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

Using a combination of density functional theory calculations with an on-site Coulomb repulsion term ($\mathrm{DFT}+U$) 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 ${\left({\mathrm{LaNiO}}_{3-\delta }\right)}_1/{\left({\mathrm{LaAlO}}_3\right)}_1\left(001\right)$ superlattices (SLs). For the pristine SL ($\delta =0$), an antiferromagnetic charge-disproportionated (AFM-CD) ($d^8\underline{L}{}^2{)}_{S=0}{\left(d^8\right)}_{S=1}$ phase is stabilized, irrespective of strain. At $\delta =0.125$ and 0.25, the localization of electrons released from the oxygen defects in the ${\mathrm{NiO}}_2$ plane triggers a charge-disproportionation, leading to a ferrimagnetic insulator both at $a_{{\mathrm{SrTiO}}_3}$ (tensile strain) and $a_{{\mathrm{LaSrAlO}}_4}$ (compressive strain). At $\delta =0.5$, an insulating phase emerges with alternating stripes of ${\mathrm{Ni}}^{2+}$ (high-spin) and ${\mathrm{Ni}}^{2+}$ (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 $a_{\mathrm{STO}}$ and $14\mu W/K^2$ cm at $a_{\mathrm{LSAO}}$ 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.