Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L15: Focus Session: X-ray and Neutron Instruments and Measurement Science II |
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Sponsoring Units: GIMS Chair: Timothy Graber, University of Chicago Room: B114 |
Tuesday, March 16, 2010 2:30PM - 2:42PM |
L15.00001: Decoherence Free Neutron Interferometer Dmitry Pushin, Michael Huber, Muhammad Arif, David Cory A neutron interferometer (NI) is a unique example of the macroscopic quantum coherence and has been used to test fundamental principles of quantum mechanics. In practice, neutron interferometers are not widely used because of their extreme sensitivity to environmental noise which is in part due to the slow velocity (relative to light) of the neutron. We show that a neutron interferometer design can benefit from concepts of quantum information processing. We have machined a Decoherence Free (DF) neutron interferometer that was designed based on a quantum error correction code and is much less sensitive to mechanical vibrations than is the standard Mach-Zehnder (MZ) interferometer. We demonstrate improvements of the DF design with a five blade single crystal NI that incorporates both the MZ and DF geometries in one crystal. We have found that the DF interferometer is much less sensitive to vibrations. We believe that our results and related quantum information approaches will enable a new series of compact neutron interferometers that can be tailored to specific applications in soft condensed matter and spintronics. [Preview Abstract] |
Tuesday, March 16, 2010 2:42PM - 2:54PM |
L15.00002: Solid Oxygen: Ultra-Cold Neutron Production from Magnetic Excitations as Illuminated by Inelastic Neutron Scattering Christopher M. Lavelle, Chen-Yu Liu, Patrick M. McChesney, Dan J. Salvat, Greg Manus, Mark Makela, Andy Saunders, Aaron Couture, Chris Morris, Albert Young, Craig M. Brown Ultra Cold Neutrons (UCN, E$\sim $100 neV) are supremely useful in the study of the weak interaction physics and provide a sensitive platform for theories beyond the standard model via measurement of the neutron electric dipole moment. UCN are produced from a cold neutron beam via excitation of collective modes in materials such as solid deuterium and liquid helium. Unfortunately, UCN sources are limited by low intensity. This provides an interesting point of collaboration between condensed matter and nuclear physics as we attempt to develop stronger sources. We have investigated the magnetic phases of solid oxygen as a potential UCN source by measuring UCN production from solid oxygen directly. Complete understanding of the results was elusive until we conducted a recent series of inelastic neutron studies scattering using the Disc Chopper Spectrometer time-of-flight instrument at NIST. Of particular interest is the high efficiency of magnetic inelastic scattering for UCN production, and the powerful influence of the spin ordering transition. [Preview Abstract] |
Tuesday, March 16, 2010 2:54PM - 3:06PM |
L15.00003: Exciting Science being done on the CG-2 Small Angle Neutron Scattering beam line at HFIR Lisa DeBeer-Schmitt, Kathy Bailey, Yuri Melnichenko, George Wignall, Ken Littrell The small-angle neutron scattering (SANS) beam line, CG-2, has been in operation since 2007. CG-2 has been optimized so that structures from 0.5 to 200 nm can be thoroughly investigated. HFIR's cold source places the flux at CG-2 among the best in the world. Along with high flux, many varied sample environments can easily be integrated into the beam line which gives the user a versatile temperature range from 1.5 K to 1000K. In addition there are two cryomagents (horizontal 4.5 T and vertical 7 T), pressure cells and load frames available to users allowing for the availability of multiple configurations of experimental setups. Due to all the above equipment and the flux at CG-2, there have been many diverse and intriguing scientific developments. One such outcome is the study of flux- line lattices found in Type-II superconductors including the highly touted iron pnictides. Besides superconductors, other science studied on CG2 ranges from molecular self-assembly and interactions in complex fluids to phase separation, grain growth and orientation in metallurgical alloys. [Preview Abstract] |
Tuesday, March 16, 2010 3:06PM - 3:18PM |
L15.00004: Differential Evolution Optimization of Diffraction Pattern Models with Clones Vickie Lynch, Hans-Beat Buergi, Juerg Hauser, Christina Hoffmann, Tara Michels-Clark, Steve Miller With the TOPAZ single crystal diffractometer at the Spallation Neutron Source in operation, new computational methods are needed for analyzing the three-dimensional diffraction patterns recorded from disordered crystals. One such method uses a combination of differential evolution and Monte Carlo techniques to model the disorder and analyze the diffuse scattering. Software implementing this method originally developed at the University of Bern has been modified to use TeraGrid high performance computers. Since different model crystals produced from a given set of disorder parameters differ, the error of the fit to the data of such clones differs too. The performance of the differential evolution optimization in improving the fit of the model is being tested by using a variable number of clones for each individual gene set in a generation of differential evolution. A reference dataset with minimal noise has been generated for this purpose. Results of tests varying the number of clones will be presented. [Preview Abstract] |
Tuesday, March 16, 2010 3:18PM - 3:30PM |
L15.00005: ABSTRACT WITHDRAWN |
Tuesday, March 16, 2010 3:30PM - 3:42PM |
L15.00006: Defects Self-Organization in a Free Standing Buckled GaN Film From 3D Laue X-ray Microdiffraction Rozaliya Barabash, Gene Ice, Wenjun Liu A novel white-beam microdiffraction analysis of defects, strains and tilts in a free standing $m$-plane GaN film grown via hydride vapor phase epitaxy is presented. Depth-resolved measurements reveal dilatational strain gradient along the film normal and formation of large 3D stress fields. It is shown that misfit dislocations self-organize within cell boundaries creating local lattice rotations between the growing cells. Growth cells are highly misoriented relative to each other. Misorientation angle fluctuates in the range of 1.5 - 4.5 degrees. Distribution of lattice rotations in the film is not homogeneous. Regions of large rotations are separated by low rotations regions. The dominating rotation axis is parallel [11-20] direction. There is little overall lattice rotation around the [0001] axis. In contrast the maximal in plane shear stress component is observed along [0001]. Research supported by the Division of Materials Sciences and Engineering, U.S. Department of Energy. Use of the APS supported by Division of Scientific Users Facilities, U.S. D.O. E. [Preview Abstract] |
Tuesday, March 16, 2010 3:42PM - 3:54PM |
L15.00007: Defect Structure Information Possible from Sub-Picosecond Bragg Diffuse Scattering Measurements of Displacement Cascade Dynamics B.C. Larson, J.Z. Tischler, R.E. Stoller The $\sim $100 femtosecond hard x-ray pulses from the Linac Coherent Light Source (LCLS) provide for the first time the capability of time-resolved measurements of the defect structural dynamics associated with energetic (i.e. tens of keV) atomic displacement cascades in crystalline materials. The local Bragg scattering origin of so-called ``asymptotic'' diffuse scattering near Bragg reflections provides a basis for performing detailed investigations of the structural evolution associated with cascades through the analysis of sub-picosecond time-slice diffraction measurements made at varying times during and after the initiation of cascades by energetic ions. The results of asymptotic diffuse scattering calculations performed using molecular dynamics displacement cascade simulations, for varying times during the generation of cascades and during the cascade annealing process will be presented. The correspondence of the calculated diffuse scattering patterns with the defect structures present in the molecular dynamics simulations will be discussed in terms of the information that could be obtained from 100 femtosecond time-resolved measurements of cascade dynamics using the LCLS. [Preview Abstract] |
Tuesday, March 16, 2010 3:54PM - 4:06PM |
L15.00008: Determination of porosity distribution in nanoporous Au by Fresnel coherent diffractive imaging Sangsoo Kim, Yu-chen Karen Chen, Xianghui Xiao, Mark Pfeifer, Garth J. Wiliams, Corey T. Putkunz, David C. Dunand, Ian Mcnulty Nanoporous metallic foams have potential applications as chemical sensors, actuators, catalytic conversion systems, and voltage-tunable components. Due to the complex internal structure, the kinetics behind nanofoam formation by dealloying is not well understood, especially for nanoporous metals with pore radii less than 50 nm. We studied the morphology and local structure of a sample of nanoporous Au at 25 nm resolution by Fresnel coherent diffractive imaging with 2.2 keV x-rays. This approach enables thickness and density maps to be obtained at length scales approaching that possible with electrons with the penetration afforded by x-rays. We quantified the pore size distribution, an essential parameter to understand the kinematic mechanisms behind nanofoam formation, from the reconstructed images and exact thickness information. Extending this method to three dimensions will provide a complete picture of the pore distribution and local pore curvature, enabling greater understanding of the formation and properties of metallic nanofoams at the nanoscale. [Preview Abstract] |
Tuesday, March 16, 2010 4:06PM - 4:18PM |
L15.00009: Systematic Distortions in Nanoparticles as Modeled by EXAFS Analysis Aaron Yevick, Anatoly Frenkel We modeled EXAFS from cuboctahedral nanoclusters with radii of 11-15 {\AA}. Such clusters may undergo significant surface distortion under the influence of ligands, substrate and/or surface tension. These effects can reduce the average nearest neighbor distance in the cluster by 2-4{\%} and cause enhanced disorder in the interatomic distance as a result of the stronger relaxation of surface bonds relative to the cluster interior. Such effects can degrade the accuracy of EXAFS analysis where the standard approach is to assume a quasi-Gaussian distribution of nearest neighbors. We present a quantitative analysis of the systematic errors that arise in EXAFS measurements as a result of neglecting such surface-induced disorder, in particular the measurement of the coordination number, which could be affected by up to 6{\%}. [Preview Abstract] |
Tuesday, March 16, 2010 4:18PM - 4:30PM |
L15.00010: Core-hole propagator in Wiener-Hopf sum equation approach Nandan Pakhira, James Freericks We apply Wiener-Hopf sum equation approach to calculate the propagator for a core-hole, added as an additional localized level to the Falicov-Kimball model. The Wiener-Hopf sum equation approach was resently applied by Shvaika {\it et. al.} [1] to calculate the finite temperature real time $f$-electron propagator. We present further iterative improvement of the method and its application to the core-hole propagator. Finally, time permitting, we will show some results on the application of the core-hole propagator on Resonant Inelastic X-ray Scattering (RIXS) spectra.\\[4pt] [1] A.M.Shvaika and J.K.Freericks, Condensed Matter Physics, {\bf 11}, 425-442 (2008). [Preview Abstract] |
Tuesday, March 16, 2010 4:30PM - 4:42PM |
L15.00011: Effect of exit beam phase aberrations on coherent x-ray reconstructions of Au nanocrystals Stephan Hruszkewycz, Ross Harder, Paul Fuoss Current studies in coherent x-ray diffractive imaging (CXDI) are focusing on in-situ imaging under a variety of environmental conditions. Such studies often involve environmental sample chambers through which the x-ray beam must pass before and after interacting with the sample: i.e. cryostats or high pressure cells. Such sample chambers usually contain polycrystalline x-ray windows with structural imperfections that can in turn interact with the diffracted beam. A phase object in the near field that interacts with the beam exiting the sample can introduce distortions at the detector plane that may affect coherent reconstructions. We investigate the effects of a thin beryllium membrane on the coherent exit beam of a gold nanoparticle. We compare three dimensional reconstructions from experimental diffraction patterns measured with and without a 380 micron thick Be dome and find that the reconstructions are reproducible within experimental errors. Simulated near-field distortions of the exit beam consistent with micron sized voids in Be establish a ``worst case scenario'' where distorted diffraction patterns inhibit accurate inversions. [Preview Abstract] |
Tuesday, March 16, 2010 4:42PM - 4:54PM |
L15.00012: Precision measurements of Van der Waals atom-surface potentials using nano-gratings Vincent Lonij, Cathy Klauss, Will Holmgren, Alex Cronin Van der Waals and Casimir-Polder potentials are the dominant interactions between charge-neutral objects at nano- to micrometer length scales. As such they have attracted considerable interest in the field of nanotechnology. Understanding of these potentials is important in searches for new forces such as deviations from Newtonian gravity at very short length scales and vacuum friction. We report precision measurements of the Van der Waals atom-surface potential strength (C3). Using atom diffraction from nano-gratings we are able to determine the interaction strength between an atom and a nano-grating with a precision of 6\%. This is a factor of 5 improvement over previous diffraction experiments. We also report ratios of C3 for different atoms with a precision of better than 3\%. At this level of precision we are sensitive to the contribution of core electrons in the atom as well as the geometry of the surface. The ratios of C3 are insensitive to surface impurities and contamination. [Preview Abstract] |
Tuesday, March 16, 2010 4:54PM - 5:06PM |
L15.00013: Acoustic Surface Evanescent Wave and Its Dominant Contribution to Extraordinary Acoustic Transmission and Collimation of Sound Minghui Lu, Yu Zhou, Liang Feng, Xu Ni, Cheng He, Yanfeng Chen Recently, extraordinary acoustic transmissions (EAT) through both one dimensional and two dimensional subwavelength acoustic gratings were demonstrated and investigated in both experiment and theory. In this talk, we demonstrate both theoretically and experimentally the physical mechanism that underlies the novel effects of EAT and collimation of sound through a one-dimensionally decorated plate. A microscopic theory considers the total field as sum of the scattered waves by every periodically aligned groove on the plate, which divides the total field into far-field radiative cylindrical waves and acoustic surface evanescent waves (ASEWs). Different from the well-known acoustic surface waves like Rayleigh waves and Lamb waves, ASEW is closely analogous to surface plasmon polariton (SPP) in the optical case. By mapping the total field, the experiments well confirm the theoretical calculations as well as the fact that EAT and sound collimation take place at the proper frequency when ASEWs satisfy the phase matching condition. The establishment of the concept of ASEW provides a new route for the integration of subwavelength acoustic devices with structured solid surface. [Preview Abstract] |
Tuesday, March 16, 2010 5:06PM - 5:18PM |
L15.00014: An investigation of the backscattered acoustic response from a suspension of clay aggregates Clementina Russo, Wayne Slade, Emmanuel Boss Particle aggregation in the ocean is a common phenomenon that has significant implications for measuring particle concentration and particle dynamics. It is recognized (Hatcher, 2001 \& Boss et al., 2009) that flocculated particles compromise certain optical measurements, but there is disagreement over the extent that aggregation affects the acoustic response. This work investigates the nature of the backscattered acoustic response to a suspension of flocculated clay particles. The character of this response is compared to the response predicted using scattering theory for singular, solid clay spheres and for clay aggregates, following an acoustical model for porous, aggregated particles (Dukhin et al., 2007). An expected linear relationship between the acoustic response and measured samples of concentration is shown and the mass normalized acoustic response to changes in particle size is compared with that predicted using scattering theory. We demonstrate that the mass normalized acoustic response exhibits a sensitivity to porous particle packaging and we discuss the implications that aggregation may have for an acoustic approach to the study of particle dynamics. [Preview Abstract] |
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