A Multiband Tight-Binding Model for Intact Dirac Cones in Graphene on Ni and Co

When the A-B sublattice symmetry in graphene is broken its linear bands of Dirac Fermions are destroyed and a parabolic dispersion arises [1]. Recent ARPES and SARPES measurements in graphene/Ni(Co) in the AC stacking configuration have shown to exhibit intact Dirac cones instead, as well as, spin separation in the graphene’s bands [2,3]. This would indicate that there is still some physics missing in the current models that describe these systems. In this work, we present an analytical multi-orbital tight-binding model for a monolayer of graphene over a monolayer of Co(Ni). We take into account the spin-orbit coupling of the intrinsic and Rashba type, as well as the magnetization of Ni(Co) through a Stoner-Wohlfarth type coupling and consider hybridization of the d_xy, d_xz, d_zy, d_3z²-r² and d_x²-y² orbitals of Co(Ni) atoms with those of graphene’s p_z orbitals within the Slater-Koster approximation. We show that under certain physical conditions, a gapless linear dispersion in the minority spin π-bands can indeed arise in such systems.
[1] A. Varykhalov et. al., PRX 2, 041017 (2012)
[2] D. Marchenko et. al., PRB 91, 235431 (2015)
[3] D. Usachov et. al., Nano Letters 15, 2396 (2015)