Density functional theory investigation of the electronic reconstruction in spin-orbit coupled double perovskites and oxide superlattices

Transition metal oxides exhibit intriguing physical and chemical properties such as metal- insulator transitions or charge transfer due to the strong coupling between lattice, spins, charge, and orbital degrees of freedom. A prominent class of materials are the perovskites which can be magnetic, multiferroic, superconducting. The study of the magnetic and electronic properties of superlattices, to reveal their intrinsic coupling mechanism and the related physical effects, is an important issue for perovskite research. Density functional theory (DFT) calculations, being accurate, fast, and not limited by experimental condi- tions, are widely used to explain various experimental results and phenomena. This thesis gives an insight into the structure, electronic, magnetic and topological properties of perovskite superlattices by employing DFT calculation including on-site Columb repulsion U term and spin-orbit coupling. The double perovskite (DP) Sr2CoIrO6 (SCIO), (001)- and (110)-oriented superlattices are highly interesting as they show an electronic reconstruction induced by heterostructuring and spin-orbit coupling effects.

The structural, electronic and magnetic properties of the bulk perovskites SrCoO3 (SCO), SrIrO3 (SIO) and SrRuO3 (SRO) were calculated with the exchange-correlation functionals PBEsol and an additonal Hubbard U parameter. The ground state of SCO and SRO is fer- romagnetic (FM) matallic, while bulk SIO is a semimetal with a quenched spin and orbital moment. Combining the 3d-Co and 5d-Ir elements into the double perovskite Sr2CoIrO6 (which can be regarded as a (111)- oriented superlattice) results in antiferromagnetic (AFM) insulator with Co spin and orbital moment of ∼2.4 and 0.3 μB. In contrast, the (001)- and (110)-oriented (SrCoO3)1/(SrIrO3)1 superlattices also are antiferromagnetic (AFM) insula- tors with Co spin and orbital moment of 2.96-2.99 and 0.13-0.09 μB ((001)-oriented super- lattice) ∼2.96 and 0.07 μB ((110)-oriented superlattice), respectively. Analysis of the orbital occupation of the DP Sr2CoIrO6 indicates an electronic reconstruction due to a substantial charge transfer from minority to majority spin states in Ir leading to a substantial spin and orbital magnetic moment of 0.96-1.2 and 0.09-0.17 μB at the Ir sites, respectively, and from Ir to Co, signaling an Ir4+δ, Co4−δ configuration. Varying the strain from compressive to tensile leads to increase of the dxyoccupation versus decrease of the dxz and dyz occupation. The band gap also changes nonmonotonically with biaxial strain, from 163 meV (aNdGaO3 ) to 235 meV (aSrTiO3 ) and 187 meV (aGdScO3 ). The optical properties of the strained dou- ble perovskite SCIO are assessed in the independent-particle (I.P.) approximation, as well as including quasiparticle and excitonic effects, by performing a single-shot G0W0 and by solving the Bethe-Salpeter equation (BSE). The absorption coefficient is compared to the one obtained from transmission measurements.

In a further DFT study, the electronic, magnetic and topological properties of (SrRuO3)n/ (SrIrO3)2(001)(n=4 and 5)superlatticesatthelaterallatticeconstantaSrTiO3 wereexplored. The results reveal an itinerant ferromagnetic state of the ruthenate layers, an electronic reconstruction and flat Ru-4dxzbands near the interface, leading to different spin and orbital moment at the Ru sites at the interface and inner layers. We analyze the topological proper- ties for out-of-plane magnetization and compare to in-plane magnetization in both the n=4 and 5 superlattice. A positive Berry curvature contribution to the anomalous Hall conduc- tivity (AHC) is found in the (SrRuO3)4/(SrIrO3)2(001) superlattice, in contrast, a negative Berry curvature contribution to the AHC is obtained in the (SrRuO3)5/(SrIrO3)2(001) superlattice. These two cases emphasize the dependence of the nonvanishing Berry curvature on the number of ruthenate layers in (SrRuO3)n/(SrIrO3)2(001) superlattices.

Beyond the (001)-oriented superlattices, the electronic, magnetic and topological proper- ties in the buckled honeycomb (Sr2IrXO6)/(SrTiO3)4(111) superlattice with X spanning the 3d, 4d and 5d transition-metal elements (X=Cr, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Os, Ir and Pt) were explored. In particular, a strong SOC effect leading to a metal-to-semiconductor (X=Mn, Rh, Pd and Pt), semiconductor-to-metal (X=Ni), metal-to-semimetal (X=Sc, Fe, Co, Cu and Ir) and semiconductor-to-semiconductor (X=Ru, Os) transition, was identified. Analysis of the electronic and magnetic properties of superlattices shows a slight difference between the in-plane and out-of-plane magnetization in X=Co, Ni and Cu due to a charge transfer from Ir to X due to the SOC effect. In addition, the magnetic anisotropy energy (MAE) in X=Cr, Mn, Fe and Co was evaluated and an in-plane easy axis was found. Fur- thermore, for X=Cu, the bands crossing at the Fermi level and a Dirac-like point above the Fermi level lead to a negative AHC for out-of-plane magnetization, while a zero AHC for easy axis in-plane magnetization is obtained. The study confirms that 3d superlattices are strongly correlated electron systems while the spin-orbit coupling effect is dominant in 4d and 5d superlattices.



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