Confinement-driven electronic, magnetic and topological phases in corundum-derived oxide honeycomb superlattices

Based on density functional theory calculations with a Hubbard U parameter, we explore electronic, magnetic and possibly topologically non-trivial phases in X₂O₃ honeycomb layers confined in the corundum structure Al₂O₃ (0001). For X=3d cation the ground state is in most cases a trivial antiferromagnetic Mott insulator. However, with imposed symmetry of the two sublattices the ferromagnetic phases of Ti, Mn, Co and Ni exhibit a characteristic set of four bands associated with the single occupation of e_g’ (Ti) and e_g (Mn, Co, Ni). The Dirac point can be tuned to the Fermi level by strain and a substantial anomalous Hall conductivity arises when spin-orbit coupling (SOC) is switched on. Moreover, a particularly strong effect of SOC is identified for X=Ti at a_Al2O3 = 4.81Å with an unusually high orbital moment of -0.88 μ_B nearly compensating the spin moment of 1.01 μ_B. Thus, this system emerges as a possible realization of Haldane’s model of spinless fermions. We further extend our work to the 4d and 5d series and identify cases of high orbital moment (Os) or candidates for Chern insulators (CI), i.e. X=Tc and Pt with C=-2 and -1, as a function of the Coulomb repulsion strength.
[1] O. Koeksal et al., arXiv:1704.08981.