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The electronic structure of the honeycomb lattice iridates Na(2)IrO(3) and Li(2)IrO(3) has been investigated using resonant inelastic x-ray scattering (RIXS). Crystal-field-split d-d excitations are resolved in the high-resolution RIXS spectra. In particular, the splitting due to noncubic crystal fields, derived from the splitting of j(eff)=3/2 states, is much smaller than the typical spin-orbit energy scale in iridates, validating the applicability of j(eff) physics in A(2)IrO(3). We also find excitonic enhancement of the particle-hole excitation gap around 0.4 eV, indicating that the nearest-neighbor Coulomb interaction could be large. These findings suggest that both Na(2)IrO(3) and Li(2)IrO(3) can be described as spin-orbit Mott insulators, similar to the square lattice iridate Sr(2)IrO(4).

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A neutron scattering study of the Mott-Hubbard insulator LaTiO3 (TN = 132 K) reveals a spin wave spectrum that is well described by a nearest-neighbor superexchange constant J = 15.5 meV and a small Dzyaloshinskii-Moriya interaction (D = 1.1 meV). The nearly isotropic spin wave spectrum is surprising in view of the absence of a static Jahn-Teller distortion that could quench the orbital angular momentum, and it may indicate strong orbital fluctuations. A resonant x-ray scattering study has uncovered no evidence of orbital order in LaTiO3.In the layered cuprates exemplified by the series La 2−x Sr x CuO 4+δ , the transition from a 3d 9 antiferromagnetic (AF) insulator at x = δ = 0 into an unconventional metallic and superconducting state with increasing hole concentration (x, δ > 0) has received an enormous amount of attention. The magnetic spectra of these materials, revealed by inelastic neutron scattering, have played a key role in efforts to arrive at a theoretical explanation of this transition. The pseudocubic perovskite La 1−x Sr x TiO 3+δ undergoes an analogous transition from a 3d1 AF insulator at x = δ = 0 to a metallic state with increasing hole concentration [1]. In the titanates, however, the metallic state shows conventional Fermi liquid behavior, and no superconductivity is found [1]. Momentum-resolved probes such as angle-resolved photoemission spectroscopy and inelastic neutron scattering have thus far not been applied to the titanates, and the origin of the very different behavior of the metallic cuprates and titanates is still largely unexplored. Here we report an inelastic neutron scattering and anomalous x-ray scattering study of the parent compound of the titanate series, LaTiO 3 , that provides insight into the microscopic interactions underlying this behavior.Orbital degrees of freedom, quenched in the layered cuprates by a large Jahn-Teller (JT) distortion of the CuO 6 octahedra, are likely to be a key factor in the phenomenology of the titanates. While the TiO 6 octahedra are tilted in a GdFeO 3 -type structure, their distortion is small and essentially undetectable in neutron powder diffraction experiments on LaTiO 3 (Ref.[2]). The crystal field acting on the Ti 3+ ion is therefore nearly cubic, and heuristically one expects a quadruply degenerate single-ion ground state with unquenched orbital angular momentum opposite to the spin angular momentum due to the spin-orbit interaction. In other perovskites such as LaMnO 3 , such spin-orbital degeneracies are broken by successive orbital and magnetic ordering transitions [3]. In the orbitally and magnetically ordered state of LaMnO 3 , the spin wave spectrum is highly anisotropic reflecting the different relative orientations of the orbitals on nearest-neighbor Mn atoms in different crystallographic directions [4].The reduced ordered moment (µ 0 ∼ 0.45µ B , Ref.[5]) in the G-Type AF structure of LaTiO 3 (inset in Fig. 1) at first sight appears consistent with a conventional scenario in which the orbital occupancies at every site are establi...

Using resonant inelastic x-ray scattering, we observe in the bilayer iridate Sr3Ir2O7, a spin-orbit coupling driven magnetic insulator with a small charge gap, a magnon gap of ≈92 meV for both acoustic and optical branches. This exceptionally large magnon gap exceeds the total magnon bandwidth of ≈70 meV and implies a marked departure from the Heisenberg model, in stark contrast to the case of the single-layer iridate Sr2IrO4. Analyzing the origin of these observations, we find that the giant magnon gap results from bond-directional pseudodipolar interactions that are strongly enhanced near the metal-insulator transition boundary. This suggests that novel magnetism, such as that inspired by the Kitaev model built on the pseudodipolar interactions, may emerge in small charge-gap iridates.

A complete, continuous transition from discrete macroions to blackberry structures, and then back to discrete macroions, is reported for the first time in the system of {Mo132}/water/acetone, with {Mo132} (full formula (NH4)42[Mo132O372(CH3COO)30(H2O)72].ca.300H2O.ca.10CH3COONH4) as the C60-like anionic polyoxomolybdate molecular clusters. Laser light scattering studies reveal the presence of the self-assembled {Mo132} blackberry structures in water/acetone mixed solvents containing 3 vol % to 70 vol % acetone, with the average hydrodynamic radius (Rh) of blackberries ranging from 45 to 100 nm with increasing acetone content. Only discrete {Mo132} clusters are found in solutions containing <3 vol % and >70 vol % acetone. The complete discrete macroion (cluster)-blackberry-discrete macroion transition helps to identify the driving forces behind the blackberry formation, a new type of self-assembly process. The charge density on the macroions is found to greatly affect the blackberry formation and dissociation, as the counterion association is very dominant around blackberries. The transitions between single {Mo132} clusters and blackberries, and between the blackberries with different sizes, are achieved by only changing the solvent quality.

Electrons in graphene behave like Dirac fermions, permitting phenomena from high energy physics to be studied in a solid state setting. A key question is whether or not these Fermions are critically influenced by Coulomb correlations. We performed inelastic x-ray scattering experiments on crystals of graphite, and applied reconstruction algorithms to image the dynamical screening of charge in a freestanding, graphene sheet. We found that the polarizability of the Dirac fermions is amplified by excitonic effects, improving screening of interactions between quasiparticles. The strength of interactions is characterized by a scale-dependent, effective fine structure constant,, whose value approaches 0.14 0.092~1 / 7  at low energy and large distances. This value is substantially smaller than the nominal 2.2 g   , suggesting that, on the whole, graphene is more weakly interacting than previously believed.

Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and high-temperature superconductivity. Recently, photo-excitation has been used to induce similarly exotic states transiently. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. Here, we implement magnetic resonant inelastic X-ray scattering at a free-electron laser to directly determine the magnetic dynamics after photo-doping the Mott insulator Sr2IrO4. We find that the non-equilibrium state, 2 ps after the excitation, exhibits strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. These two-dimensional (2D) in-plane Néel correlations recover within a few picoseconds, whereas the three-dimensional (3D) long-range magnetic order restores on a fluence-dependent timescale of a few hundred picoseconds. The marked difference in these two timescales implies that the dimensionality of magnetic correlations is vital for our understanding of ultrafast magnetic dynamics.