Stephen P. Collins

Size-Dependent Shape and Tilt Transitions in In2O3 Nanoislands Grown on Cubic Y-Stabilized ZrO2(001) by Molecular Beam Epitaxy

The growth of In(2)O(3) on cubic Y-stabilized ZrO(2)(001) by molecular beam epitaxy leads to formation of nanoscale islands which may tilt relative to the substrate in order to help accommodate the 1.7% tensile mismatch between the epilayer and the substrate. High-resolution synchrotron-based X-ray diffraction has been used in combination with atomic force microscopy to probe the evolution in island morphology, orientation, and tilt with island size. Very small islands formed at low substrate coverage are highly strained but exhibit no tilt, while intermediate islands are tilted randomly in all directions, giving rise to distinctive doughnut-shaped structure in three-dimensional reciprocal space isosurfaces. The largest islands with lateral sizes on the order of 1 μm tilt away from the four equivalent in-plane <110> directions, giving three-dimensional scattering isosurfaces dominated by structure at the four corners of a square. Spatially resolved reciprocal space mapping using an X-ray beam with dimensions on the order of 1 μm suggests that the four-fold symmetry observed using a larger beam arises from averaging over an ensemble of islands, each with an individual tilt down one direction, rather than from the coexistence of differently tilted domains within a given island.

Coherent X-ray diffraction investigation of twinned microcrystals

Coherent X-ray diffraction has been used to study pseudo-merohedrally twinned manganite microcrystals. The analyzed compositions were Pr(5/8)Ca(3/8)MnO(3) and La(0.275)Pr(0.35)Ca(3/8)MnO(3). The prepared loose powder was thermally attached to glass (and quartz) capillary walls by gentle heating to ensure positional stability during data collection. Many diffraction data sets were recorded and some of them were split as expected from the main observed twin law: 180° rotation around [101]. The peak splitting was measured with very high precision owing to the high-resolution nature of the diffraction data, with a resolution (Δd/d) better than 2.0 × 10(-4). Furthermore, when these microcrystals are illuminated coherently, the different crystallographic phases of the structure factors induce interference in the form of a speckle pattern. The three-dimensional speckled Bragg peak intensity distribution has been measured providing information about the twin domains within the microcrystals. Research is ongoing to invert the measured patterns. Successful phase retrieval will allow mapping out the twin domains and twin boundaries which play a key role in the physical properties.

Counting statistics of X-ray detectors at high counting rates

Modern synchrotron radiation sources with insertion devices and focusing optics produce high fluxes of X-rays at the sample, which leads to a requirement for photon-counting detectors to operate at high counting rates. With high counting rates there can be significant non-linearity in the response of the detector to incident X-ray flux, where this non-linearity is caused by the overlap of the electronic pulses that are produced by each X-ray. A model that describes the overlap of detector pulses is developed in this paper. This model predicts that the correction to the counting rate for pulse overlap is the same as a conventional dead-time correction. The model is also used to calculate the statistical uncertainty of a measurement and predicts that the error associated with a measurement can be increased significantly over that predicted by Poisson (N(-1/2)) statistics. The error differs from that predicted by a conventional dead-time treatment.

An Investigation of the Cross-Section for Magnetic Compton Scattering

The existence of a term in the cross section for inelastic photon scattering of circularly polarised photons which is dependent on orbital magnetisation has been proposed and supported by evidence from one Compton scattering experiment carried out at 45 keV. The integrated intensity of the Compton scattering is studied as a function of the angle between the incident beam and the direction of magnetisation. Further measurements of a similar nature on the same samples, Fe, Co, HoFe 2 , which have produced negative results, are reported here. These latest results, which were obtained at two different synchrotron rings (the Daresbury Storage Ring and the KEK Accumulation Ring), throw doubt upon the importance of such effects in the Compton limit.

Identification of inversion domains in KTiOPO4 via resonant X-ray diffraction

The identification and high-resolution mapping of the absolute crystallographic structure in multi-domain ferroelectric KTiOPO4 is achieved through a novel synchrotron X-ray diffraction method. On a single Bragg reflection, the intensity ratio in resonant diffraction below and above the Ti absorption K edge demonstrates a domain contrast up to a factor of ∼270, thus implementing a non-contact, non-destructive imaging technique with micrometre spatial resolution, applicable to samples of arbitrarily large dimensions.

Synthesis, properties, and catalytic behavior of zeolite EU-12

Abstract The synthesis and characterization of EU-12, a novel thermally stable high-silica zeolite, are described. Sorption measurements indicate that access to the zeolitic void space is through windows with a free diameter of ∼ 0.40 nm. Ion exchange gives a hydrogen form that shows preferential formation of ethene in methanol conversion and the selective formation of cis -but-2-ene in but-1-ene isomerization.

Modelling Detector Deadtime with the Pulse Overlap Model

Photon counting detectors have to work at high count rates at synchrotron radiation sources. It is well known that at high count rates, counts may be lost because the time difference between the arrival of two X‐rays in the detector can be too small to resolve each count. This is coventionally modelled in the paralyzable deadtime model. In a previous paper, we introduced a new model — the pulse overlap model. This model predicts that there is a limit to the statistical accuracy that can be achieved by a measurement made in fixed counting time by a given photon counting detector. At counting rates above the point where maximum statistical accuracy is achieved, the statistical accuracy decreases. We present data obtained from computer Monte Carlo simulation and measurements with a real detector and compare with the predictions of the pulse overlap model.

Chiral Properties of BiFeO3Inferred from Resonant X-ray Bragg Diffraction

A new chiral phase of ferric ions in bismuth ferrite (BiFeO3), the only material known to support multiferroic behaviour at room temperature, is inferred from extensive sets of data gathered by resonant x-ray diffraction. Values of all ferric multipoles participating in a minimal model of Fe electronic structure are deduced from azimuthal-angle scans. Paramagnetic (700 K) and magnetically ordered (300 K) phases of a single crystal of BiFeO3 have been studied with x-rays tuned near to the iron K-edge (7.1135 keV). At both temperatures, intensities at a Bragg spot forbidden in the nominal space-group, R3c, are consistent with a chiral motif of ferric ions in a circular cycloid propagating along (1, 1, 0)H. Templeton and Templeton scattering at 700 K is attributed in part to charge-like quadrupoles in a cycloid. The contribution is not present in a standard, simplified model of electronic states of the resonant ion with trivial cylindrical symmetry.A new chiral phase of ferric ions in bismuth ferrite (BiFeO3), the only material known to support multiferroic behaviour at room temperature, is inferred from extensive sets of data gathered by resonant x-ray diffraction. Values of all ferric multipoles participating in a minimal model of Fe electronic structure are deduced from azimuthal-angle scans. Paramagnetic (700 K) and magnetically ordered (300 K) phases of a single crystal of BiFeO3 have been studied with x-rays tuned near to the iron K-edge (7.1135 keV). At both temperatures, intensities at a Bragg spot forbidden in the nominal space-group, R3c, are consistent with a chiral motif of ferric ions in a circular cycloid propagating along (1, 1, 0)H. Templeton and Templeton scattering at 700 K is attributed in part to charge-like quadrupoles in a cycloid. The contribution is not present in a standard, simplified model of electronic states of the resonant ion with trivial cylindrical symmetry.

A unified formulation of dichroic signals using the Borrmann effect and twisted photon beams

Dichroicxa0X-ray signals derived from the Borrmann effect and a twisted photon beam with topological charge lu2009=u20091 are formulated with an effective wavevector. The unification applies for non-magnetic and magnetic materials. Electronic degrees of freedom associated with an ion are encapsulated in multipoles previously used to interpret conventional dichroism and Bragg diffraction enhanced by an atomic resonance. A dichroic signal exploiting the Borrmann effect with a linearly polarized beam presents charge-like multipoles that include a hexadecapole. A difference between dichroic signals obtained with a twisted beam carrying spin polarization (circular polarization) and opposite winding numbers presents charge-like atomic multipoles, whereas a twisted beam carrying linear polarization alone presents magnetic (time-odd) multipoles. Charge-like multipoles include a quadrupole, and magnetic multipoles include a dipole and an octupole. We discuss the practicalities and relative merits of spectroscopy exploiting the two remarkably closely-related processes. Signals using beams with topological charges l ≥ 2 present additional atomic multipoles.

Conventional and unconventional studies of magnetic structures with X-rays

This talk will provide a brief introduction to modern synchrotron facilities and techniques for studies of magnetic structures, highlighting some examples, limitations and future directions. We will focus on the use of hard x-ray probes of single crystal samples. The talk will then move into less well-charted territory, highlighting some recent investigations where higher-order multipole resonances and induced charge scattering play important roles, along with new approaches to observing magnetoelectric sample properties.