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

The topological and thermoelectric properties of (EuO)$_{n}$/(MgO)$_{m}$(001) superlattices (SLs) are explored using density functional theory calculations including a Hubbard $U$ term together with Boltzmann transport theory. In (EuO)$_{1}$/(MgO)$_{3}$(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 $5d$ conduction electrons. This inversion between bands of opposite parity is accompanied by a spin reorientation in the spin-texture along the contour of band crossing surrounding the $Γ$ 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 $μ$VK$^{-1}$. A similar SOC-induced band inversion takes place also in the ferromagnetic semimetallic (EuO)$_{2}$/(MgO)$_{2}$(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 thermoelectric properties. Both systems emphasize the relation between non-trivial topological bands and thermoelectricity also in systems with broken inversion symmetry.