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 ...

The antiferromagnetic structure of Sr3Ir2O7, the bilayer analogue of a spin-orbital Mott insulator Sr2IrO4, was revealed by resonant magnetic x-ray diffraction. Contrasting intensities of the magnetic diffraction at the Ir LIII and LII edges show a J eff = 1/2 character of the magnetic moment as is argued in Sr2IrO4. The magnitude of moment, however, was found to be smaller than that of Sr2IrO4 by a factor of 5-6, implying that Sr3Ir2O7 is no longer a Mott insulator but a weak antiferromagnet. An evident change of the temperature dependence of the resistivity at TN, from almost temperatureindependent resistivity to insulating, strongly suggests that the emergent weak magnetism controls the charge gap. The magnetic structure was found to be an out-of-plane collinear antiferromagnetic ordering in contrast to the inplane canted antiferromagnetism in Sr2IrO4, originating from the strong bilayer antiferromagnetic coupling.Recently, novel interplay of spin-orbit coupling and electron correlations in heavy 5d transition metal oxides has been attracting considerable interest as a new paradigm of oxide physics. In Sr 2 IrO 4 , a layered Ir 4+perovskite with five d-electrons in its t 2g orbitals, such interplay is particularly pronounced. Sr 2 IrO 4 is an insulator with a charge gap of the order of 0.5 eV and shows a canted antiferromagnetism below T N ∼ 230 K. 1-3The insulating behavior even well above T N and the presence of a large localized magnetic moment of ∼ 0.5µ B , estimated from the large canted moment 0.075 µ B /Ir, indicate that Sr 2 IrO 4 can be viewed as a Mott insulator. The Mottness of Sr 2 IrO 4 is believed to be associated with a formation of a half-filled J eff = 1/2 band, created by a very strong spin-orbit splitting as large as ∼ 0.7 eV comparable to the width of the t 2g band ∼ 1.5 eV. The polarization dependence of x-ray absorption spectra and the contrasting resonances at L II and L III edges in the magnetic x-ray diffraction firmly evidence J eff = 1/2 character of the magnetic moment. 3-5The Mottness of Sr 2 IrO 4 characterized by the charge gap as small as 0.5 eV is marginal. Like some other tetravalent transition metal ions such as Ti 4+ , Mn 4+ and Ru 4+ , Ir 4+ forms a series of perovskite-based structures called Ruddlesden-Popper series, Sr n+1 Ir n O 3n+1 , with stacking of n-IrO 2 layers as structural units. Reflecting the marginal Mottness in Sr 2 IrO 4 (n = 1), the system becomes more and more itinerant with increasing n. The three dimensional analogue of the series, SrIrO 3 (n = ∞) is known to show a metallic transport.6 Recent study revealed that the ground state of SrIrO 3 is a semimetal close to a band insulator, 7 where the lifting of band degeneracy by spin-orbit coupling plays a key role in producing the incomplete charge gap.8 Very likely due to the presence of strong spin orbit coupling, the semimetal state is located almost right next to the Mott insulator. A question arises how the Mott insulator evolves into the semimetal in Sr n+1 Ir n O 3n+1 .Sr 3 Ir 2 O 7 , the n = 2 membe...

We report 13 C NMR spectra and nuclear spin lattice relaxation rate 1/T1 for a quasi-onedimensional quarter-filled organic material (TMTTF)2AsF6, which undergoes charge ordering (TCO = 102 K) and spin-Peierls phase transitions (TSP = 14 K). The ratio of two 1/T1 for the charge accepting and donating TMTTF sites which grows from TCO finally saturates in approaching TSP, indicating one spin correlation function even in the charge ordered state. Below TSP, however, the doubly split NMR lines from inequivalently charged molecules merge into one line, originated from the variation in charge densities. This shows that a rearrangement of the charge configuration occurs at TSP.

The ratio of orbital (L) and spin (S) contributions to the magnetically ordered moments of a 5d transition metal oxide, Sr2IrO4 was evaluated by nonresonant magnetic x-ray diffraction. We applied an improved experimental setting that minimized the experimental error, in which we varied only the linear polarization of incident x ray at a fixed scattering angle. Strong polarization dependence of the intensity of magnetic diffraction was observed, from which we conclude that the ordered moments contain substantial contribution from the orbital degree of freedom with the ratio of /∼5.0, evidencing the pronounced effect of spin-orbit coupling. The obtained ratio is close to, but slightly larger than the expected value for the ideal J(eff) = 1/2 moment of a spin-orbital Mott insulator, ||/|| = 4, which cannot be accounted for by the redistribution of orbital components within the t(2g) bands associated with the elongation of the IrO6 octahedra.

We report nuclear quadrupole resonance (NQR) studies on the chain Cu sites of PrBa2Cu4O8, a quasi-one-dimensional conductor with a nearly quarter-filled band. The nuclear spin-lattice relaxation rate 1/T1 shows a pronounced peak near 100 K caused by fluctuations of electric field gradient (EFG). Similar peak was observed for the spin-echo decay rate 1/T2, however, at a different temperature near 50 K. These results and broadening of the NQR spectrum at low temperatures indicate that slow charge fluctuations of either electronic or ionic origin freeze gradually at low temperatures.There has been increasing interest in quasi-onedimensional (Q1D) correlated electrons. Theoretical studies on generalized Hubbard or t-J models for chains and ladders have revealed rich phase diagram associated with various instabilities towards Mott localization, spin density wave, charge order, and superconductivity [1,2,3] These two compounds show contrasting transport behavior. Pr124 is a highly anisotropic Q1D metal showing extremely large in-plane resistivity anisotropy ρ a /ρ b ∼ 1000 at 4 K [12,13,14], where b is along the chain direction and a is perpendicular to it within the CuO 2 plane. Although ρ b shows monotonic metallic temperature dependence, ρ a exhibits a broad peak near 130 K, indicating incoherent transport perpendicular to the chains at high temperatures. The angle resolved photoemission spectra (ARPES) [15] also revealed a clear one-dimensional dispersion which crosses the Fermi level near the momentum k b ≃ π/4, pointing to a nearly quarter-filled conduction band. In Pr123, the spectral weight of the chain contribution to the optical conductivity [9] as well as the ARPES spectra [16] are also consistent with an approximately quarter-filled conduction band. However, semiconducting behavior of the dc-resistivity, steep suppression of the optical conductivity below 0.1 eV and vanishing ARPES intensity near the Fermi level all indicate the existence of a charge gap. Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments in Pr123 [17] revealed line broadening and anomalies in relaxation rates, which were ascribed to a charge ordering instability.Apparent absence of static charge order in Pr124 may be due to slight deviation from the quarter-filling [15] or to the geometrical frustration of intersite Coulomb interactions in zigzag chains [18]. However, dynamic signature towards charge order instability is indicated by the optical conductivity in Pr124, which splits into a zero-energy Drude part with a small weight and a finite energy mode centered at 40 meV [10]. In this letter, we report results of NQR experiments for the chain Cu sites in Pr124. We found a pronounced peak in the temperature dependences of the nuclear spin-lattice relaxation rate (1/T 1 ) and spin-echo decay rate (1/T 2 ), as well as broadening of the NQR spectra, indicating gradual freezing of fluctuations of electric field gradient (EFG) with decreasing temperature.The powder sample of Pr124 was synthesized by solid stat...