Confinement- and strain-induced enhancement of thermoelectric properties in LaNiO3/LaAlO3 (001) superlattices

By combining ab initio simulations including an on-site Coulomb repulsion term and Boltzmann theory, we explore the thermoelectric properties of (LaNiO3)(n)/(LaAlO3)(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 LaNiO3 region and the LaAlO3 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 a control parameter: Under tensile strain corresponding to the lateral lattice constant of SrTiO3 we predict in- and cross-plane Seebeck coefficients of +/- 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 SrTiO3 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.