title: Two-dimensional Heisenberg behavior of J(eff)=1/2 isospins in the paramagnetic state of the spin-orbital Mott insulator Sr2IrO4.
abstract: The dynamical correlations of J(eff)=1/2 isospins in the paramagnetic state of spin-orbital Mott insulator Sr2IrO4 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∼0.1  eV that is comparable to the small Mott gap (<0.5  eV) points out the weak and marginal Mott character of this spin-orbital entangled system.
authors.affiliation: RIKEN Advanced Science Institute, Wako 351-0198, Japan. fujiyama@riken.jp
authors.forename: S
authors.initials: S
authors.lastname: Fujiyama
authors.forename: H
authors.initials: H
authors.lastname: Ohsumi
authors.forename: T
authors.initials: T
authors.lastname: Komesu
authors.forename: J
authors.initials: J
authors.lastname: Matsuno
authors.forename: B J
authors.initials: BJ
authors.lastname: Kim
authors.forename: M
authors.initials: M
authors.lastname: Takata
authors.forename: T
authors.initials: T
authors.lastname: Arima
authors.forename: H
authors.initials: H
authors.lastname: Takagi