  Time-of-flight inelastic neutron scattering measurements on Sr2IrO4 single
crystals were performed to access the spin Hamiltonian in this canonical
Jeff=1/2 spin-orbital Mott insulator. The momentum of magnetic scattering at
all inelastic energies that were measured is revealed to be $L$-independent,
indicative of idealized two-dimensional in-plane correlations. By probing the
in-plane energy and momentum dependence up to ~80 meV we model the magnetic
excitations and define a spin-gap of 0.6(1) meV. Collectively the results
indicate that despite the strong spin-orbit entangled isospins an isotropic
two-dimensional S=1/2 Heisenberg model Hamiltonian accurately describes the
magnetic interactions, pointing to a robust analogy with unconventional
superconducting cuprates.
