Optical properties and Γ-X crossover of electron states in Sr₂IrO₄ under strain: A density-functional study

We study the effect of epitaxial strain on the electronic properties of the strong spin-orbit coupled Sr₂IrO₄ using the first principles methods. The optimized structures show a considerable effect of strain in the structural parameters such as the Ir-O bond lengths and the Ir-O-Ir bond angles. These structural modifications lead to important changes in the electronic structure. The band widths of the upper and the lower J_eff = 1/2 Hubbard bands increase with compressive strain, leading to a reduction in the band gap, while the opposite happens for the tensile strain. An interesting observation is the Γ-X crossover of the valence band maximum with the strain. These points are nearly degenerate for the unstrained structure, but under the compressive epitaxial strain, the valence band maximum occurs only at X and Γ for the tensile strain. This strain dependent Γ-X crossover, in turn, suggests the possibility of the tuning of the transport properties in the hole-doped iridates with strain. We discuss the calculated optical absorption coefficients and how they change with the strain, and compare the results with the experimental absorption spectra.