  We theoretically investigate the mechanism to generate large intrinsic spin
Hall effect 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,
SrIrO$_3$, which is a three-dimensional semimetal with nodal line spectrum. It
is shown that large intrinsic spin Hall effect 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 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 spin Hall conductivity 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 spin Hall conductivity in certain cases. We compare our findings
with the recent experimental report on the discovery of large spin Hall effect
in SrIrO$_3$ thin films.
