Two-dimensional heisenberg behavior of J _eff=1/2 isospins in the paramagnetic state of the spin-orbital mott insulator Sr ₂IrO ₄

The dynamical correlations of J _eff=1/2 isospins in the paramagnetic state of spin-orbital Mott insulator Sr ₂IrO ₄ were revealed by resonant magnetic x-ray diffuse scattering. We found a two-dimensional antiferromagnetic fluctuation with a large in-plane correlation length exceeding 100 lattice spacings at even 20 K above the magnetic ordering temperature. In marked contrast to the naive expectation of the strong magnetic anisotropy associated with an enhanced spin-orbit coupling, we discovered an isotropic isospin correlation that is well described by the two-dimensional S=1/2 quantum Heisenberg model. The estimated antiferromagnetic coupling constant as large as J∼0.1eV that is comparable to the small Mott gap (<0.5eV) points out the weak and marginal Mott character of this spin-orbital entangled system.