  We investigated electronic structure of 5d transition-metal oxide Sr2IrO4
using angle-resolved photoemission, optical conductivity, and x-ray absorption
measurements and first-principles band calculations. The system was found to be
well described by novel effective total angular momentum Jeff states, in which
relativistic spin-orbit (SO) coupling is fully taken into account under a large
crystal field. Despite of delocalized Ir 5d states, the Jeff-states form so
narrow bands that even a small correlation energy leads to the Jeff = 1/2 Mott
ground state with unique electronic and magnetic behaviors, suggesting a new
class of the Jeff quantum spin driven correlated-electron phenomena.
