  Dynamical correlations of Jeff = 1/2 isospins in the paramagnetic state of
spin-orbital Mott insu- lator Sr2IrO4 was revealed by resonant magnetic x-ray
diffuse scattering. We found two-dimensional antiferromagnetic fluctuation with
a large in-plane correlation length exceeding 100 lattice spacings at even 20 K
above the mangnetic ordering temperature. In marked contrast to the naive
expecta- tion of strong magnetic anisotropy associated with an enhanced
spin-orbit coupling, we discovered isotropic isospin correlation that is well
described by the two-dimensional S = 1/2 quantum Heisen- berg model. The
estimated antiferromagnetic coupling constant as large as J ~ 0.1 eV that is
comparable to the small Mott gap (< 0.5 eV) points the weak and marginal Mott
character of this spin-orbital entangled system.
