Dirac Nodal Lines and Density-Functional Prediction of a Large Spin Hall Effect in 6H-perovskite Iridate Ba₃TiIr₂O₉

Topological semi-metals have received considerable attention recently due to their nontrivial band structure protected by symmetry, resulting in a number of emerging quantum phenomena such as Fermi arcs, anomalous Hall effect, spin Hall effect (SHE), etc. While the Dirac and Weyl semi-metals, the typical examples of 3D topological semi-metals, are well studied in the literature, currently an interest has been emerging in the Dirac nodal line semi-metals (DNLS) because they are good candidates for SHE. Here, based on the density-functional calculations, we predict a new iridate material Ba₃TiIr₂O₉ with nodal lines present in the band structure, which with the introduction of SOC become gapped along certain high symmetry directions in the Brillouin zone, but not along the other directions, being protected by the non-symmorphic symmetry of the crystal structure. The latter type of nodal lines are characterized by the quantized Berry phase π, while the former kind of nodal lines are responsible for a strong SHE. The magnitude of the SHE is calculated using a tight-binding fit to the DFT band structure. Our work suggests Ba₃TiIr₂O₉ as a potential SHE material for spintronics applications and motivates experimental work on the system.