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67 items found (0 items not approved), displaying 41 to 50.[First/Prev] 1, 2, 3, 4, 5, 6, 7 [Next/Last]

Instruments / Technical Area	Publication Details	Published
Theoretical Physics	Symmetry-protected hidden order and magnetic neutron Bragg diffraction by URu2Si2 D. D. Khalyavin , S. Lovesey , A. N. Dobrynin , E. Ressouche , R. Ballou , J. Flouquet Journal Of Physics: Condensed Matter 26 (4) DOI: 10.1088/0953-8984/26/4/046003 Show Abstract ▼ Abstract: We investigate how the order parameter of a continuous phase transition can be protected from view by symmetry in a magnetic crystal. The symmetry in question forbids atomic displacements and formation of magnetic dipoles, rendering the order parameter invisible in standard x-ray and magnetic neutron Bragg diffraction. Analysis of the allowed magnetic space-groups reveals exact properties of the hidden order parameter. We demonstrate that Bragg spots forbidden by the chemical structure can unveil magnetic hidden order. The method is applied to URu2Si2, which has been thoroughly investigated in the past few decades using all manner of experimental techniques. Starting from the established chemical structure of URu2Si2, we have performed a critical analysis of available data for magnetic neutron Bragg diffraction.	Jan 2014
Electrical Engineering Mechanical Engineering Optics Theoretical Physics	Predicting the coupling properties of axially-textured materials Luis Fuentes-Cobas , Alejandro Muñoz-Romero , María Montero-Cabrera , Luis Fuentes-Montero , María Fuentes-Montero Materials 6 (11), 4967 - 4984 DOI: 10.3390/ma6114967 Open Access Show Abstract ▼ Abstract: A description of methods and computer programs for the prediction of “coupling properties” in axially-textured polycrystals is presented. Starting data are the single-crystal properties, texture and stereography. The validity and proper protocols for applying the Voigt, Reuss and Hill approximations to estimate coupling properties effective values is analyzed. Working algorithms for predicting mentioned averages are given. Bunge’s symmetrized spherical harmonics expansion of orientation distribution functions, inverse pole figures and (single and polycrystals) physical properties is applied in all stages of the proposed methodology. The established mathematical route has been systematized in a working computer program. The discussion of piezoelectricity in a representative textured ferro-piezoelectric ceramic illustrates the application of the proposed methodology. Polycrystal coupling properties, predicted by the suggested route, are fairly close to experimentally measured ones.	Nov 2013
B16-Test Beamline Optics	X-ray standing wave induced Compton and elastic scattering from thin periodic multilayer structures M. K. Tiwari , H. Wang , K. J. S. Sawhney , M. Nayak , G. S. Lodha Physical Review B 87 (23) DOI: 10.1103/PhysRevB.87.235401 Show Abstract ▼ Abstract: We report determination of structural parameters on angstrom length scale of nanostructured periodic multilayers using x-ray standing wave (XSW) enhanced elastic and Compton scattering. We show that the elastic scattered x-ray intensities emitted from the thin periodic multilayer structures, under strong XSW condition, are largely sensitive to the structural parameters of high-z layers, whereas the Compton scattered intensities are sensitive to the parameters of low-z layers. The utility of the methodology is demonstrated by analyzing two repetitive W/B4C multilayer structures with different surface-interface properties. The results are compared with those obtained using x-ray reflectivity and conventional x-ray standing wave fluorescence techniques. It is further shown that the Bragg angle can be derived with high accuracy (∼0.002∘) from the ratio of the scattering intensities, which in turn improves the accuracy of the derived multilayer periodicity. The method presents an opportunity to probe structures on angstrom length scale of any periodic multilayer structure comprising of low atomic number layers. Unlike the conventional XSW fluorescence measurements, the present method has an advantage that it permits the determination of structural parameters of both the high- and the low-z layers independently.	Jun 2013
Theoretical Physics	Applications of soft x-ray magnetic dichroism Gerrit Van Der Laan Journal Of Physics: Conference Series 430 15th International Conference on X-ray Absorption Fine Structure (XAFS15) DOI: 10.1088/1742-6596/430/1/012127 Show Abstract ▼ Abstract: Applications of x-ray magnetic circular and linear dichroism (XMCD and XMLD) are reviewed in the soft x-ray region, covering the photon energy range 0.4-2 keV, which includes important absorption edges such as the 3d transition metal L2,3 and rare earth M4,5. These techniques enable a broad range of novel and exciting studies such as on the electronic properties and magnetic ordering of novel nanostructured systems. XMCD has a sensitivity better than 0.01 monolayer (at the surface) and due to simple detection methods, such as electron yield and fluorescence yield, it has become a workhorse technique in physics and materials science. It is the only element-specific technique able to distinguish between the spin and orbital parts of the magnetic moments. The applications are vast, e.g., in x-ray holographic imaging, XMCD gives a spatial resolution of tens of nm. While many studies in the past were centered on physics, more recently new applications have emerged in areas such as chemistry, biology and earth and environmental sciences. For instance, XMCD allows the determination of the cation occupations in spinels and other ternary oxides. In scanning transmission x-ray microscopy (STXM), XMCD enables us to map biogenic magnetite redox changes resulting in a surprising degree of variation on the nanoscale. Another recent development is ferromagnetic resonance (FMR) detected by time-resolved XMCD which opens the door to element-, site- and layer-specific dynamical measurements. By exploiting the time structure of the pulsed synchrotron radiation from the storage ring the relative phase of precession in the individual magnetic layers of a multilayer stack can be determined.	Apr 2013
B16-Test Beamline	On the resolution and linearity of lensless in situ X-ray beam diagnostics using pixelated sensors Anton Kachatkou , Roelof Van Silfhout Optics Express 21 (4) DOI: 10.1364/OE.21.004291 PMID: 23481962 Open Access Show Abstract ▼ Abstract: We present a theoretical model that describes the resolution and linearity of a novel transparent X-ray beam imaging and position measurement method. Using a pinhole or coded aperture camera with pixelated area sensors to image a small fraction of radiation scattered by a thin foil placed at oblique angles with respect to the beam, a very precise measurement of the beam position is made. We show that the resolution of the method is determined by incident beam intensity, beam size, camera parameters, sensor pixel size and noise. The model is verified experimentally showing a sub-micrometer resolution over a large linear range.	Feb 2013
	Birefringence and polarization rotation in resonant x-ray diffraction Yves Joly , Stephen P. Collins , Stephane Grenier , Helio C. N. Tolentino , Maurizio De Santis Physical Review B 86 (22) DOI: 10.1103/PhysRevB.86.220101 Show Abstract ▼ Abstract: Birefringence can contribute to x-ray resonant Bragg diffraction and likely explains recent novel data collected on CuO. We prove these statements using ab initio simulations which reproduce the experimental polarization effects quantitatively. We show that an unrotated polarization signalruled out in resonant magnetic scattering within the electric dipole approximationarises from the dynamic change in polarization inside the material. We are able to reproduce all the related behavior with circular polarization and its dependence on the angle of rotation about the Bragg wave vector. We provide a tool to disentangle the various physical origins of the polarization rotation, providing a more complete understanding of the illuminated material.	Dec 2012
Theoretical Physics	Magnetic correlations in a layered iridate, Na2IrO3 S. W. Lovesey , A. N. Dobrynin Journal Of Physics. Condensed Matter : An Institute Of Physics Journal 24 (38) DOI: 10.1088/0953-8984/24/38/382201 Show Abstract ▼ Abstract: Analysis of published data gathered on a sample of Na2IrO3, held deep inside the antiferromagnetic phase at 1.58 K, shows that iridium magnetic dipole moments, measured in resonant x-ray Bragg diffraction, lie in the a–c plane of the monoclinic crystal and enclose an angle ≈118° with the c-axis. These findings, together with bulk measurements, are united in a plausible magnetic ground state for an iridium ion constructed from a Kramers doublet. A magnetic space group, derived from the chemical space group C2/m (unique axis b), possesses an anti-translation, to accommodate antiferromagnetic order, and an odd, two-fold axis of rotation symmetry on the b-axis, placing Ir magnetic dipoles perpendicular to the b-axis. Anapoles (toroidal dipoles) are predicted to be likewise confined to the a–c plane, and magnetic charges forbidden.	Sep 2012
Theoretical Physics	Nonresonant inelastic x-ray scattering from actinides and rare earths G. Van Der Laan Physical Review B 86 (3) DOI: 10.1103/PhysRevB.86.035138 Show Abstract ▼ Abstract: Calculated multiplet spectra are presented for the electric-multipole transitions in nonresonant inelastic x-ray scattering (NIXS) at the O 4,5 edges of actinides and the N 4,5 edges of rare earths. The multiplet structure is characteristic for the f count as well as for the angular momentum coupling. As a remarkable peculiarity, the calculations for the actinide O 4,5 edge show that the higher multipole spectra resemble jj coupling, whereas the dipole spectrum is close to LS coupling. The exchange integral G 1 (5d,5f) is responsible for a shift of ? 15 eV of the dipole spectrum to higher energy. This high energy is conserved by a sum rule for the average energy of the accessible final states. In the higher multipole spectra, where the allowed final states are different, the splitting by the 5d spin-orbit interaction exceeds that of the electrostatic interaction. This leads to a distinct spin-orbit-split doublet structure, with a broad first peak and a narrower second peak. According to the spin-orbit sum rule, the 5f spin-orbit interaction per hole is linearly related to the core-level branching ratio, where the proportionality factor is opposite in sign for the k=3 and 5 spectra. Compared to the k=3 spectrum, the k=5 spectrum has overall a lower intensity in the low-energy region but a higher intensity in the high-energy region, especially for the heavier elements in the series. The NIXS at the rare-earth N 4,5 edge is quite different since the electrostatic and core spin-orbit splitting in the k=3 and 5 spectra are of similar size. The pre-edge region of the rare-earth spectra, where the linewidth is narrow, shows a rich multiplet structure that depends strongly on the initial-state J value.	Jul 2012
I16-Materials and Magnetism	Ab initio calculations of the forbidden Bragg reflections energy spectra in wurtzites versus temperature A. P. Oreshko , E. N. Ovchinnikova , G. Beutier , S. P. Collins , G. Nisbet , A. M. Kolchinskaya , V. E. Dmitrienko Journal Of Physics: Condensed Matter 24 (24) DOI: 10.1088/0953-8984/24/24/245403 Show Abstract ▼ Abstract: Thermal-motion induced (TMI) scattering is caused by the influence of atomic displacements on electronic states in crystals and strongly depends on temperature. It corresponds to dipole–dipole resonant x-ray scattering, but is usually accompanied by dipole–quadrupole scattering. The phenomenological theory supposes the dipole–quadrupole term to be temperature independent (TI). As a result, the transformation of the energy spectra with temperature observed experimentally in ZnO and GaN corresponds to the interference between the TMI and TI terms. In the present paper the direct confirmation of this theoretical prediction is given. Ab initio molecular dynamics was used to simulate the sets of atomic sites at various temperatures followed by quantum mechanical calculation of resonant Bragg reflection energy spectra. The results of simulation are in excellent coincidence with experimental energy spectra of forbidden reflections and confirm the earlier phenomenological conjecture about the interference between the TI dipole–quadrupole and TMI dipole–dipole contributions to the resonant atomic factor.	Jun 2012
Theoretical Physics	Spin-orbit sum rule for electric multipole transitions in nonresonant inelastic X-Ray scattering Gerrit Van Der Laan Physical Review Letters 108 (6) DOI: 10.1103/PhysRevLett.108.077401 PMID: 22401253 Show Abstract ▼ Abstract: The sum rule for the branching ratio of dipole-excited core-valence transitions in isotropic x-ray absorption spectroscopy is extended to electric multipole transitions. The derived sum rule not only shows that the branching ratio is linearly related to the expectation value of the angular part of the spin-orbit operator in the valence states, but also shows a strong dependence on the rank of the multipole. The spin orbit dependence vanishes in the weighted sum over the branching ratios. The effect can be an important diagnostic tool for high-energy spectroscopies.	Feb 2012

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