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74 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
Theoretical Physics	Reply to Comment on 'Quantum correlations between protons in potassium bicarbonate' D. A. Keen , S. W. Lovesey Journal Of Physics: Condensed Matter 16 (30), 5637-5638 DOI: 10.1088/0953-8984/16/30/N03 Show Abstract ▼ Abstract: Allegations in a Comment (Fillaux and Cousson 2004 J. Phys.: Condens. Matter 16 1007) that our calculation of the intensity of neutrons diffracted by potassium bicarbonate is incorrect are shown to be ill-founded. The downfall of the Comment is not to apply quantum mechanics to the calculation of scattering by indistinguishable nuclei with overlapping degrees of freedom (protons or deutrons).	Jul 2004
Theoretical Physics	Comment on 'Quantum entanglement and neutron scattering experiments' E. B. Karlsson , S. W. Lovesey Journal Of Physics: Condensed Matter 16 (30), 5631-5633 DOI: 10.1088/0953-8984/16/30/N01 Show Abstract ▼ Abstract: Allegations that our theoretical description of neutron scattering by entangled spin and spatial degrees of freedom is incorrect are shown to be misplaced for the allegations are derived from a naively incomplete, quantum mechanical calculation of neutron scattering by identical nuclei.	Jul 2004
Theoretical Physics	The giant magneto-volume effect in solid oxygen K. Katsumata , S. Kimura , U. Staub , Y. Narumi , Yoshi Tanaka , S. Shimomura , T. Nakamura , S. W. Lovesey , T. Ishikawa , H. Kitamura Journal Of Physics: Condensed Matter 17 (23), L235-L239 DOI: 10.1088/0953-8984/17/23/L03 Show Abstract ▼ Abstract: X-ray diffraction measurements on a-solid oxygen under an applied magnetic field are reported. The volume of alpha-solid O-2 is found to expand with increasing magnetic field: the lattice constants a and b elongate, while the lattice constant c and the angle are almost independent of the magnetic field. The maximum volume expansion observed is about 1% under a 7.5 T field, which is a giant effect compared to the volume change observed in ferromagnetic metals, such as Fe, Co and Ni. This finding indicates that a spin-dependent intermolecular potential is essential in determining the structure of alpha-solid O-2. The experimental results conform with predictions based on a magneto-elastic interaction derived from an antiferromagnetic exchange coupling of O-2 spins.	May 2005
Theoretical Physics	Dichroism and resonant diffraction in x-ray scattering by complex materials S. P. Collins , S. W. Lovesey , E. Balcar Journal Of Physics: Condensed Matter 19 DOI: 10.1088/0953-8984/19/21/213201 Show Abstract ▼ Abstract: We survey universal concepts that influence dichroism and resonant Bragg diffraction, aiming to reach across the range of scientific disciplines that benefit from x-ray techniques, namely, chemistry, physics, life-sciences, and the science of materials. To this end, we adopt a top down discussion of the aspects of symmetry and concomitant selection rules. Starting from selection rules that can be deduced from the global condition that an observable quantity is unchanged on reversing the directions of both space and time separately, to selection rules that flow from bulk symmetry properties of electrons imposed by elements of a point group or crystal class to, finally, atomic selection rules that emerge from the details of the electronic structure. As a motivation for the latter we discuss, with a new calculation of the x-ray scattering length, E 1–M 1 absorption and scattering events that particularly interest scientists studying the chirality of life. In the main text there is modest use of mathematics, with appropriate details relegated to a few appendices.	May 2007
Theoretical Physics	Polar and magnetoelectric multipoles in gallium ferrate inferred from optical and x-ray measurements Kevin S. Knight , Ewald Balcar , Stephen W. Lovesey Journal Of Physics: Condensed Matter 19 (37), 2-13 DOI: 10.1088/0953-8984/19/37/376205 Show Abstract ▼ Abstract: Observations utilizing optical and x-ray techniques of coexisting magnetoelectric, piezoelectric and magnetic modifications in gallium ferrate (GaFeO3) are successfully analysed with an atomic model that complies with the established chemical structure and the motifs of spontaneous polar and ferrimagnetic order. Electron variables in the corresponding scattering length, which is common to an analysis of dichroism and diffraction, are expressed as multipoles for charge, magnetic, magnetoelectric and polar electron degrees of freedom. Polar and magnetoelectric multipoles are parity-odd and cannot exist in a compound with a centrosymmetric structure. Magnetoelectric multipoles are time-odd and vanish with loss of long-range magnetic order. Dichroic signals measured in the energy region 1.0-2.5 eV and near the Fe K edge are consistent with a sum of magnetochiral and non-reciprocal linear signals caused by magnetoelectric multipoles in E1-M1 and E1-E2 absorption events. Resonant x-ray Bragg diffraction near the Fe K edge shows interference not previously reported between Thomson scattering and resonant E1, M1 and E2 processes.	Aug 2007
	Probing medium-range structural correlations by fluctuation microscopy M. M. J. Treacy , D. Kumar , A. Rougée , G. Zhao , P. R. Buseck , I. Mcnulty , L. Fan , C. Rau , J. M. Gibson Journal Of Physics: Condensed Matter 19 (45) DOI: 10.1088/0953-8984/19/45/455201 Show Abstract ▼ Abstract: We outline recent advances in the fluctuation microscopy technique for probing medium-range structural correlations in disordered materials. The technique was originally developed for electron microscopy, but has now been extended to optical and x-ray microscopies. We show that fluctuation microscopy can detect trace quantities of C60 in a disordered graphite matrix, even though the diffraction signature from the C60 is essentially undetectable. This result indicates that the technique can be used to discern dilute distributions of macromolecules in an otherwise disordered matrix. We also report preliminary studies of interferometric fluctuation microscopy using cross-correlations in diffraction between coherent double probes. This is a form of holography where the diffraction patterns from two neighboring regions are allowed to overlap and interfere. Young's fringes appear wherever both regions scatter strongly. The cross-correlation can be examined as a function of probe separation to estimate a structure correlation length. This method holds much promise for studying medium-range order, since it isolates the essential four-body terms underpinning the fluctuation microscopy technique.	Nov 2007
	Iron deposition on the tenfold surface of the Al72Ni11Co17 decagonal quasicrystal L. H. Wearing , J. A. Smerdon , L. Leung , Ju. Ledieu , I. Fisher , C. J. Jenks , R. Mcgrath , S. S. Dhesi , P. Bencok Journal Of Physics: Condensed Matter 20 (1) DOI: 10.1088/0953-8984/20/01/015005 Show Abstract ▼ Abstract: The adsorption behaviour of Fe on the tenfold surface of the decagonal quasicrystal Al72.6Ni10.5Co16.9 has been studied using scanning tunnelling microscopy, low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and x-ray magnetic circular dichroism (XMCD). The results show the growth of a disordered film up to a coverage of 9 MLE (monolayer equivalent) after which polycrystalline island growth is observed. These islands are interconnected, and the LEED pattern indicates that they preferentially align along five directions. The AES results indicate that the film is composed of Fe intermixed with the substrate elements. The XMCD results point to an induced magnetic moment for Co and Ni in the intermixed layer/interface region.	Jan 2008
Theoretical Physics	Resonant diffraction of circularly polarized x-rays by a chiral crystal (low quartz) Stephen W. Lovesey , Ewald Balcar , Yoshikazu Tanaka Journal Of Physics: Condensed Matter 20 DOI: 10.1088/0953-8984/20/27/272201 PMID: 21694361 Show Abstract ▼ Abstract: A correlation in x-ray resonant scattering between crystal chirality and circular polarization (helicity) is explored in the context of an analysis of Bragg diffraction from low quartz (alpha-SiO2). There is a one-to-one correlation between chirality and helicity when one resonant event is present in diffraction and thus, in this simple case, resonant Bragg diffraction of circularly polarized x-rays is a direct probe of crystal chirality. The presence of more than one resonant event is shown to add phase relations to the scattering amplitude and then coupling of helicity and chirality is no longer transparent.	Jan 2008
I06-Nanoscience	Loss of long-range magnetic order in a nanoparticle assembly due to random anisotropy Chris Binns , Paul Howes , Steve Baker , Paul Steadman , Mervyn Roy , Mark Everard , Andrew Rushforth , Sarnjeet Dhesi , Helder Marchetto , Alessandro Potenza Journal Of Physics: Condensed Matter 20 (5) DOI: 10.1088/0953-8984/20/5/055213 Diamond Proposal Number(s): [051] Show Abstract ▼ Abstract: We have used soft x-ray photoemission electron microscopy (XPEEM) combined with x-ray magnetic circular dichroism (XMCD) and DC SQUID (superconducting quantum interference device) magnetometry to probe the magnetic ground state in Fe thin films produced by depositing size-selected gas-phase Fe nanoparticles with a diameter of 1.7 nm (~200 atoms) onto Si substrates. The depositions were carried out in ultrahigh vacuum conditions and thicknesses of the deposited film in the range 550 nm were studied. The magnetometry data are consistent with the film forming a correlated super-spin glass with a magnetic correlation length ~5 nm. The XPEEM magnetic maps from the cluster-assembled films were compared to those for a conventional thin Fe film with a thickness of 20 nm produced by a molecular beam epitaxy (MBE) source. Whereas a normal magnetic domain structure is observed in the conventional MBE thin film, no domain structure could be observed in any of the nanoparticle films down to the resolution limit of the XMCD based XPEEM (100 nm) confirming the ground state indicated by the magnetometry measurements. This observation is consistent with the theoretical prediction that an arbitrarily weak random anisotropy field will destroy long-range magnetic order.	Feb 2008
	Polar multipoles in wurtzite-like crystals (ZnO, GaN) Stephen Lovesey , Ewald Balcar Journal Of Physics: Condensed Matter 20 (12) DOI: 10.1088/0953-8984/20/12/122201 Show Abstract ▼ Abstract: A theoretical analysis and simulation of resonance-enhanced x-ray Bragg diffraction by wurtzite-like crystals is presented for parity-even and polar (parity-odd) atomic resonant processes. Compounds with this crystal structure are of great interest for a range of technologies. The electronic properties of a crystal are represented as ground-state expectation values of atomic multipoles, and the analysis respects all selection rules imposed on diffraction by atomic and crystal symmetry. Our main focus is directed at weak, space-group forbidden reflections that are a result of angular anisotropy in the cation electron distribution. Expected variations of the diffracted intensity with orientation of the crystal due to angular anisotropy of the electron distribution are simulated. The crystal symmetry allows the intensity to be coupled to circular polarization in the primary beam and illustrative examples for the creation of circular polarization in diffraction are provided. These polarization effects are unique probes of electronic structure. Moreover, we prove that recent diffraction experiments on ZnO reveal the pyroelectric (dipole) and polar octupole moment of the cation.	Feb 2008

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