Confinement- and strain-induced enhancement of thermoelectric properties in LaNiO${}_3$/LaAlO${}_3(001)$ superlattices

Abstract

By combining ab initio simulations including an on-site Coulomb repulsion term and Boltzmann theory, we explore the thermoelectric properties of (LaNiO${}_3$)${}_n$/(LaAlO${}_3$)${}_n$(001) superlattices ($n=1,3$) and identify a strong dependence on confinement, spacer thickness, and epitaxial strain. While the system with $n=3$ shows modest values of the Seebeck coefficient and power factor, the simultaneous reduction of the LaNiO${}_3$ region and the LaAlO${}_3$ spacer thickness to single layers results in a strong enhancement, in particular of the in-plane values. This effect can be further tuned by using epitaxial strain as control parameter: Under tensile strain corresponding to the lateral lattice constant of SrTiO${}_3$ we predict in- and cross-plane Seebeck coefficients of $\pm 600$ $\mu$V/K and an in-plane power factor of $11$ $\mu$W/K${}^2$cm for an estimated relaxation time of $\tau =4$ fs around room temperature. These values are comparable to some of the best performing oxide systems such as La-doped SrTiO${}_3$ or layered cobaltates and are associated with the opening of a small gap ($0.29$ eV) induced by the concomitant effect of octahedral tilting and Ni-site disproportionation. This establishes oxide superlattices at the verge of a metal-to-insulator transition driven by confinement and strain as promising candidates for thermoelectric materials.

8 pages, 7 figures; corrected typos