Ab initio study of nontrivial topological phases in corundum structured $($M$_2$O$_3)/($Al$_2$O$_3)_5$ multilayers

Abstract: \textit{Ab initio} calculations have been performed on hexagonal layers of M$_2$O$_3$ (M being several transition metals of the $5d$ series) sandwiched by a band insulator such as Al$_2$O$_3$ that provides the honeycomb lattice where the $5d$ electrons reside. This corundum-structure-based superlattice is the most obvious way to design a honeycomb lattice with transition metal cations avoiding the use of largely polar surfaces. We obtain that this system supports the presence of Dirac cones at the Fermi level that open up with the introduction of spin-orbit coupling at various fillings of the $5d$ band. The DFT calculations performed in this work show that the $5d^5$ situation is always a trivial insulator, whereas the $5d^8$ filling presents topological insulating configurations which evolve into a trivial state with increasing tensile strain or on-site Coulomb potential U. However, LDA+U calculations show a stable antiferromagnetic solution for the $5d^8$ case at every U value, which would break time reversal symmetry and could affect the topological properties of the system. We also discuss the similarities with the buckled honeycomb lattice obtained using perovskite (111) bilayers previously studied in literature, in particular for the $5d^5$ and $5d^8$ configurations. This work provides some clues on the stability of topological phases using metal oxides in general.