Tuning the carrier localization, magnetic and thermoelectric properties of ultrathin (LaNiO_{3−δ})_{1}/(LaAlO_{3})_{1}(001) superlattices by oxygen vacancies

Abstract

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−δ})_{1}/(LaAlO_{3})_{1}(001) superlattices (SLs). For the pristine SL (δ=0), an antiferromagnetic charge-disproportionated (AFM-CD) (d^{8}[under L]̲^{2})_{S=0}(d^{8})_{S=1} phase is stabilized, irrespective of strain. At δ=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_{SrTiO_{3}} (tensile strain) and a_{LaSrAlO_{4}} (compressive strain). At δ=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 24µW/K^{2} cm at a_{STO} and 14µW/K^{2} cm at a_{LSAO} at 300 K (assuming relaxation time τ=4 fs). Additionally, the pristine and δ = 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.