Tuning of the carrier localization, magnetic and thermoelectric properties in ultrathin (LaNiO${}_{3-\delta }$)${}_1$/(LaAlO${}_3$)${}_1$(001) superlattices by oxygen vacancies

Using a combination of density functional theory calculations with an on-site Coulomb repulsion term (DFT+$U$) and Boltzmann transport theory within the constant relaxation time approximation, we explore the effect of oxygen vacancies on the electronic, magnetic, and thermoelectric properties in ultrathin (LaNiO${}_{3-\delta }$)${}_1$/(LaAlO${}_3$)${}_1$(001) superlattices (SLs). For the pristine SL, an antiferromagnetic charge-disproportionated (AFM-CD) ($d^8${$\underset{‾}{L}$}${}^2$)${}_{S=0}$($d^8$)${}_{S=1}$ phase is stabilized, irrespective of strain. At $\delta$ = 0.125 and 0.25, the localization of electrons released from the oxygen defects in the NiO${}_2$ plane triggers a charge-disproportionation, leading to a ferrimagnetic insulator both at $a_{\mathrm{S}\mathrm{T}\mathrm{O}}$ (tensile strain) and $a_{\mathrm{L}\mathrm{S}\mathrm{A}\mathrm{O}}$ (compressive strain). At $\delta$ = 0.5, an insulating phase emerges with alternating stripes of Ni${}^{2+}$ (high-spin) and Ni${}^{2+}$ (low-spin) and oxygen vacancies ordered along the [110] direction (S-AFM), irrespective of strain. This results in a robust $n$-type in-plane power factor of 24~$\mu$W/K${}^2$ cm at $a_{\mathrm{S}\mathrm{T}\mathrm{O}}$ and 14~$\mu$W/K${}^2$ cm at $a_{\mathrm{L}\mathrm{S}\mathrm{A}\mathrm{O}}$ at 300~K (assuming relaxation time $\tau =4$~fs). Additionally, the pristine and $\delta$ = 0.5 SLs are shown to be dynamically stable. This demonstrates the fine tunability of electronic, magnetic, and thermoelectric properties of ultrathin nickelate superlattices by oxygen vacancies.