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ABSTRACT: A spherical tensor expansion of resonant inelastic x-ray scattering (RIXS) is
carried out to express the RIXS cross-section in terms of a sum of products of
fundamental RIXS spectra and tensors involving incident and scattered x-ray
wavevectors and polarization vectors. Specific terms of this expansion are
discussed in detail, in particular magnetic x-ray dichroism and powder samples.
02/2013;
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ABSTRACT: After a review of temperature-dependent experimental x-ray absorption
near-edge structure (XANES) and related theoretical developments, we present
the Al K-edge XANES spectra of corundum and beryl for temperature ranging from
300K to 930K. These experimental results provide a first evidence of the role
of thermal fluctuation in XANES at the Al K-edge especially in the pre-edge
region. The study is carried out by polarized XANES measurements of single
crystals. For any orientation of the sample with respect to the x-ray beam, the
pre-edge peak grows and shifts to lower energy with temperature. In addition
temperature induces modifications in the position and intensities of the main
XANES features. First-principles DFT calculations are performed for both
compounds. They show that the pre-edge peak originates from forbidden 1s to 3s
transitions induced by vibrations. Three existing theoretical models are used
to take vibrations into account in the absorption cross section calculations:
i) an average of the XANES spectra over the thermal displacements of the
absorbing atom around its equilibrium position, ii) a method based on the crude
Born-Oppenheimer approximation where only the initial state is averaged over
thermal displacements, iii) a convolution of the spectra obtained for the atoms
at the equilibrium positions with an approximate phonon spectral function. The
theoretical spectra so obtained permit to qualitatively understand the origin
of the spectral modifications induced by temperature. However the correct
treatment of thermal fluctuation in XANES spectroscopy requires more
sophisticated theoretical tools.
Physical Review B 06/2012; 85(22). · 3.69 Impact Factor
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ABSTRACT: Angular dependent core-hole screening effects have been found in the cobalt K-edge x-ray absorption spectrum of LiCoO2, using high-resolution data and parameter-free GGA+U calculations. The Co 1s core-hole on the absorber causes strong local attraction. The core-hole screening on the nearest neighbours cobalt induces a 2 eV shift in the density of states with respect to the on-site 1s-3d transitions, as detected in the Co K pre-edge spectrum. Our DFT+U calculations reveal that the off-site screening is different in the out-of-plane direction, where a 3 eV shift is visible in both calculations and experiment. The detailed analysis of the inclusion of the core-hole potential and the Hubbard parameter U shows that the core-hole is essential for the off-site screening, while U improves the description of the angular dependent screening effects. In the case of oxygen K-edge, both the core-hole potential and the Hubbard parameter improve the relative positions of the spectral features.
12/2009;
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ABSTRACT: The influence of atomic vibrations on x-ray absorption near edge structure (XANES) is calculated by assuming that vibrational energies are small with respect to the instrumental resolution. The resulting expression shows that, at the K-edge, vibrations enable electric dipole transitions to 3s and 3d final states. The theory is applied to the K-edge of Al in Al2O3 corundum and of Ti in TiO2 rutile and compared with experiment. At the Al K-edge, sizeable transitions towards 3s final states are obtained, leading to a clear improvement of the agreement with experimental spectra. At the Ti K-edge, electric dipole transitions towards 3d final states explain the temperature dependence of the pre-edge features. Comment: 4 pages, 2 figures
12/2009;
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ABSTRACT: We present a detailed analysis of the pre-edge peak present in the Al K-edge XANES spectra of corundum (α-Al2O3) and diaspore (α-AlOOH), as measured at room temperature. This is achieved by XANES and DOS calculations performed using the density functional theory in a pseudopotential plane-wave framework. The XANES calculations carried out for the equilibrium atomic positions do not reproduce the pre-edge of corundum and partially reproduce it in the case of diaspore. It is shown that the electronic transitions occuring in the pre-edge involves the 3s empty states of the aluminium absorbing atom. The Al 3s states can be probed in the electric dipole approximation via a p-s mixing, which is possible only if the Al site is not centrosymmetric. Although Al does not occupy an inversion center in the two minerals under study, the p-s mixing is too weak to provide a pre-edge feature in good agreement with experiment. The deviation from centrosymmetry can be enhanced by the atomic vibrations. We develop a theory that takes into account the atomic vibrations directly in the calculation of the absorption cross section, based on the Born-Oppenheimer approximation. This theory is applied to corundum and diaspore and yields satisfactory results in the pre-edge region.
Journal of Physics Conference Series 11/2009; 190(1):012003.
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ABSTRACT: The angular dependence (x-ray linear dichroism) of the Cr K pre-edge in MgAl2O4:Cr3+ spinel is measured by means of x-ray absorption near edge structure spectroscopy (XANES) and compared to calculations based on density functional theory (DFT) and ligand field multiplet theory (LFM). We also present an efficient method, based on symmetry considerations, to compute the dichroism of the cubic crystal starting from the dichroism of a single substitutional site. DFT shows that the electric dipole transitions do not contribute to the features visible in the pre-edge and provides a clear vision of the assignment of the 1s-->3d transitions. However, DFT is unable to reproduce quantitatively the angular dependence of the pre-edge, which is, on the other side, well reproduced by LFM calculations. The most relevant factors determining the dichroism of Cr K pre-edge are identified as the site distortion and 3d-3d electronic repulsion. From this combined DFT, LFM approach is concluded that when the pre-edge features are more intense than 4 % of the edge jump, pure quadrupole transitions cannot explain alone the origin of the pre-edge. Finally, the shape of the dichroic signal is more sensitive than the isotropic spectrum to the trigonal distortion of the substitutional site. This suggests the possibility to obtain quantitative information on site distortion from the x-ray linear dichroism by performing angular dependent measurements on single crystals.
06/2008;
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ABSTRACT: The relation between the properties of a specific crystallographic site and the properties of the full crystal is discussed by using spherical tensors. The concept of spherical tensors is introduced and the way it transforms under the symmetry operations of the site and from site to site is described in detail. The law of spherical tensor coupling is given and illustrated with the example of the electric dipole and quadrupole transitions in x-ray absorption spectroscopy. The main application of the formalism is the reduction of computation time in the calculation of the properties of crystals by band structure methods. The general approach is illustrated by the examples of substitutional chromium in spinel and substitutional vanadium in garnet. Comment: 27 pages, 3 figures
06/2008;
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ABSTRACT: We discuss how measurements of x-ray circular intensity differentials (XCIDs) could be used to probe the Voigt–Fedorov vector part of optical activity (OA) due to electric dipole–electric quadrupole (E 1E 2) interferences. The first experiment was carried out on a single crystal of zincite (hexagonal ZnO: class 6mm). XCID spectra were recorded in the x-ray resonant diffraction regime near the Zn K edge using the (300) reflection at Bragg angles near 45°. In the x-ray range, the effective operator responsible for the vector part of OA can be assigned to the vector product , in which L and ΩL are time-reversal odd operators associated with the orbital angular momentum and the orbital anapole respectively, whereas n is a true time-reversal even electric dipole. This is consistent with the pyroelectric properties of zincite crystals.
Journal of Physics Condensed Matter 03/2007; 19(15):156201. · 2.55 Impact Factor
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ABSTRACT: We determine the structural relaxations around paramagnetic impurities (Ti, Cr, Fe) in corundum (alpha-Al2O3), by combining x-ray absorption near edge structure (XANES) experiments and ab initio calculations. The structural relaxations are found to be very local. We then show that XANES is sensitive to small variations in interatomic distances within the coordination shell of the absorbing atom. The experiments were carried out on single crystals of ruby and sapphires. Linear dichroic signals are essential to characterize the geometry of the impurity site. The calculations were performed within a self-consistent ``non muffin-tin'' framework, that uses pseudopotentials, plane-wave basis set, and the continued fraction for the absorption cross section.
02/2005;
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Microscopy and Microanalysis 07/2004; 10:90 - 91. · 3.01 Impact Factor
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ABSTRACT: Optical activity (OA) was only recently discovered in the x-ray range. It is caused mainly by the electric dipole–electric quadrupole E1E2 interference terms which mix multipole moments of opposite parity and exist only in structures with odd space parity. OA related phenomena can be either even or odd with respect to time reversal: natural OA is concerned with time-reversal even effects whereas non-reciprocal OA refers to time-reversal odd contributions. Various types of x-ray dichroisms related to either natural or non-reciprocal OA have been detected and are reviewed in the present paper. Extra emphasis is put on two different non-reciprocal effects that were observed in selected magnetoelectric materials: (i) x-ray magnetochiral dichroism (XMχ D) and (ii) the non-reciprocal x-ray magnetic linear dichroism. Edge-selective OA sum rules have recently been established which show that the effective operator of XMχ D is the orbital anapole moment at the absorbing centre.
Journal of Physics Condensed Matter 02/2003; 15(5):S633. · 2.55 Impact Factor
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ABSTRACT: Chrysotile asbestos is formed by densely packed bundles of multiwall hollow nanotubes. Each wall in the nanotubes is a cylindrically wrapped layer of Mg(3)Si(2)O(5)(OH)(4). We show by experiment and theory that the infrared spectra of chrysotile present multiple ionic-plasmon resonances in the Si-O stretching bands. These collective charge excitations are universal features of the nanotubes that are obtained by cylindrically wrapping an anisotropic material. The multiple plasmons can be observed if the width of the resonances is sufficiently small as in chrysotile.
Physical Review Letters 11/2002; 89(17):177401. · 7.37 Impact Factor
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ABSTRACT: Chrysotile asbestos is formed by densely packed bundles of multiwall hollow nanotubes. Each wall in the nanotubes is a cylindrically wrapped layer of $Mg_3 Si_2 O_5 (OH)_4$. We show by experiment and theory that the infrared spectrum of chrysotile presents multiple plasmon resonances in the Si-O stretching bands. These collective charge excitations are universal features of the nanotubes that are obtained by cylindrically wrapping an anisotropic material. The multiple plasmons can be observed if the width of the resonances is sufficiently small as in chrysotile. Comment: 4 pages, 5 figures. Revtex4 compuscript. Misprint in Eq.(6) corrected
05/2002;
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ABSTRACT: We have produced recently the very first experimental evidence that X-ray Natural Circular Dichroism (XNCD) can be detected in both uniaxial and biaxial gyrotropic single crystals. XNCD signals are to be assigned to electric dipole-electric quadrupole E1.E2 interference terms which can be quite significant in the X-ray range whereas there can hardly be any detectable contribution of the electric dipole-magnetic dipole E1.M1 terms which dominate CD at optical wavelengths. For heavily absorbing single crystals, the differential absorption between left-handed and right-handed circularly polarized X- ray photons cannot be measured in transmission but circular dichroism can still be detected in fluorescence excitation spectra. XNCD and Fluorescence detected X-ray Natural Circular Dichroism (Fd-XNCD) spectra are strictly identical for uniaxial crystals such a lithium iodate ((alpha) -LiIO3) but this is only true to the first order for biaxial crystals such as potassium titanyl phosphate (KTP). Such difficult experiments became feasible only with the development of powerful helical undulator sources and with a careful optimization of all components of the beamline.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
11/1999;
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ABSTRACT: High-resolution sulphur K-edge x-ray absorption near-edge structure spectra have been recorded for europium and gadolinium monosulphides. A comparison of the experimental spectra with the element-projected partial density of unoccupied states calculated using a spin-polarized linear muffin-tin orbital method revealed a rather good agreement. Our analysis suggests that there is a strong hybridization of the 4f and 5d orbitals of the rare earths with the p orbitals of the sulphur which also contribute to the conduction band. The strong d-p and f-p interactions result in a large sulphur p-band polarization which has been confirmed experimentally by the detection of a strong x-ray magnetic circular dichroism signal at the S K edge in EuS.
Journal of Physics Condensed Matter 12/1998; 11(4):1115. · 2.55 Impact Factor
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ABSTRACT: A coordinate-free expression is derived for the scattering of an electromagnetic wave by a dielectric sphere. The scattering of a plane wave propagating along any direction and with any state of polarization is summarized in one compact formula. As an application, we present an efficient expression for the scattering of light emitted by an extended source.
Journal of Optics 12/1998; 29(2):88.
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ABSTRACT: We produce the first experimental evidence of x-ray natural circular dichroism (XNCD) in a uniaxial gyrotropic crystal of α-LiIO3 that is known to crystallize with space group P63 and to exhibit a very strong nonlinear susceptibility. For the sake of simplicity, the optical axis was set colinear to the direction of the exciting x-ray beam in order to get rid of undesirable birefringence and linear dichroism effects. The nicely structured XNCD spectra recorded at the LI, LII, and LIII edges of iodine are assigned to the electric dipole–electric quadrupole (E1.E2) interference terms which do not vanish in gyrotropic crystals but contribute to a second order polarizability. Our interpretation is consistent with a band structure calculation and is fully supported by ab initio multiple scattering simulations. The signatures recorded at the LII and LIII edges are similar and have the same sign. This is in contrast to x-ray magnetic circular dichroism (XMCD) spectra which usually exhibit opposite signs at the LII and LIII edges: while the exchange and spin-orbit interactions are the driving terms in XMCD, this is not the case in XNCD. The XNCD signal detected at the LI edge is found to be relatively more intense and has the opposite sign. These results leave very little space for a contribution of the electric dipole–magnetic dipole (E1.M1) interference terms which are usually dominant at optical wavelengths but should be barely detectable in the x-ray range. © 1998 American Institute of Physics.
The Journal of Chemical Physics 04/1998; 108(15):6394-6403. · 3.33 Impact Factor
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ABSTRACT: We have checked the x-ray magnetic circular dichroism (XMCD) sum rules through an analytical calculation of the Cu2+ L2,3 edges in octahedral symmetry. It is found that the XMCD sum rules are fully satisfied in the framework of the crystal-field multiplet approach resulting from the very general arguments developed in their derivations by Thole and Carra. Analytical expressions for the orbital magnetic moment 〈Lz〉, the spin magnetic moment 〈Sz〉, and the magnetic dipole term 〈Tz〉 are obtained as functions of crystal-field strength and spin-orbit couplings. The nullity of 〈Tz〉 is specially examined through group theory considerations, and it is found that at low temperature 〈Tz〉 contribution can be much larger than 〈Sz〉 contribution in the spin sum rule.
Phys. Rev. B. 10/1995; 52(17).
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ABSTRACT: The V K-edge spectrum of a V-bearing grossular garnet (tsavorite) single crystal has been measured by high-energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS). First-principles calculations based on the density functional theory (DFT) reproduce the experimental spectra well, including the angular dependence of the pre-edge. The latter arises from pure electric quadrupole transitions since vanadium is incorporated in a centrosymmetric site, which inhibits the electric dipole transitions. An interpretation of the spectral features is given using a monoelectronic description of the transitions involved in the X-ray absorption process. The assignment of experimental pre-edge peaks in terms of 1s-3d transitions makes it possible to determine the magnitude of the 3d-splitting, which is similar to the 10 Dq crystal-field value determined from optical absorption spectroscopy. DFT calculations also indicate a full structural relaxation around V3+ substituting Al3+ in the garnet structure. The angular dependence only causes weak variations of the pre-edge intensity, which supports the use of V-bearing grossular garnet as a reference compound for octahedral V3+.
American mineralogist.
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ABSTRACT: Angular-dependent core hole screening effects have been found in the cobalt K-edge x-ray absorption spectrum of LiCoO(2), using high-resolution data and parameter-free general gradient approximation plus U calculations. The Co 1s core hole on the absorber causes strong local attraction. The core hole screening on the cobalt nearest-neighbors induces a 2 eV shift in the density of states with respect to the on-site 1s-3d transitions, as detected in the Co K pre-edge spectrum. Our density functional theory plus U calculations reveal that the off-site screening is different in the out-of-plane direction, where a 3 eV shift is visible in both calculations and experiment. The detailed analysis of the inclusion of the core hole potential and the Hubbard parameter U shows that the core hole is essential for the off-site screening while U improves the description of the angular-dependent screening effects. In the case of oxygen K edge, both the core hole potential and the Hubbard parameter improve the relative positions of the spectral features.
PHYSICAL REVIEW B. 81(11).
