  We used resonant inelastic x-ray scattering to reveal the nature of magnetic
interactions in Sr2IrO4, a 5d transition-metal oxide with a spin-orbit
entangled ground state and Jeff=1/2 magnetic momemts, referred to as
'isospins'. The magnon dispersion in Sr2IrO4 is well described by an
antiferromagnetic Heisenberg model with isospin one-half moments on a square
lattice, which renders the low-energy effective physics of Sr2IrO4 much akin to
that in superconducting cuprates. This is further supported by the observation
of exciton modes in Sr2IrO4 whose dispersion is strongly renormalized by
magnons, which can be understood by analogy to the hole propagation in the
background of antiferromagnetically ordered spins in the cuprates.
