We report inelastic x-ray scattering studies of charge excitations in insulating Nd 2 CuO 4 as a function of incident photon energy. An excitation of ϳ6 eV is observed when the incident photon energy is tuned through the Cu K edge. This is interpreted as resonantly enhanced inelastic x-ray scattering. Numerical calculations identify the 6 eV feature as a charge-transfer excitation to the antibonding state and suggest that nonlocal effects play a role in determining the shape of the resonance. [S0031-9007(98)06181-X] PACS numbers: 74.25.Jb, 71.27. + a, Understanding the normal state electronic properties of the high-T C copper oxides is an important prerequisite for a theory of high temperature superconductivity. In addition, the properties themselves are highly unusual, exhibiting non-Fermi-like behavior [1] and they have therefore attracted much interest from the more general perspective of understanding electronic behavior in strongly correlated transition metal oxides.A useful theoretical approach has been to treat the copper oxide planes within the framework of a microscopic electronic Hamiltonian, such as the extended Hubbard model [2]. The strong electron correlations preclude the possibility of successful band structure calculations, and a variety of numerical techniques have therefore been applied, utilizing small clusters of ions for which on-site interactions can be treated explicitly [3], for example, impurity and cluster interaction models [4,5]. These models are typically local, that is, the translational symmetry of the lattice is neglected. However, as first emphasized by Veenendaal, Eskes, and Sawatzky [6], solid state (nonlocal) effects can be important, and recently calculations have been performed for a number of clusters connected in a planar geometry for which some degree of translational symmetry is restored [7].A crucial test of such treatments is to compare their predictions with measurements of the electronic excitation spectrum. To this end a variety of spectroscopies have been applied to the cuprates. However, each has its limitations. For example, photoemission experiments only probe to the photoelectron escape depth and it can be difficult to ensure that bulklike properties are measured. Also finalstate effects (i.e., those due to the presence of a core hole in the final state) are large, and electrostatic charging may be a problem for insulators. X-ray absorption spectroscopy (XAS) offers bulklike penetration, but still suffers from a final-state core hole, and optical spectroscopies are essentially limited to q 0 properties and energy transfers of a few eV. There remains a clear need for new spectroscopies to elucidate the electronic excitation spectrum of the high-T C cuprates.Inelastic x-ray scattering in the hard x-ray regime exhibits none of the above disadvantages. The scattering process is charge neutral so that final-state effects are eliminated and the excitation spectrum is measured directly. Further, bulklike properties are measured. It would thus seem an ideal tool in thi...

An energy loss feature with energy transfer close to the anion-to-cation charge-transfer energy of NiO is observed in the inelastic-x-ray-scattering spectrum as the incident x-ray energy is tuned through the Ni K absorption edge of NiO. The inelastic-scattering cross section shows large resonant enhancement and strong incident energy dependence. These observations are interpreted using a configuration-interaction cluster model of NiO.

High-resolution resonant polarized x-ray diffraction experiments near the sulfur K edge have been performed on free-standing liquid crystal films exhibiting the chiral smectic-C*FI2 phase. It is widely accepted that this phase has a four-layer repeat unit, but the internal structure of the repeat unit remains controversial. We report different resolved features of the resonant x-ray diffraction peaks associated with the smectic-C*FI2 phase that unambiguously demonstrate that the four-layer repeat unit is locally biaxial about the layer normal and that the measured angle, describing the biaxiality, is in good agreement with optical measurements.

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We report experimental and theoretical results on the Fe K edge x-ray absorption spectrum and 1s2 p resonant inelastic x-ray scattering ͑RIXS͒ spectra in ␣-Fe 2 O 3 . The results are interpreted using an FeO 6 9Ϫ cluster model with intra-atomic multiplet coupling and interatomic covalency hybridization. The 1s2 p RIXS is treated as a coherent second-order optical process. It is shown that the double-peak structure in the pre-edge region of Fe K absorption spectrum is due to the cubic crystal-field splitting, and that the intensity of the e g (t 2g ) component in the 1s2p resonant inelastic spectrum is enhanced by tuning the incident photon energy to the e g (t 2g ) component in the absorption spectrum. ͓S0163-1829͑98͒08443-4͔