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      Parity and Time-Reversal Breaking Effects on Resonant X-Ray Scattering at the Fe Pre-K-Edge in Magnetite

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          Abstract

          We study noncentrosymmetric effects on resonant x-ray scattering (RXS) in magnetite. The noncentrosymmetry at A sites in spinel structure makes the 4p states strongly hybridize with the 3d states through neighboring oxygen 2p states, giving rise to the non-vanishing contribution of the dipole-quadrupole (E1-E2) process in the RXS spectra. We substantiate this observation by introducing a microscopic model of a FeO_4 cluster with multiplets and the 4p band. We show that the hybridization changes its sign between two kinds of A sites and accordingly the local amplitude from the E1-E2 process changes its sign, resulting in non-vanishing RXS intensities at forbidden spots (002) and (006) in the pre-edge region in agreement with the experiment. A large dependence of the pre-edge intensity on the direction of the applied magnetic field is predicted as a consequence of breaking both centrosymmetry and time-reversal symmetry. Furthermore we analyze the intensity difference between two opposite directions of the applied magnetic field at the allowed spot (222) in connection with the experiment. We obtain the intensity difference of a "dispersion" form, which resembles the observed spectra at the Mn pre-K-edge in MnCr_2O_4 but is quite different from the observed one in magnetite. Although the observed spectra are claimed to arise from "magnetoelectric" amplitude, we argue that this claim has no ground.

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          Critical Reexamination of the Experimental Evidence of Orbital Ordering inLaMnO3andLa0.5Sr1.5MnO4

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            Orbital Occupancy Order inV2O3: Resonant X-Ray Scattering Results

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              Orbital ordering, Jahn-Teller distortion, and anomalous X-Ray scattering in Manganates

              We demonstrate with LSDA+U calculations that x-ray scattering at the K edge of Mn is sensitive to orbital ordering in one energy range and Jahn-Teller distortion in another. Contrary to what is suggested by atomic or cluster models used to date we show that band structure effects rather than local Coulomb interactions dominate the polarization dependence of the K edge scattering and therefore it is sensitive to nearest neighbour bond length distortions and next nearest neighbour orbital occupation. Based on this we propose a new mechanism for K edge x-ray scattering in the manganates which we suggest is also applicable to transition metal compounds in general.
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                Author and article information

                Journal
                10 June 2008
                Article
                10.1143/JPSJ.77.084706
                0806.1626
                3af41de3-5105-4b0a-ba12-b62dc79269dc

                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

                History
                Custom metadata
                J. Phys. Soc. Jpn., vol. 77 (2008) 084706.
                26 pages, 7 figures, to be published in J. Phys. Soc. Jpn. vol. 77, No 8
                cond-mat.str-el

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