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

TL;DR

This study uses ab initio simulations and Boltzmann theory to show how confinement, spacer thickness, and epitaxial strain can significantly enhance the thermoelectric properties of LaNiO3/LaAlO3 superlattices, making them promising thermoelectric materials.

Contribution

It demonstrates that confinement and strain can be used to tune and greatly improve the thermoelectric performance of oxide superlattices, revealing new design strategies.

Findings

Enhanced in-plane Seebeck coefficient of ±600 μV/K under tensile strain.

In-plane power factor of 11 μW/K^2·cm at room temperature.

Small gap of 0.29 eV associated with octahedral tilting and Ni-site disproportionation.

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.