  There has been a rapidly growing interest on the interplay between spin-orbit
coupling (SOC) and Hubbard interaction U in correlated materials. A current
consensus is that the stronger the SOC, the smaller is the critical interaction
Uc required for a spin-orbit Mott insulator, because the atomic SOC splits a
band into different total angular momentum bands narrowing the effective
bandwidth. It was further claimed that at large enough SOC, the stronger the
SOC, the weaker the Uc because in general the effective SOC is enhanced with
increasing electron-electron interaction strength. Contrary to this
expectation, we find that, in orthorhombic perovskite oxides (Pbnm), the
stronger the SOC, the bigger the Uc. This is originated from a line of Dirac
node in Jeff=1/2 bands near the Fermi level inherited from a combination of the
lattice structure and a large SOC. Due to this protected line of nodes, there
are small hole and electron pockets in SrIrO3, and such a small density of
states makes Hubbard interaction less efficient in building a magnetic
insulator. The full phase diagram in U vs. SOC is obtained, where non-magnetic
semimetal, magnetic metal, and magnetic insulator are found. Magnetic ordering
patterns beyond Uc are also presented. We further discuss implications of our
finding in relation to other perovskites such as SrRhO3 and SrRuO3.
