Spin-orbit coupling triggered Mott insulator Sr$_{2}$IrO$_{4}$.

Electronic structures of 5d transition-metal oxide (TMO) Sr$_{2}$IrO$_{4 }$are investigated by angle-resolved photoemission spectroscopy and density-functional electronic structure calculations. The insulating nature of this compound and its measured valence band symmetry are correctly accounted for by the calculation only when both spin-orbit coupling and electronic correlation effects are included. It is shown that the spin-orbit coupling plays a crucial role in stabilizing the Mott-insulating ground state, which is unexpected in a compound with extended 5d valence band. The spin-orbit coupling leads to symmetry-split-off bands near the Fermi level which are narrow enough to be gapped by the Coulomb repulsion of moderate strength. This results in a conduction band as narrow as $\sim $0.5 eV, defying its general character expected for 5$d$ TMO. Our finding marks an establishment of a new type of Mott insulator and suggests possibility of novel Mott-derived phenomena in 5$d$ based materials.