Theory of Large Intrinsic Spin Hall Effect in Iridate Semimetals

We theoretically investigate the mechanism to generate large intrinsic spin Hall effect (SHE) in iridates or more broadly in 5d transition metal oxides with strong spin-orbit coupling. We demonstrate such a possibility by taking the example of orthorhombic perovskite iridate with nonsymmorphic lattice symmetry, SrIrO3, which is a three-dimensional semimetal with nodal line spectrum. It is shown that large intrinsic SHE arises in this system via the spin-Berry curvature originating from the nearly degenerate electronic spectra surrounding the nodal line. This effect exists even when the nodal line is gently gapped out, due to the persistent nearly degenerate electronic structure, suggesting a distinct robustness. The magnitude of the spin Hall conductivity (SHC) is shown to be comparable to the best known example such as doped topological insulators and the biggest in any transition metal oxides. To gain further insight, we compute the intrinsic SHC in both of the bulk and thin film systems. We find that the geometric confinement in thin films leads to significant modifications of the electronic states, leading to even bigger SHC in certain cases. We compare our findings with the recent experimental report on the discovery of large SHE in SrIrO3 thin films.