Effect of confinement and octahedral rotations on the electronic, magnetic, and thermoelectric properties of correlated SrXO3/SrTiO3(001) superlattices ( X=V , Cr, or Mn)

Abstract

By using density functional theory calculations with an on-site Coulomb repulsion term combined with Boltzmann transport theory, we explore the effect of ${t}_{2g}$ orbital occupation on the electronic, magnetic, and thermoelectric properties of ${(\mathrm{Sr}X{\mathrm{O}}_{3})}_{1}/{({\mathrm{SrTiO}}_{3})}_{n}(001)$ superlattices with $n=1,3$ and $X=\mathrm{V}$, Cr, and Mn. In order to disentangle the effect of quantum confinement and octahedral rotations and to account for a wider temperature range, $P4/mmm$ (untilted) and $P{2}_{1}/c$ (tilted) phases are considered. We find that the ground-state superlattice geometries always display finite octahedral rotations, which drive an orbital reconstruction and a concomitant metal-to-insulator transition in confined ${\mathrm{SrVO}}_{3}$ and ${\mathrm{SrCrO}}_{3}$ single layers with ferro- and antiferromagnetic spin alignments, respectively. On the other hand, the confined ${\mathrm{SrMnO}}_{3}$ single layer exhibits electronic properties similar to bulk. We show that confinement enhances the thermoelectric properties, particularly for ${\mathrm{SrVO}}_{3}$ and ${\mathrm{SrCrO}}_{3}$ due to the emergent Mott phase. Large room-temperature Seebeck coefficients are obtained for the tilted superlattices, ranging from 500 to $600\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}/\mathrm{K}$ near the band edges. The estimated attainable power factors of $27.9\phantom{\rule{0.28em}{0ex}}(26.6)\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in plane for the ${({\mathrm{SrCrO}}_{3})}_{1}/{({\mathrm{SrTiO}}_{3})}_{1}(001)$ superlattice with $P4/mmm\phantom{\rule{0.28em}{0ex}}(P{2}_{1}/c)$ symmetry and $28.1\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ cross plane for the ${({\mathrm{SrMnO}}_{3})}_{1}/{({\mathrm{SrTiO}}_{3})}_{1}(001)$ superlattice with $P{2}_{1}/c$ symmetry compare favorably with some of the best-performing oxide thermoelectrics. This demonstrates that the idea to use quantum confinement to enhance the thermoelectric response in correlated transition-metal oxide superlattices [Phys. Rev. Mater. 2, 055403 (2018)] can be applied to a broader class of materials combinations.