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Register a new userCoherent X-ray Scattering from Manganite Charge and Orbital Domains
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We report coherent x-ray scattering studies of charge and orbital domains in manganite systems. The experiments were carried out on LaMnO_3 and Pr_{0.6}Ca_{0.4}MnO_3, with the incident photon energy tuned near the Mn K edge. At room temperature, the orbital speckle pattern of LaMnO_3 was observed to be constant over a timescale of at least minutes, which is indicative of static orbital domains on this timescale. For Pr_{0.6}Ca_{0.4}MnO_3, both charge and orbital speckle patterns were observed. The observation of the latter rules out the presence of fast orbital fluctuations, while long time series data-- on the order of several minutes-- were suggestive of slow dynamic behavior. In contrast, the charge order speckle patterns were static.
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This article reports a detailed x-ray resonant scattering study of the bilayer iridate compound, Sr3Ir2O7, at the Ir L2 and L3 edges. Resonant scattering at the Ir L3 edge has been used to determine that Sr3Ir2O7 is a long-range ordered antiferromagnet below TN 230K with an ordering wavevector, q=(1/2,1/2,0). The energy resonance at the L3 edge was found to be a factor of ~30 times larger than that at the L2. This remarkable effect has been seen in the single layer compound Sr2IrO4 and has been linked to the observation of a Jeff=1/2 spin-orbit insulator. Our result shows that despite the modified electronic structure of the bilayer compound, caused by the larger bandwidth, the effect of strong spin-orbit coupling on the resonant magnetic scattering persists. Using the programme SARAh, we have determined that the magnetic order consists of two domains with propagation vectors k1=(1/2,1/2,0) and k2=(1/2,-1/2,0), respectively. A raster measurement of a focussed x-ray beam across the surface of the sample yielded images of domains of the order of 100 microns size, with odd and even L components, respectively. Fully relativistic, monoelectronic calculations (FDMNES), using the Greens function technique for a muffin-tin potential have been employed to calculate the relative intensities of the L2,3 edge resonances, comparing the effects of including spin-orbit coupling and the Hubbard, U, term. A large L3 to L2 edge intensity ratio (~5) was found for calculations including spin-orbit coupling. Adding the Hubbard, U, term resulted in changes to the intensity ratio <5%.
We analyze the resonant inelastic x-ray scattering (RIXS) spectra at the Cu and Ni K edges in La2CuO4 and La2NiO4, respectively. We make use of the Keldysh-Green-function formalism, in which the RIXS intensity is described by a product of incident-photon-dependent factor and density-density correlation function in the 3d states. The former factor is calculated using the $4p$ density of states given by an ab initio band structure calculation and the latter using the wavefunctions given by a Hartree-Fock calculation of a multi-orbital tight-binding model. The initial state is described within the Hartree-Fock approximation and the electron correlations on charge excitations are treated within the random phase approximation. The calculated RIXS spectra well reproduce several characteristic features in the experiments. Although several groups have interpreted the RIXS peaks as bound excitons, our calculation indicates that they should be interpreted as band-to-band excitations augmented by electron correlations. The difference in RIXS spectra between La2CuO4 and La2NiO4 is explained from this point of view.
Density waves are inherent to the phase diagrams of materials that exhibit unusual, and sometimes extraordinarily useful properties, such as superconductivity and colossal magnetoresistance. While the pure charge density waves (CDW) are well described by an itinerant approach, where electrons are treated as waves propagating through the crystal, the charge-orbital ordering (COO) is usually explained by a local approach, where the electrons are treated as localized on the atomic sites. Here we show that in the half-doped manganite La0.5Sr1.5MnO4 (LSMO) the electronic susceptibility, calculated from the angle-resolved photoemission spectra (ARPES), exhibits a prominent nesting-driven peak at one quarter of the Brillouin zone diagonal, that is equal to the reciprocal lattice vector of the charge-orbital pattern. Our results demonstrate that the Fermi surface geometry determines the propensity of the system to form a COO state which, in turn, implies the applicability of the itinerant approach also to the COO.
We report a resonant inelastic x-ray scattering study of the dispersion relations of charge transfer excitations in insulating La$_2$CuO$_4$. These data reveal two peaks, both of which show two-dimensional characteristics. The lowest energy excitation has a gap energy of $sim 2.2$ eV at the zone center, and a dispersion of $sim 1$ eV. The spectral weight of this mode becomes dramatically smaller around ($pi$, $pi$). The second peak shows a smaller dispersion ($sim 0.5$ eV) with a zone-center energy of $sim 3.9$ eV. We argue that these are both highly dispersive exciton modes damped by the presence of the electron-hole continuum.
We investigate the high temperature phase of layered manganites, and demonstrate that the charge-orbital phase transition without magnetic order in La$_{0.5}$Sr$_{1.5}$MnO$_4$ can be understood in terms of the density wave instability. The orbital ordering is found to be induced by the nesting between segments of Fermi surface with different orbital characters. The simultaneous charge and orbital orderings are elaborated with a mean field theory. The ordered orbitals are shown to be $d_{x^2-y^2} pm d_{3z^2-r^2}$.
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