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67 items found (0 items not approved), displaying 31 to 40.[First/Prev] 1, 2, 3, 4, 5, 6, 7 [Next/Last]

Instruments / Technical Area	Publication Details	Published
Theoretical Physics	Ferro-type order of magneto-electric quadrupoles as an order-parameter for the pseudo-gap phase of a cuprate superconductor Stephen Lovesey , Dmitry Khalyavin , Urs Staub Journal Of Physics: Condensed Matter 27 DOI: 10.1088/0953-8984/27/29/292201 PMID: 26153665 Show Abstract ▼ Abstract: There is general agreement within the community of researchers that investigate high-Tc materials that it is most important to understand the pseudo-gap phase. To this end, many experiments on various cuprates have been reported. Two prominent investigations—Kerr effect and neutron Bragg diffraction—imply that underdoped YBCO samples possess longrange magnetic order of an unusual kind. However, other measurements do not support the existence of magnetic order. Here we show that the Kerr effect and magnetic Bragg diffraction data are individual manifestations of ordered magneto-electric quadrupoles at Cu sites. While the use of magneto-electric multipoles is new in studies of the electronic properties of cuprates, they are not unknown in other materials, including an investigation with x-rays of the parent compound CuO. We exploit the recent prediction that neutrons are deflected by magneto-electric multipoles. The outcome of our study is a theory for the order-parameter of the pseudo-gap phase without the aforementioned conflict with other measurements, and the first experimental evidence that neutrons interact with multipoles belonging to a state of magnetic charge.	Jul 2015
Theoretical Physics	Zen and the art of dichroic photoemission Gerrit Van Der Laan Journal Of Electron Spectroscopy And Related Phenomena DOI: 10.1016/j.elspec.2015.06.001 Open Access Show Abstract ▼ Abstract: The discovery of magnetic dichroism in photoemission is celebrating its 25th anniversary this year. Here a review of the underlying general theory for the angular and spin dependence of dichroic core-level photoemission is presented using both a single-particle model and a many-body approach. The established methods of angular momentum coupling offer an elegant and powerful way to analyse the magnetic dichroism and spin polarization in photoemission from core and localized valence levels. In the presence of core-valence interactions one can distinguish different fundamental spectra, which via sum rules are related to physical properties described by coupled tensor operators for spin and orbital moments. By separating the angular dependence from the physical information, different geometries can be distinguished to measure the magnetic circular dichroism (MCD), linear dichroism (LD), circular dichroism in the angular dependence (CDAD), and magnetic linear dichroism in the angular dependence (MLDAD). Various ways to probe the core-hole polarization are discussed, such as using the angular dependence, moment analysis of the spectral distribution, and resonant photoemission decay.	Jun 2015
B16-Test Beamline Metrology Optics Theoretical Physics	Theoretical analysis of the background intensity distribution in X-ray Birefringence Imaging using synchrotron bending-magnet radiation John Sutter , Igor Dolbnya , Steve Collins , Kenneth D. M. Harris , Gregory Edwards-Gau , Benjamin Palmer Journal Of Applied Physics 117 (16), 164902 DOI: 10.1063/1.4918925 Diamond Proposal Number(s): [8323] Open Access Show Abstract ▼ Abstract: In the recently developed technique of X-ray Birefringence Imaging, molecular orientational order in anisotropic materials is studied by exploiting the birefringence of linearly polarized X-rays with energy close to an absorption edge of an element in the material. In the experimental setup, a vertically deflecting high-resolution double-crystal monochromator is used upstream from the sample to select the appropriate photon energy, and a horizontally deflecting X-ray polarization analyzer, consisting of a perfect single crystal with a Bragg reflection at Bragg angle of approximately 45°, is placed downstream from the sample to measure the resulting rotation of the X-ray polarization. However, if the experiment is performed on a synchrotron bending-magnet beamline, then the elliptical polarization of the X-rays out of the electron orbit plane affects the shape of the output beam. Also, because the monochromator introduces a correlation between vertical position and photon energy to the X-ray beam, the polarization analyzer does not select the entire beam, but instead selects a diagonal stripe, the slope of which depends on the Bragg angles of the monochromator and the polarization analyzer. In the present work, the final background intensity distribution is calculated analytically because the phase space sampling methods normally used in ray traces are too inefficient for this setup. X-ray Birefringence Imaging data measured at the Diamond Light Source beamline B16 agree well with the theory developed here.	Apr 2015
Theoretical Physics	A model of magneto-electric multipoles S. W. Lovesey , E. Balcar Journal Of Physics: Condensed Matter 27 DOI: 10.1088/0953-8984/27/10/106001 PMID: 25705914 Show Abstract ▼ Abstract: A long-known Hamiltonian of electrons with entangled spin and orbital degrees of freedom is re-examined as a model of magneto-electric multipoles (MEs). In the model, a magnetic charge and simple quantum rotator are tightly locked in action, some might say they are enslaved entities. It is shown that MEs almost perfectly accord with those inferred from an analysis of magnetic neutron diffraction data on a ceramic superconductor (YBCO) in the pseudo-gap phase. Nigh on perfection between Stone's model and inferred MEs is achieved by addition to the original model of a crystal-field potential appropriate for the magnetic space group used in the published data analysis. An impression of thermal properties of multipoles is sought from a molecular-field model.	Feb 2015
Theoretical Physics	Magneto-electric operators in neutron scattering from electrons S. W. Lovesey Journal Of Physics: Condensed Matter 26 DOI: 10.1088/0953-8984/26/35/356001 PMID: 25105987 Show Abstract ▼ Abstract: We succeed in deriving an exact expression for the magnetic interaction of neutrons and electrons including magneto-electric operators, allowed in the absence of a centre of inversion symmetry. Central characters are a spin anapole and an orbital (toroidal) analogue, in addition to familiar parity-even operators like the magnetic moment. A simulation of neutron diffraction by antiferromagnetic copper oxide makes full use of information inferred from a thorough investigation with resonant x-ray Bragg diffraction.	Sep 2014
Theoretical Physics	Strange magnetic multipoles and neutron diffraction by an iridate perovskite (Sr2IrO4) Stephen Lovesey , Dmitry Khalyavin Journal Of Physics: Condensed Matter 26 (32) DOI: 10.1088/0953-8984/26/32/322201 PMID: 25055164 Show Abstract ▼ Abstract: A theoretical investigation of a plausible construct for electronic structure in iridate perovskites demonstrates the existence of magnetic multipoles hitherto not identified. The strange multipoles, which are parity-even, time-odd and even rank tensors, are absent from the so-called j(eff) = 1/2 model. We prove that the strange multipoles contribute to magnetic neutron diffraction, and we estimate their contribution to intensities of Bragg spots for Sr2IrO4. The construct encompasses the j(eff) = 1/2 model, and it is consistent with the known magnetic structure, ordered magnetic moment, and published resonant x-ray Bragg diffraction data. Over and above time-odd quadrupoles and hexadecapoles, whose contribution changes neutron Bragg intensities by an order of magnitude, according to our estimates, are relatively small triakontadipoles recently proposed as the primary magnetic order-parameter of Sr2IrO4.	Aug 2014
Theoretical Physics	Chirality, birefringence, and polarization effects in alpha-quartz studied by resonant elastic x-ray scattering Yves Joly , Yoshikazu Tanaka , Delphine Cabaret , Stephen P. Collins Physical Review B 89 (22) DOI: 10.1103/PhysRevB.89.224108 Show Abstract ▼ Abstract: We show that enantiomers are extremely sensitive to the quality of the light polarization in resonant elastic x-ray scattering experiments. In alpha- -quartz, the (001) reflection shows a very strong sensitivity to a small fraction of linear component included in nominally circularly polarized light. This fraction induces an important phase difference between the angular dependence of the recorded intensity from each enantiomer. We are thus able to model all aspects of the resonant scattering and determine the dominant physics, extract the crystal chirality, and obtain very precise values for the Stokes polarization parameters. Effects of birefringence and of higher-order electric transitions are also quantified and it is shown that in comparison they are negligible.	Jun 2014
Theoretical Physics	Modelling ferromagnetic resonance in magnetic multilayers: Exchange coupling and demagnetisation-driven effects Alexander Baker , Carl Davies , Adriana Figueroa-Garcia , Leigh Shelford , Thorsten Hesjedal , Gerrit Van Der Laan Journal Of Applied Physics 115 (17), 17D140 DOI: 10.1063/1.4868185 Show Abstract ▼ Abstract: Ferromagnetic resonance in thin films and heterostructures is studied by micromagnetic simulation. Using a Fourier transform, we obtain the power spectrum of modes of precession and demonstrate that they agree with experimental results. Additional resonance modes associated with demagnetization effects are observed and studied as a function of the sample dimensions. Sufficiently confined geometries lead to a reorientation of the resonance mode, and a dramatic change in its frequency. Finally, the simulations are extended to exchange-coupled bilayers where energy is transferred between magnetic materials.	May 2014
Theoretical Physics	Effects of dispersion and absorption in resonant Bragg diffraction of x-rays S. W. Lovesey , V. Scagnoli , A. N. Dobrynin , Y. Joly , S. P. Collins Journal Of Physics: Condensed Matter 26 (12), 332-337 DOI: 10.1088/0953-8984/26/12/125504 PMID: 24599265 Show Abstract ▼ Abstract: Resonant diffraction of x-rays by crystals with anisotropic optical properties is investigated theoretically, to assess how the intensity of a Bragg spot is influenced by effects related to dispersion (birefringence) and absorption (dichroism). Starting from an exact but opaque expression, simple analytic results are found to expose how intensity depends on dispersion and absorption in the primary and secondary beams and, also, the azimuthal angle (rotation of the crystal about the Bragg wavevector). If not the full story for a given application, our results are more than adequate to explore consequences of dispersion and absorption in the intensity of a Bragg spot. Results are evaluated for antiferromagnetic copper oxide, and low quartz. For CuO, one of our results reproduces all salient features of a previously published simulation of the azimuthal-angle dependence of a magnetic Bragg peak. It is transparent in our analytic result that dispersion and absorption effects alone cannot reproduce published experimental data. Available data for the azimuthal-angle dependence of space-group forbidden reflections. (0, 0, l), with l not equal 3n, of low quartz depart from symmetry imposed by the triad axis of rotation symmetry. The observed asymmetry can be induced by dispersion and absorption even though absorption coefficients are constant, independent of the azimuthal angle, in this class of reflections.	Mar 2014
	Resonant X-ray scattering and the jeff=1/2 electronic ground state in iridate perovskites M. Moretti Sala , S. Boseggia , D. f. Mcmorrow , G. Monaco Physical Review Letters 112 DOI: 10.1103/PhysRevLett.112.026403 PMID: 24484032 Show Abstract ▼ Abstract: The resonant x-ray scattering (magnetic elastic, RXMS, and inelastic, RIXS) of Ir 4+ at the L 2,3 edges relevant to spin-orbit Mott insulators A n+1 Ir n O 3n+1 (A=Sr , Ba, etc.) are calculated using a single-ion model which treats the spin-orbit and tetragonal crystal-field terms on an equal footing. Both RXMS and RIXS in the spin-flip channel are found to display a nontrivial dependence on the direction of the magnetic moment, μ . Crucially, we show that for μ in the ab plane, RXMS in the cross-polarized channel at the L 2 edge is zero irrespective of the tetragonal crystal field; spin-flip RIXS, relevant to measurements of magnons, behaves reciprocally, being zero at L 2 when μ is perpendicular to the ab plane. Our results have important implications for the assignment of a j eff =1/2 ground state on the basis of resonant x-ray experiments.	Jan 2014

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