Chern insulating phases and thermoelectric properties of EuO/MgO(001) superlattices

The topological and thermoelectric properties of ${(\mathrm{Eu}\mathrm{O})}_n/{(\mathrm{Mg}\mathrm{O})}_m$(001) superlattices (SLs) are explored using density functional theory calculations including a Hubbard $U$ term together with Boltzmann transport theory. In ${(\mathrm{Eu}\mathrm{O})}₁/{(\mathrm{Mg}\mathrm{O})}₃$(001) SL at the lattice constant of MgO a sizable band gap of 0.51 eV is opened by spin-orbit coupling (SOC) due to a band inversion between occupied localized Eu $4f$ and empty $5d$ conduction states. This inversion between bands of opposite parity is accompanied by a reorientation in the spin texture along the contour of band inversion surrounding the $\mathrm{\ensuremath{\Gamma}}$ point and leads to a Chern insulator with $C$ = --1, also confirmed by the single edge state. Moreover, this Chern insulating phase shows promising thermoelectric properties, e.g., a Seebeck coefficient between 400 and $800\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}{\mathrm{VK}}^{\ensuremath{-}1}$. A similar SOC-induced band inversion takes place also in the ferromagnetic semimetallic ${(\mathrm{Eu}\mathrm{O})}₂/{(\mathrm{Mg}\mathrm{O})}₂$(001) SL. Despite the vanishing band gap, it leads to a substantial anomalous Hall conductivity with values up to --1.04 ${e}²/h$ and somewhat lower Seebeck coefficient. Both cases emphasize the relation between nontrivial topological bands and thermoelectricity also in systems with broken inversion symmetry.