D. Casa

Interplay between Charge, Orbital, and Magnetic Order in Pr{sub 1-x}Ca{sub x}MnO{sub 3}

We report resonant x-ray scattering studies of charge and orbital order in Pr{sub 1-x}Ca {sub x}MnO{sub 3} with x=0.4 and 0.5. Below the ordering temperature, T{sub O}=245 K , the charge and orbital order intensities follow the same temperature dependence, including an increase at the antiferromagnetic ordering temperature, T{sub N} . High resolution measurements reveal, however, that long range orbital order is never achieved. Rather, an orbital domain state is formed. Above T{sub O} , the charge order fluctuations are more highly correlated than the orbital fluctuations. We conclude that the charge order drives the orbital order at the transition. (c) 1999 The American Physical Society.

Magnetic excitation spectra of Sr2IrO4 probed by resonant inelastic x-ray scattering: establishing links to cuprate superconductors.

Jungho Kim, D. Casa, M. H. Upton, T. Gog, Young-June Kim, J. F. Mitchell, M. van Veenendaal, M. Daghofer, J. van den Brink, G. Khaliullin, B. J. Kim Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7 Material Science Division, Argonne National Laboratory, Argonne, IL 60439, USA Department of Physics, Northern Illinois University, De Kalb, IL 60115, USA Institute for Theoretical Solid Sate Physics, IFW Dresden, Helmholtzstr. 20, 01069 Dresden, Germany and Max Planck Institute for Solid State Research, Heisenbergstrae 1, D-70569 Stuttgart, Germany (Dated: December 11, 2012)

Small angle x-ray scattering for sub-100 nm pattern characterization.

Characterization of sub-100 nm photolithographic patterns with nanometer scale resolution is demonstrated using small angle x-ray scattering. The transmission scattering geometry employed potentially enables high throughput measurements for future technology nodes of the semiconductor industry, organic and inorganic nanoscale devices, and three-dimensional structures. The method is demonstrated through the characterization of a series of polymer photoresist gratings using a synchrotron x-ray source. Quantities, such as periodicity and line width, are extracted using minimal modeling. Additional quantities and the potential of a laboratory-based x-ray system are briefly discussed.

The Effective Fine-Structure Constant of Freestanding Graphene Measured in Graphite

Weakly Interacting Graphene Many unusual properties of graphene are a consequence of the Dirac dispersion of its electrons—a linear relationship between an electrons momentum and energy. Naïvely, this dispersion leads to the conclusion that electrons in graphene are strongly affected by mutual electrostatic interactions; however, there is little experimental evidence for strong interaction. Reed et al. (p. 805) resolved this discrepancy by using inelastic x-ray scattering spectra of graphite (which consists of loosely bound layers of graphene) to estimate how much the electric field was damped by the presence of mobile charge carriers. In fact, damping was strong at distances in excess of 1 nanometer, suggesting that graphene is more weakly interacting than was assumed. Spectral analysis of graphite reveals an unexpectedly low influence of electron interactions in graphene. 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, αg*(k,ω), the value of which approaches 0.14±0.092~1/7 at low energy and large distances. This value is substantially smaller than the nominal αg=2.2, suggesting that, on the whole, graphene is more weakly interacting than previously believed.

Crystal-field splitting and correlation effect on the electronic structure of A2IrO3.

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).

Large spin-wave energy gap in the bilayer iridate Sr3Ir2O7: evidence for enhanced dipolar interactions near the mott metal-insulator transition.

Jungho Kim, A. H. Said, D. Casa, M. H. Upton, T. Gog, M. Daghofer, G. Jackeli, J. van den Brink, G. Khaliullin, B. J. Kim Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA Institute for Theoretical Solid Sate Physics, IFW Dresden, Helmholtzstr. 20, 01069 Dresden, Germany Max Planck Institute for Solid State Research, Heisenbergstraße 1, D-70569 Stuttgart, Germany and Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA (Dated: May 1, 2014)

Ultrafast energy- and momentum-resolved dynamics of magnetic correlations in the photo-doped Mott insulator Sr2IrO4

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.

Resonant Inelastic X-Ray Scattering of the Holon-Antiholon Continuum in SrCuO~2

We report a resonant inelastic x-ray scattering study of charge excitations in the quasi-one-dimensional Mott insulator SrCuO2. We observe a continuum of low-energy excitations, the onset of which exhibits a small dispersion of approximately 0.4 eV. Within this continuum, a highly dispersive feature with a large sinusoidal dispersion (approximately 1.1 eV) is observed. We have also measured the optical conductivity, and studied the dynamic response of the extended Hubbard model with realistic parameters, using a dynamical density-matrix renormalization group method. In contrast to earlier work, we do not find a long-lived exciton, but rather these results suggest that the excitation spectrum comprises a holon-antiholon continuum together with a broad resonance.

RESONANT X-RAY DIFFRACTION OF THE MAGNETORESISTANT PEROVSKITE PR0.6CA0.4MNO3

We report a resonant x-ray-diffraction study of the magnetoresistant perovskite Pr 0 . 6 Ca 0 . 4 MnO 3 . We discuss the spectra measured above and below the semiconductor-insulator transition temperature with the aid of a detailed formal analysis of the energy and polarization dependences of the structure factors and ab initio calculations of the spectra. In the low-temperature insulating phase, we find that inequivalent Mn atoms order in a CE-type pattern and that the crystallographic structure of La 0 . 5 Ca 0 . 5 MnO 3 [Radaelli et al., Phys. Rev. B 55, 3015 (1997)] can also describe this system in detail. Instead, the alternative structure proposed for the so-called Zener-polaron model [Daoud-Aladine et al., Phys. Rev. Lett. 89, 097205 (2002)] is ruled out by crystallographic and spectroscopic evidence. Our analysis supports a model involving orbital ordering. However, we confirm that there is no direct evidence of charge disproportionation in the Mn K-edge resonant spectra. Therefore, we consider a CE-type model in which there are two Mn sublattices, each with partial e g occupancy. One sublattice consists of Mn atoms with the 3x 2 -r 2 or 3y 2 -r 2 orbitals partially occupied in an alternating pattern, the other sublattice with the x 2 -y 2 orbital partially occupied.

X-ray Spectroscopic Characterization of Co(IV) and Metal–Metal Interactions in Co4O4: Electronic Structure Contributions to the Formation of High-Valent States Relevant to the Oxygen Evolution Reaction

The formation of high-valent states is a key factor in making highly active transition-metal-based catalysts of the oxygen evolution reaction (OER). These high oxidation states will be strongly influenced by the local geometric and electronic structures of the metal ion, which are difficult to study due to spectroscopically active and complex backgrounds, short lifetimes, and limited concentrations. Here, we use a wide range of complementary X-ray spectroscopies coupled to DFT calculations to study Co(III)4O4 cubanes and their first oxidized derivatives, which provide insight into the high-valent Co(IV) centers responsible for the activity of molecular and heterogeneous OER catalysts. The combination of X-ray absorption and 1s3p resonant inelastic X-ray scattering (Kβ RIXS) allows Co(IV) to be isolated and studied against a spectroscopically active Co(III) background. Co K- and L-edge X-ray absorption data allow for a detailed characterization of the 3d-manifold of effectively localized Co(IV) centers and provide a direct handle on the t2g-based redox-active molecular orbital. Kβ RIXS is also shown to provide a powerful probe of Co(IV), and specific spectral features are sensitive to the degree of oxo-mediated metal-metal coupling across Co4O4. Guided by the data, calculations show that electron-hole delocalization can actually oppose Co(IV) formation. Computational extension of Co4O4 to CoM3O4 structures (M = redox-inactive metal) defines electronic structure contributions to Co(IV) formation. Redox activity is shown to be linearly related to covalency, and M(III) oxo inductive effects on Co(IV) oxo bonding can tune the covalency of high-valent sites over a large range and thereby tune E(0) over hundreds of millivolts. Additionally, redox-inactive metal substitution can also switch the ground state and modify metal-metal and antibonding interactions across the cluster.