Topological insulators in transition-metal intercalated graphene: The role of d electrons in significantly increasing the spin-orbit gap

We study the effect of transition-metal intercalation of graphene on the formation of a two-dimensional topological insulator with experimentally measurable edge states. Our first-principles calculations reveal that the spin-orbit-coupling (SOC) gap in Re-intercalated graphene on SiC(0001) substrate can be as large as 100 meV. This value is five orders of magnitude larger than that of pristine graphene. A similar effect should also exist in Mn- or Tc-intercalated graphene. A tight-binding model Hamiltonian analysis establishes the role of orbital coupling between graphene p states and transition-metal d states on the formation of the SOC gap. Remarkably, the gap can be larger than atomic SOC, as is the case for Mn. This finding opens the possibility of designing topological insulators comprised of only relatively light elements. Our study also reveals that the presence of the substrate should induce a splitting between the K and K' valleys with the potential to integrate spintronics with valleytronics.