict isolated calcene to be a ferromagnetic with a magnetic moment of ~0.37 B/Ca and to possess topological nodal-line band near the Fermi level. In addition, a similar Ca honeycomb structure is reproduced on Ag(111) substrate, demonstrating that the observation of calcene is not isolated case on Cu(111). The discovery of sd2 graphene electronic structure alkaline earth honeycomb monolayer expands the scope of 2D quantum materials with novel electronic properties for nanoscience and technology.