Electronic reconstruction and charge transfer in strained Sr\(_2\)CoIrO\(_6\) double perovskite

The electronic, magnetic and optical properties of the double perovskite Sr\(_2\)CoIrO\(_6\) (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 SrIrO\(_3\) is a semimetal with a quenched spin and orbital moment and bulk SrCoO\(_3\) is a ferromagnetic (FM) metal with spin and orbital moment of 2.50 and 0.13 \(\mu_{B}\), 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 \(\sim 2.35-2.45\) and \(0.31-\)0.45 \(\mu_{B}\), respectively. Most remarkably, Ir acquires a significant spin and orbital moment of 1.21-1.25 and 0.13 \(\mu_{B}\), 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\(^{4+\delta}\), Co\(^{4-\delta}\) configuration. Biaxial strain, varied from -1.02% (\(a_{\rm NdGaO_3}\)) through 0% (\(a_{\rm SrTiO_3}\)) to 1.53% (\(a_{\rm GdScO_3}\)), influences in partcular the orbital polarization of the \(t_{2g}\) 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 \(t_{2g}\) and the higher lying broader \(e_{g}\) bands. Inclusion of many body effects, in particular, excitonic effects by solving the Bethe-Salpeter equation (BSE), increases the band gap by \(\sim0.2\) and improves the agreement with the measured spectrum concerning the position of the second peak at \(\sim 2.6\) eV.