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649 items found (0 items not approved), displaying 1 to 10.[First/Prev] 1, 2, 3, 4, 5, 6, 7, 8 [Next/Last]

Instruments / Technical Area	Publication Details	Published
	Invalidity of low-pass filtering in atom-resolving x-ray holography S. Fanchenko , D. Novikov , A. Schley , G. Materlik Physical Review B 66 (6) DOI: 10.1103/PhysRevB.66.060104 Show Abstract ▼ Abstract: Atom-resolving x-ray holography is a recently developed method for direct imaging of local three-dimensional structures at the atomic level. We investigate analytically and numerically additional effects arising from the long-range order in an object. It is shown that they are not correctly taken into account by existing image reconstruction procedures used commonly in the analysis of experimental data. It is proven that low-pass filtering cannot be used for extracting information about the short-range order in crystalline samples.	Aug 2002
	Temperature-dependent forbidden resonant x-ray scattering in zinc oxide S. Collins , D. Laundy , V. Dmitrienko , D. Mannix , P. Thompson Physical Review B 68 (6) DOI: 10.1103/PhysRevB.68.064110 Show Abstract ▼ Abstract: We have measured the resonance spectrum of glide-plane forbidden x-ray diffraction in hexagonal ZnO as a function of temperature. This is the only method to observe deformations of the electronic states caused by thermal atomic motion. The results provide the first evidence for a complex line shape in the spectrum of thermal-motion-induced scattering and the first observation of a dramatic change in resonance spectrum with temperature. The measurements are in agreement with a phenomenological model, based on a combination of constant (most probably dipole-quadrupole) and temperature-dependent amplitudes. This model provides a means of separating the two components, including their relative phase.	Aug 2003
	Ultrathin Pt films on Ni(111): Structure determined by surface x-ray diffraction O. Robach , H. Isern , P. Steadman , K. Peters , C. Quiros , S. Ferrer Physical Review B 68 (21) DOI: 10.1103/PhysRevB.68.214416 Show Abstract ▼ Abstract: The growth of platinum on a nickel ~111! single crystal under ultrahigh vacuum conditions was studied using surface x-ray diffraction on the ID03 beamline of the ESRF. Film thickness ranged from one to eight monoatomic layers ~ML!. Specular reflectivity was used to determine the growth mode and vertical lattice parameter of the Pt film. A two-dimensional ~2D! growth up to 1 ML followed by more 3D growth was found. A small expansion of the Pt vertical lattice parameter was found. The Pt in-plane lattice parameter was measured. Its relaxation was found to be very slow, with a residual contraction of 2.3% in an 8-ML-thick film ~with respect to bulk Pt!. A Ni crystal truncation rod measured before and after growing 1 ML of Pt revealed the presence of a small amount of pseudomorphic Pt, adsorbed on both fcc and hcp sites. The stacking of the ~111! Pt planes was investigated by measuring stacking-sensitive Pt diffraction rods. A strong tendency to stacking reversal was found at room temperature, with an amount of ‘ ‘reversed’’ Pt about ten times higher than the amount of Pt with the same stacking as the Ni. An eight-layer Ni film on Pt~111! was also studied for comparison	Dec 2003
Theoretical Physics	Quadrupole and hexadecapole ordering in DyB2C2: Direct observation with resonant x-ray diffraction Yoshikazu Tanaka , Toshiya Inami , Stephen Lovesey , Kevin Knight , Flora Yakhou , Danny Mannix , Jun Kokubun , Masayuki Kanazawa , Kohtaro Ishida , Susumu Nanao , Tetsuya Nakamura , Hiroki Yamauchi , Hideya Onodera , Kenji Ohoyama , Yasuo Yamaguchi Physical Review B 69 (2) DOI: 10.1103/PhysRevB.69.024417 Show Abstract ▼ Abstract: Direct evidence of the spatial ordering of Dy 4 f quadrupole and hexadecapole moments in DyB2C2 is demonstrated by resonant x-ray diffraction enhanced by an electric quadrupole event (E2 resonance! at the Dy LIII absorption edge. The diffraction data show that the structural phase transition at TQ524.7 K is accompanied by a reduction in the symmetry of the Dy site to 2/m, from 4/m, and the spatial ordering of the time-even Dy multipoles with Ag character. Below TQ the crystal structure is described by the space group P42 /mnm and Dy ions occupy sites (4c). The distortion at TQ involves the lattice occupied by B and C ions, and it amounts to a buckling of B-C planes that are normal to the two-fold rotation axis of 2/m. An immediately plausible model of low-energy Dy states correlates data on the specific heat, our x-ray diffraction signals, and magnetic ordering below 15.3 K which has been observed in magnetic neutron diffraction.	Jan 2004
	Unraveling Orbital Ordering in La0.5Sr1.5MnO4 S. Dhesi , A. Mirone , C. De Nadaï , P. Ohresser , P. Bencok , N Brookes , P. Reutler , A. Revcolevschi , A. Tagliaferri , O. Toulemonde , G. Van Der Laan Physical Review Letters 92 (5) DOI: 10.1103/PhysRevLett.92.056403 PMID: 14995324 Show Abstract ▼ Abstract: Orbital ordering (OO) in the layered perovskite La0.5Sr1.5MnO4 has been investigated using the enhanced sensitivity of soft x-ray resonant diffraction at the Mn L edges. The energy dependence of an OO diffraction peak over the L2,3 edges is compared to ligand-field calculations allowing a distinction between the influences of Jahn-Teller distortions and spin correlations. The energy dependence of the diffraction peak at the Mn L1 edge is remarkably different from that observed at the Mn K edge.	Feb 2004
	Extinction effects in x-ray holographic imaging with internal reference P. Korecki , D. Novikov , M. Tolkiehn , G. Materlik Physical Review B 69 (18) DOI: 10.1103/PhysRevB.69.184103 Show Abstract ▼ Abstract: Secondary extinction effects in x-ray holography with internal atomic reference are examined in the frame of multiple-scattering power transfer equations for a mosaic crystal. The calculations are compared with experimental holograms of an imperfect Cu3Au single crystal measured using x-ray fluorescence and total electron yields. It is shown that extinction effects in imperfect crystals can influence reconstructed real-space images in x-ray atom-resolving holography.	May 2004
	Circular Dichroism in Core Level Photoemission from an Adsorbed Chiral Molecule M. Polcik , F. Allegretti , D. I. Sayago , G. Nisbet , C. L. A. Lamont , Daniel Woodruff Physical Review Letters 92 DOI: 10.1103/PhysRevLett.92.236103 Show Abstract ▼ Abstract: The results of experimental measurements and theoretical simulations of circular dichroism in the angular dependence (CDAD) of photoemission from atomic core levels of each of the enantiomers of a chiral molecule, alanine, adsorbed on Cu(110) are presented. Measurements in, and out of, substrate mirror planes distinguish CDAD due to the chirality of the sample and the experimental geometry. The effect due to sample chirality is relatively weak, so such measurements may not provide a routine spectral fingerprint of adsorbate chirality.	Jul 2004
	Reply to Comment on Invalidity of low-pass filtering in atom-resolving x-ray holography S. Fanchenko , M. Tolkiehn , D. Novikov , A. Schley , G. Materlik Physical Review B 70 (10) DOI: 10.1103/PhysRevB.70.106102 Show Abstract ▼ Abstract: In their Comment, G. Faigel, M. Tegze, and G. Bortel claim that low-pass filtering can be used to remove the long-range order contributions to the reconstructed images of objects with crystalline internal structure. In this Reply, we present further results, clarifying the problem and supporting the conclusions drawn in the commented paper. We show that the Comment does not provide any evidence to contradict our results.	Sep 2004
	Tracking the local reversal processes in nanostructures by magnetic speckles K. Chesnel , M. Belakhovsky , G. Van Der Laan , F. Livet , A. Marty , G. Beutier , S. Collins , A. Haznar Physical Review B 70 (18) DOI: 10.1103/PhysRevB.70.180402 Show Abstract ▼ Abstract: We have used coherent soft x-ray resonant magnetic scattering to locally track reversal processes in magnetic nanostructures. Coherent illumination of a limited number of nano-objects in a CoPt nanoline grating produces a specific speckle pattern, whose evolution under in situ magnetic field reveals the true local magnetic ordering. While each nanoline behaves as a single macrospin whose direction depends on the dipolar coupling with neighbors, the global reversal of the line array is successively governed by two effects: first, by the random distribution of defects, followed by the dipolar coupling favoring antiferromagnetic ordering.	Nov 2004
	Phase transitions, noncollinear magnetism, and magnetoelectric symmetry in gadolinium tetraboride S. W. Lovesey , J. Fernández Rodríguez , J. A. Blanco Rodriguez , P. J. Brown Physical Review B 70 DOI: 10.1103/PhysRevB.70.172414 Show Abstract ▼ Abstract: Resonant x-ray Bragg diffraction data for gadolinium tetraboride (GdB4), gathered at the Gd L3 edge above and below a transition temperature TN=42K, is shown to imply that a structural phase transition occurs at temperatures above the onset of long-range magnetic order in the Gd moments. Specifically, the room-temperature tetragonal structure P4∕mbm would appear to change to orthorhombic P21212, which is a continuous structural phase transition in Landau and renormalization-group theories. Below TN, a configuration of Gd moments is established, which is consistent with the magnetic Shubnikov group P21212, where moments are constrained to lie in the plane normal to the c axis. The crystal structure and magnetic point symmetry (222) inferred, is one of the class of 58 magnetic groups that can show the magnetoelectric effect.	Nov 2004

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