There has been a rapidly growing interest in the interplay between spin-orbit coupling (SOC) and the Hubbard interaction $U$ in correlated materials. A current consensus is that the stronger the SOC, the smaller is the critical interaction $U_c$ 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 $U_c$, 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 $U_c$. This originates from a line of Dirac nodes in $J_{\mathrm{eff}}=1/2$ 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 SrIrO${}_3$, and such a small density of states makes the Hubbard interaction less efficient in building a magnetic insulator. The full phase diagram in $U$ vs SOC is obtained, where nonmagnetic semimetal, magnetic metal, and magnetic insulator are found. Magnetic ordering patterns beyond $U_c$ are also presented. We further discuss implications of our finding in relation to other perovskites such as SrRhO${}_3$ and SrRuO${}_3$.

• Received 27 June 2012

DOI:https://doi.org/10.1103/PhysRevB.86.085149