Electronic reconstruction and charge transfer in strained Sr$_2$CoIrO$_6$ double perovskite

Abstract

The electronic, magnetic and optical properties of the double perovskite Sr2CoIrO6 (SCIO) under biaxial strain are explored in the framework of density functional theory (DFT) including a Hubbard U term and spin-orbit coupling (SOC) in combination with absorption spectroscopy measurements on epitaxial thin films. While the end member SrIrO3 is a semimetal with a quenched spin and orbital moment and bulk SrCoO3 is a ferromagnetic (FM) metal with spin and orbital moment of 2.50 and 0.13 µB, respectively, the double perovskite SCIO emerges as an antiferromagnetic Mott insulator with antiparallel alignment of Co, Ir planes along the [110]-direction. Co exhibits a spin and enhanced orbital moment of ∼ 2.35 − 2.45 and 0.31−0.45 µB, respectively. Most remarkably, Ir acquires a significant spin and orbital moment of 1.21-1.25 and 0.13 µB, respectively. Analysis of the orbital occupation indicates an electronic reconstruction due to a substantial charge transfer from minority to majority spin states in Ir and from Ir to Co, signaling an Ir 4+δ , Co 4−δ configuration. Biaxial strain, varied from -1.02% (a NdGaO 3 ) through 0% (aSrTiO 3 ) to 1.53% (a GdScO 3 ), influences in partcular the orbital polarization of the t2g states and leads to a nonmonotonic change of the band gap between 163 and 235 meV. The absorption coefficient reveals a two plateau fearure due to transitions from the valence to the lower lying narrow t2g and the higher lying broader eg bands. Inclusion of many body effects, in particular, excitonic effects by solving the Bethe-Salpeter equation (BSE), increases the band gap by ∼ 0.2 and improves the agreement with the measured spectrum concerning the position of the second peak at ∼ 2.6 eV.