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

Abstract

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 control parameter: Under tensile strain corresponding to the lateral lattice constant of SrTiO3 we predict in-and cross-plane Seebeck coefficients of ±600 µV/K and an in-plane power factor of 11 µW/K 2 cm for an estimated relaxation time of τ = 4 fs around room temperature. These values are comparable to some of the best performing oxide systems such as Ladoped 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.

Inducing n - and p -Type Thermoelectricity in Oxide Superlattices by Strain Tuning of Orbital-Selective Transport Resonances

“…43 Static correlation effects were considered within DFT+U 44 U = 4 and J = 0.7 eV for Ni 3d, in line with previous work by us and others. 15,16,27,40,45,46 In order to take octahedral tilts fully into account, 40-atom √ 2a × √ 2a × c supercells were used to model the (LNO) 3 /(LAO) 1 (001) SLs. Epitaxial strain is considered by varying the in-plane lattice parameter a from 3.65 to 3.95 Å.…”

“…20,[31][32][33] The origin of this MIT is, however, not related to orbital polarization, as initially proposed, but to a disproportionation into two inequivalent Ni sites. 27 With increasing LNO thickness, e.g., in (LNO) n /(LAO) 3 (001) SLs on STO (n = 3, 5, 10) 16,27,30 or (LNO) 4 /(LAO) 4 (001) SLs on STO, 21,34 the metallic behavior of the nickelate is restored.…”

Competition of defect ordering and site disproportionation in strained LaCoO3 on SrTiO3 (001)

“…3(a)], similar as in (LaNiO 3 ) 1 /(LaAlO 3 ) 1 (001) SLs. 26,29 In the majority spin channel this occurs at the Fermi energy, and an indirect band gap emerges between the filled and the empty subset, clearly visible in the band structure shown in Fig. 2(c).…”

“…We find that the formation of brownmillerite is impeded due to the tendency to decompose to CoO and La 2 O 3 under the corresponding growth conditions. We also report a novel IS phase of bulk LCO, in which the excited electron is accommodated in the e g states via a charge-and bond-disproportionation mechanism, which bears similarities to nickelate systems, [26][27][28][29] while orbital order occurs exclusively for the hole in the t 2g manifold. Breathingmode distortions lead to O-Co-O distances that are close to xray diffraction results at elevated temperature, 6 whereas Jahn-Teller distortions are found to be minute.…”

“…A similar type of electronic reconstruction is well known for nickelate films and heterostructures. [26][27][28][29] For LCO, spin disproportionation has been suggested in model studies. 53 The CD causes a splitting of the e g manifold into two subsets [ Fig.…”