Two-Dimensional Heisenberg Behavior ofJeff=1/2Isospins in the Paramagnetic State of the Spin-Orbital Mott InsulatorSr2IrO

Abstract

Dynamical correlations of J eff = 1/2 isospins in the paramagnetic state of spin-orbital Mott insulator 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 expectation 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 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 the weak and marginal Mott character of this spin-orbital entangled system. PACS numbers:In magnetic oxides with 3d transition metal ions, the energy scale of spin-orbit coupling (SOC) is usually smaller than those of crystal field splitting and on-site Coulomb repulsion. SOC can be treated as a perturbation, of which a primal role is to give rise to a magnetic anisotropy. In 5d transition metal oxides, however, SOC is more than one order of magnitude larger than that of 3d due to the pronounced relativistic effect, as large as a half eV, and can modify the electronic structure drastically. A spin-orbital Mott insulator that was recently identified in a layered perovskite Sr 2 IrO 4 is a novel state of correlated electrons [1,2]. Here, a strong SOC, inherent to heavy 5d transition metal Ir 4+ , splits the Ir t 2g bands into a half-filled J eff = 1/2 bands and completely filled J eff = 3/2 bands, which gives rise to a J eff = 1/2 Mott state induced by a moderate Coulomb repulsion, U . The uniqueness of such a spin-orbital Mott insulator is that J eff = 1/2 isospins with the wave function(|xy, ± ± |yz, ∓ + i|zx, ± ) are accommodated in the outermost 5d orbitals and, therefore, exotic magnetic couplings representing the complex spin-orbital states are anticipated [3,4].Sr 2 IrO 4 is a canted antiferromagnet below 230 K [5][6][7]. The J eff = 1/2 moments order approximately antiferromagnetically but significantly canted to cause a ferromagnetic moment of ∼ 0.1µ B /Ir within each IrO 2 (ab) plane as shown in Fig. 1(a). With applying a small magnetic field of ∼ 0.2 T parallel to the layer, the system shows a metamagnetic transition and becomes a weak ferromagnet. Note that the canting moment is one to two orders of magnitude larger than that of an analogous canted antiferromagnet with 3d copper, La 2 CuO 4 . This large canting moment is produced apparently by the interplay between the large SOC and lattice distortion and, at a glance, would imply strongly anisotropy in the isospin coupling.Recently Jackeli and Khaliullin discussed theoretically the effective Hamiltonian for J eff = 1/2 isospins in Sr 2 IrO 4 [3]. They found that, in the absence of Hund's coupling, the isospin Hamiltonian, including antisymmetric (Dzyloshinskii-Moriya) and symmetric anisotropy terms produced by the interplay between ...

Crystal growth and intrinsic magnetic behaviour of Sr2IrO4

“…The former surprisingly indicates that a system with very strong spin-orbit coupling can mimic a system without it. Experimentally, it has been confirmed that the canted antiferromagnetic structure of Sr 2 IrO 4 derives from a predominantly isotropic nearest-neighbor magnetic exchange coupling [8][9][10] . Thus, Sr 2 IrO 4 becomes the first non-cuprate material to realize a (iso)spin-1/2 Heisenberg antiferromagnet in a quasitwo-dimensional square lattice-a possible platform for high temperature superconductivity [11][12][13][14][15] .…”

Noncollinear versus collinear description of the Ir-based one-t2g-hole perovskite-related compounds:SrIrO3andSr2Lado1, Pardo2 2015Phys. Rev. B12580View full textAdd to dashboardCite

“…23 Some papers indicate the ordered moment can be produced via slight canting of the otherwise AF moments. [24][25][26][27] The canting angle (see Fig. 1b) occurs due to the deviation of the Ir-O-Ir angle from 180 • via Dzyaloshinskii-Moriya interaction.…”

Excitonic quasiparticles in a spin–orbit Mott insulator

“…Motivated by the recent discovery of a new pseudospin-1/2 Heisenberg AF on a square lattice, Sr 2 IrO 4 (refs [13][14][15][16][17][18], we demonstrate a novel experimental approach using resonant inelastic X-ray scattering (RIXS), which is a rapidly evolving tool especially well suited for such 5d transition-metal oxides, to address this longstanding problem. On the basis of its remarkable similarity to superconducting cuprates in structural 18 , electronic 13 and magnetic aspects [14][15][16] , superconductivity has been predicted in Sr 2 IrO 4 (ref.…”