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Iowa State University

2009

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Articles 1 - 30 of 68

Full-Text Articles in Physics

Flux Growth At Ambient Pressure Of Millimeter-Sized Single Crystals Of Lafeaso, Lafeaso1−Xfx, And Lafe1−Xcoxaso, J.-Q. Yan, Shibabrata Nandi, Jerel L. Zarestky, W. Tian, Andreas Kreyssig, Brandt A. Jensen, A. Kracher, Kevin W. Dennis, Robert J. Mcqueeney, Alan I. Goldman, R. William Mccallum, Thomas A. Lograsso Dec 2009

Flux Growth At Ambient Pressure Of Millimeter-Sized Single Crystals Of Lafeaso, Lafeaso1−Xfx, And Lafe1−Xcoxaso, J.-Q. Yan, Shibabrata Nandi, Jerel L. Zarestky, W. Tian, Andreas Kreyssig, Brandt A. Jensen, A. Kracher, Kevin W. Dennis, Robert J. Mcqueeney, Alan I. Goldman, R. William Mccallum, Thomas A. Lograsso

Ames Laboratory Publications

Millimeter-sized single crystals of LaFeAsO, LaFeAsO1−xFx, and LaFe1−xCoxAsO were grown in NaAs flux at ambient pressure. The detailed growth procedure and crystal characterizations are reported. The as-grown crystals have typical dimensions of3×4×0.05–0.3 mm3 with the crystallographic c-axis perpendicular to the plane of the platelike single crystals. Various characterizations confirmed the high quality of our LaFeAsO crystals. Co and F were introduced into the lattice leading to superconductingLaFe1−xCoxAsO and LaFeAsO1−xFx single crystals, respectively. This growth protocol ...


Magnetostructural Transition In Gd5sb0.5ge3.5, A. S. Chernyshov, Yaroslav Mudryk, Durga Paudyal, Vitalij K. Pecharsky, Karl A. Gschneidner Jr., Deborah L. Schlagel, Thomas A. Lograsso Nov 2009

Magnetostructural Transition In Gd5sb0.5ge3.5, A. S. Chernyshov, Yaroslav Mudryk, Durga Paudyal, Vitalij K. Pecharsky, Karl A. Gschneidner Jr., Deborah L. Schlagel, Thomas A. Lograsso

Ames Laboratory Publications

Magnetic and crystallographic properties of Gd5Sb0.5Ge3.5 were investigated using dc magnetization, ac magnetic susceptibility, and heat capacity of an oriented single crystal, combined with temperature and magnetic field dependent x-ray powder diffraction. The compound undergoes an unusual magnetostructural transition at 40 K and a nonmagnetic second-order transition around 63 K. The detailed crystallographic study of Gd5Sb0.5Ge3.5 shows that contrary to the R5(SixGe1−x)4 systems (R is a rare-earth metal), the structural transition occurs without shear displacements of the 2[R ...


Electrical Resistivity And Magnetoresistance Of Single-Crystal Tb5si2.2ge1.8, Min Zou, Vitalij K. Pecharsky, Karl A. Gschneidner Jr., Yaroslav Mudryk, Deborah L. Schlagel, Thomas A. Lograsso Nov 2009

Electrical Resistivity And Magnetoresistance Of Single-Crystal Tb5si2.2ge1.8, Min Zou, Vitalij K. Pecharsky, Karl A. Gschneidner Jr., Yaroslav Mudryk, Deborah L. Schlagel, Thomas A. Lograsso

Ames Laboratory Publications

A positive colossal magnetoresistance (CMR) of 160% has been observed in Tb5Si2.2Ge1.8 with the magnetic field applied parallel to the a axis. When the magnetic field is applied parallel to the b and c axes, the magnetoresistance (MR) is less than 8% and 5%, respectively. The CMR effect originates from intrinsic crystallographic phase coexistence. The anisotropy of the MR effect is due to a unique geometric arrangement of the interphase boundaries and large magnetocrystalline anisotropy of the compound.


Analysis Of Nonequilibrium Hcp Precipitate Growth In Fcc Matrices: Application To Al–Ag, Daniel Finkenstadt, Duane D. Johnson Nov 2009

Analysis Of Nonequilibrium Hcp Precipitate Growth In Fcc Matrices: Application To Al–Ag, Daniel Finkenstadt, Duane D. Johnson

Duane D. Johnson

Hexagonal-close-packed (hcp) γ-precipitates with large aspect ratios form rapidly in some face-centered cubic (fcc) solid-solutions. No model explains the observed time-dependent increase in aspect ratio, nor irregular intermediate growth shapes. We propose a nonequilibrium process involving trapezoidal offshoots (controlled only by energetics) that governs the growth behavior (kinetics) and yields nonequilibrium structures in agreement with observation. Then, combining nucleation theory and diffusion-limited growth both of secondary nuclei and ledges, we derive a general growth equation for γ-precipitates due to solute-segregation to precipitate–matrix interfaces that includes our modification of the Jones–Trivedi model for thickening to account for the slow ...


High-P(T) Pi(0) Production With Respect To The Reaction Plane In Au Plus Au Collisions At S(Nn)=200 Gev, S. Afanasiev, Sergey Belikov, Paul Constantin, Nathan C. Grau, John C. Hill, John G. Lajoie, Alexandre Lebedev, Craig Ogilvie, H. Pei, Jan Rak, Marzia Rosati, S. Skutnik, Carla Vale, Et Al., Phenix Collaboration Nov 2009

High-P(T) Pi(0) Production With Respect To The Reaction Plane In Au Plus Au Collisions At S(Nn)=200 Gev, S. Afanasiev, Sergey Belikov, Paul Constantin, Nathan C. Grau, John C. Hill, John G. Lajoie, Alexandre Lebedev, Craig Ogilvie, H. Pei, Jan Rak, Marzia Rosati, S. Skutnik, Carla Vale, Et Al., Phenix Collaboration

Physics and Astronomy Publications

Measurements of the azimuthal anisotropy of high-p(T) neutral pion (pi(0)) production in Au+Au collisions at s(NN)=200 GeV by the PHENIX experiment are presented. The data included in this article were collected during the 2004 Relativistic Heavy Ion Collider running period and represent approximately an order of magnitude increase in the number of analyzed events relative to previously published results. Azimuthal angle distributions of pi(0) mesons detected in the PHENIX electromagnetic calorimeters are measured relative to the reaction plane determined event-by-event using the forward and backward beam-beam counters. Amplitudes of the second Fourier component (v ...


Gluon Saturation Effects On J/Psi Production In Heavy Ion Collisions, Dmitri Kharzeev, Eugene Levin, Marzia Nardi, Kirill Tuchin Nov 2009

Gluon Saturation Effects On J/Psi Production In Heavy Ion Collisions, Dmitri Kharzeev, Eugene Levin, Marzia Nardi, Kirill Tuchin

Physics and Astronomy Publications

We discuss a novel mechanism for J/ψ production in nuclear collisions arising due to the high density of gluons. We demonstrate that gluon saturation in the colliding nuclei is a dominant source of J/Psi suppression and can explain its experimentally observed rapidity and centrality dependence.


Single-Crystal Neutron Diffraction Study Of Short-Range Magnetic Correlations In Tb5ge4, W. Tian, Andreas Kreyssig, Jerel L. Zarestky, L. Tan, Shibabrata Nandi, Alan I. Goldman, Thomas A. Lograsso, Deborah L. Schlagel, Karl A. Gschneidner Jr., Vitalij K. Pecharsky, Robert J. Mcqueeney Oct 2009

Single-Crystal Neutron Diffraction Study Of Short-Range Magnetic Correlations In Tb5ge4, W. Tian, Andreas Kreyssig, Jerel L. Zarestky, L. Tan, Shibabrata Nandi, Alan I. Goldman, Thomas A. Lograsso, Deborah L. Schlagel, Karl A. Gschneidner Jr., Vitalij K. Pecharsky, Robert J. Mcqueeney

Ames Laboratory Publications

We present a single-crystal neutron diffraction study of the magnetic short-range correlations in Tb5Ge4 which orders antiferromagnetically below the Neel temperature TN≈92 K. Strong diffuse scattering arising from magnetic short-range correlations was observed in wide temperature ranges both below and above TN. The antiferromagnetic ordering in Tb5Ge4 is described to consist of strongly coupled ferromagnetic block layers in the ac plane that stack along the b axis with weak antiferromagnetic interlayer coupling. Diffuse scattering was observed along both a∗ and b∗ directions indicating three-dimensional short-range correlations. Moreover, the q dependence of ...


Charged Kaon Interferometric Probes Of Space-Time Evolution In Au Plus Au Collisions At S(Nn)=200 Gev, S. Afanasiev, Sergey Belikov, Paul Constantin, Nathan C. Grau, John C. Hill, John G. Lajoie, Alexandre Lebedev, Craig Ogilvie, H. Pei, Jan Rak, Marzia Rosati, S. Skutnik, Carla Vale, Et Al., Phenix Collaboration Oct 2009

Charged Kaon Interferometric Probes Of Space-Time Evolution In Au Plus Au Collisions At S(Nn)=200 Gev, S. Afanasiev, Sergey Belikov, Paul Constantin, Nathan C. Grau, John C. Hill, John G. Lajoie, Alexandre Lebedev, Craig Ogilvie, H. Pei, Jan Rak, Marzia Rosati, S. Skutnik, Carla Vale, Et Al., Phenix Collaboration

Physics and Astronomy Publications

Bose-Einstein correlations of charged kaons are used to probe Au+Au collisions at s(NN)=200 GeV and are compared to charged pion probes, which have a larger hadronic scattering cross section. Three-dimensional Gaussian source radii are extracted, along with a one-dimensional kaon emission source function. The centrality dependences of the three Gaussian radii are well described by a single linear function of N-part(1/3) with a zero intercept. Imaging analysis shows a deviation from a Gaussian tail at r greater than or similar to 10 fm, although the bulk emission at lower radius is well described by a ...


Student Ideas Regarding Entropy And The Second Law Of Thermodynamics In An Introductory Physics Course, Warren M. Christensen, David E. Meltzer, Craig Ogilvie Oct 2009

Student Ideas Regarding Entropy And The Second Law Of Thermodynamics In An Introductory Physics Course, Warren M. Christensen, David E. Meltzer, Craig Ogilvie

Physics and Astronomy Publications

We report on students’ thinking regarding entropy in an introductory calculus-based physics course. We analyzed students’ responses to a variety of questions on entropy changes of an arbitrarily defined system and its surroundings. In four offerings of the same course we found that before instruction, no more than 6% of all students could give completely correct responses to relevant questions posed in both general and concrete contexts. Nearly two-thirds of the students showed clear evidence of conservation-type reasoning regarding entropy. These outcomes were little changed even after instruction. Targeted instruction that guided students to recognize that entropy is not a ...


Competition Between Multiple Totally Asymmetric Simple Exclusion Processes For A Finite Pool Of Resources, L. Jonathan Cook, R. K. P. Zia, Beate Schmittmann Sep 2009

Competition Between Multiple Totally Asymmetric Simple Exclusion Processes For A Finite Pool Of Resources, L. Jonathan Cook, R. K. P. Zia, Beate Schmittmann

Beate Schmittmann

Using Monte Carlo simulations and a domain-wall theory, we discuss the effect of coupling several totally asymmetric simple exclusion processes (TASEPs) to a finite reservoir of particles. This simple model mimics directed biological transport processes in the presence of finite resources such as protein synthesis limited by a finite pool of ribosomes. If all TASEPs have equal length, we find behavior which is analogous to a single TASEP coupled to a finite pool. For the more generic case of chains with different lengths, several unanticipated regimes emerge. A generalized domain-wall theory captures our findings in good agreement with simulation results.


Optimal Site-Centered Electronic Structure Basis Set From A Displaced-Center Expansion: Improved Results Via A Priori Estimates Of Saddle Points In The Density, Aftab Alam, Duane D. Johnson Sep 2009

Optimal Site-Centered Electronic Structure Basis Set From A Displaced-Center Expansion: Improved Results Via A Priori Estimates Of Saddle Points In The Density, Aftab Alam, Duane D. Johnson

Duane D. Johnson

Site-centered, electronic-structure methods use an expansion inside nonoverlapping “muffin-tin” (MT) spheres plus an interstitial basis set. As the boundary separating the more spherical from nonspherical density between atoms, the “saddle-point” radii (SPR) in the density provide an optimal spherical region for expanding in spherical harmonics, as used in augmented plane wave, muffin-tin orbital, and multiple-scattering [Korringa, Kohn, and Rostoker (KKR)] methods. These MT-SPR guarantee unique, convex Voronoi polyhedra at each site, in distinction to Bader topological cells. We present a numerically fast, two-center expansion to find SPR a priori from overlapping atomic charge densities, valid also for disordered alloys. We ...


Fast-Framing Ballistic Imaging Of Velocity In An Aerated Spray, David Sedarsky, James Gord, Campbell Carter, Terrence R. Meyer, Mark Linne Sep 2009

Fast-Framing Ballistic Imaging Of Velocity In An Aerated Spray, David Sedarsky, James Gord, Campbell Carter, Terrence R. Meyer, Mark Linne

Mechanical Engineering Publications

We describe further development of ballistic imaging adapted for the liquid core of an atomizing spray. To fully understand spray breakup dynamics, one must measure the velocity and acceleration vectors that describe the forces active in primary breakup. This information is inaccessible to most optical diagnostics, as the signal is occluded by strong scattering in the medium. Ballistic imaging mitigates this scattering noise, resolving clean shadowgram-type images of structures within the dense spray region. We demonstrate that velocity data can be extracted from ballistic images of a spray relevant to fuel-injection applications, by implementing a simple, targeted correlation method for ...


Photoproduction Of J/Psi And Of High Mass E(+)E(-) In Ultra-Peripheral Au Plus Au Collisions At Root S(Nn)=200 Gev, S. Afanasiev, Sergey Belikov, Paul Constantin, Nathan C. Grau, John C. Hill, John G. Lajoie, Alexandre Lebedev, Craig Ogilvie, H. Pei, Jan Rak, Marzia Rosati, S. Skutnik, Carla Vale, Et Al., Phenix Collaboration Aug 2009

Photoproduction Of J/Psi And Of High Mass E(+)E(-) In Ultra-Peripheral Au Plus Au Collisions At Root S(Nn)=200 Gev, S. Afanasiev, Sergey Belikov, Paul Constantin, Nathan C. Grau, John C. Hill, John G. Lajoie, Alexandre Lebedev, Craig Ogilvie, H. Pei, Jan Rak, Marzia Rosati, S. Skutnik, Carla Vale, Et Al., Phenix Collaboration

Physics and Astronomy Publications

We present the first measurement of photoproduction of J/psi and of two-photon production of high-mass e(+)e(-) pairs in electromagnetic (or ultra-peripheral) nucleus-nucleus interactions, using Au + Au data at root s(NN) = 200 GeV. The events are tagged with forward neutrons emitted following Coulomb excitation of one or both Au* nuclei. The event sample consists of 28 events with m(e+e-) > 2 GeV/c(2) with zero like-sign background. The measured cross sections at midrapidity of d sigma/dy (J/psi + Xn, y = 0) = 76 +/- 33 (stat) +/- 11 (syst) pb and d(2)sigma /dm dy (e(+) e ...


Changes In Students’ Problem-Solving Strategies In A Course That Includes Context-Rich, Multifaceted Problems, Craig Ogilvie Aug 2009

Changes In Students’ Problem-Solving Strategies In A Course That Includes Context-Rich, Multifaceted Problems, Craig Ogilvie

Physics and Astronomy Publications

Most students struggle when faced with complex and open-ended tasks because the strategies taught in schools and universities simply require finding and applying the correct formulae or strategy to answer well-structured, algorithmic problems. For students to develop their ability to solve ill-structured problems, they must first believe that standardized procedural approaches will not always be sufficient for solving engineering and scientific challenges. In this paper we document the range of beliefs university students have about problem solving. Students enrolled in a physics course submitted a written reflection both at the start and the end of the course on how they ...


Magnetic And Lattice Coupling In Single-Crystal Srfe2as2: A Neutron Scattering Study, Haifeng Li, Wei Tian, Jerel L. Zarestky, Andreas Kreyssig, Ni Ni, Sergey L. Bud'ko, Paul C. Canfield, Alan I. Goldman, Robert J. Mcqueeney, David Vaknin Aug 2009

Magnetic And Lattice Coupling In Single-Crystal Srfe2as2: A Neutron Scattering Study, Haifeng Li, Wei Tian, Jerel L. Zarestky, Andreas Kreyssig, Ni Ni, Sergey L. Bud'ko, Paul C. Canfield, Alan I. Goldman, Robert J. Mcqueeney, David Vaknin

Ames Laboratory Publications

A detailed elastic neutron scattering study of the structural and magnetic phase transitions in single-crystal SrFe2As2reveals that the orthorhombic (O)-tetragonal (T) and the antiferromagnetic transitions coincide at TO=TN=(201.5±0.25) K. The observation of coexisting O-T phases over a finite temperature range at the transition and the sudden onset of the O distortion provide strong evidences that the structural transition is first order. The simultaneous appearance and disappearance within 0.5 K upon cooling and within 0.25 K upon warming, respectively, indicate that the magnetic and structural transitions are intimately coupled. We find that the ...


Measurement Of Bottom Versus Charm As A Function Of Transverse Momentum With Electron-Hadron Correlations In P Plus P Collisions At S=200 Gev, Andrew Adare, Nathan C. Grau, John C. Hill, Todd Kempel, John G. Lajoie, Alexandre Lebedev, Craig Ogilvie, H. Pei, Marzia Rosati, Alexey Yu. Semenov, S. Skutnik, Carla Vale, Feng Wei, Et Al., Phenix Collaboration Aug 2009

Measurement Of Bottom Versus Charm As A Function Of Transverse Momentum With Electron-Hadron Correlations In P Plus P Collisions At S=200 Gev, Andrew Adare, Nathan C. Grau, John C. Hill, Todd Kempel, John G. Lajoie, Alexandre Lebedev, Craig Ogilvie, H. Pei, Marzia Rosati, Alexey Yu. Semenov, S. Skutnik, Carla Vale, Feng Wei, Et Al., Phenix Collaboration

Physics and Astronomy Publications

The momentum distribution of electrons from semileptonic decays of charm and bottom quarks for midrapidity |y|< 0.35 in p+p collisions at s=200 GeV is measured by the PHENIX experiment at the Relativistic Heavy Ion Collider over the transverse momentum range 2 < p(T)< 7 GeV/c. The ratio of the yield of electrons from bottom to that from charm is presented. The ratio is determined using partial D/D -> e(+/-)K(-/+)X (K unidentified) reconstruction. It is found that the yield of electrons from bottom becomes significant above 4 GeV/c in p(T). A fixed-order-plus-next-to-leading-log perturbative quantum chromodynamics calculation agrees with the data within the theoretical and experimental uncertainties. The extracted total bottom production cross section at this energy is sigma(bb)=3.2(-1.1)(+1.2)(stat)(-1.3)(+1.4)(syst)mu b.


Photon-Hadron Jet Correlations In P Plus P And Au Plus Au Collisions At S(Nn)=200 Gev, Andrew Adare, Sergey Belikov, Paul Constantin, Nathan C. Grau, John C. Hill, Todd Kempel, John G. Lajoie, Alexandre Lebedev, Craig Ogilvie, H. Pei, Jan Rak, Marzia Rosati, Alexey Yu. Semenov, S. Skutnik, Carla Vale, Feng Wei, Et Al., Phenix Collaboration Aug 2009

Photon-Hadron Jet Correlations In P Plus P And Au Plus Au Collisions At S(Nn)=200 Gev, Andrew Adare, Sergey Belikov, Paul Constantin, Nathan C. Grau, John C. Hill, Todd Kempel, John G. Lajoie, Alexandre Lebedev, Craig Ogilvie, H. Pei, Jan Rak, Marzia Rosati, Alexey Yu. Semenov, S. Skutnik, Carla Vale, Feng Wei, Et Al., Phenix Collaboration

Physics and Astronomy Publications

We report the observation at the Relativistic Heavy Ion Collider of suppression of back-to-back correlations in the direct photon+jet channel in Au+Au relative to p+p collisions. Two-particle correlations of direct photon triggers with associated hadrons are obtained by statistical subtraction of the decay photon-hadron (gamma-h) background. The initial momentum of the away-side parton is tightly constrained, because the parton-photon pair exactly balance in momentum at leading order in perturbative quantum chromodynamics, making such correlations a powerful probe of the in-medium parton energy loss. The away-side nuclear suppression factor, I-AA, in central Au+Au collisions, is 0.32 ...


Systematic Studies Of Elliptic Flow Measurements In Au Plus Au Collisions At S(Nn)=200 Gev, S. Afanasiev, Sergey Belikov, Paul Constantin, Nathan C. Grau, John C. Hill, John G. Lajoie, Alexandre Lebedev, Craig Ogilvie, H. Pei, Jan Rak, Marzia Rosati, S. Skutnik, Carla Vale, Et Al., Phenix Collaboration Aug 2009

Systematic Studies Of Elliptic Flow Measurements In Au Plus Au Collisions At S(Nn)=200 Gev, S. Afanasiev, Sergey Belikov, Paul Constantin, Nathan C. Grau, John C. Hill, John G. Lajoie, Alexandre Lebedev, Craig Ogilvie, H. Pei, Jan Rak, Marzia Rosati, S. Skutnik, Carla Vale, Et Al., Phenix Collaboration

Physics and Astronomy Publications

We present inclusive charged hadron elliptic flow (v(2)) measured over the pseudorapidity range vertical bar eta vertical bar < 0.35 in Au+Au collisions at s(NN)=200 GeV. Results for v(2) are presented over a broad range of transverse momentum (p(T)=0.2-8.0 GeV/c) and centrality (0-60%). To study nonflow effects that are correlations other than collective flow, as well as the fluctuations of v(2), we compare two different analysis methods: (1) the event-plane method from two independent subdetectors at forward (vertical bar eta vertical bar=3.1-3.9) and beam (vertical bar eta vertical bar>6.5) pseudorapidities and (2) the two-particle cumulant method extracted using correlations between particles detected at midrapidity. The two event-plane results are consistent within systematic uncertainties over the measured p(T) and in centrality 0-40%. There is at most a 20% difference in the v(2) between the two event-plane methods in peripheral (40-60%) collisions. The comparisons between the two-particle cumulant results and the standard event-plane measurements are discussed.


Modeling A Biomass Fluidizing Bed With Side Port Air Injection, Mirka Deza, Francine Battaglia, Theodore J. Heindel Aug 2009

Modeling A Biomass Fluidizing Bed With Side Port Air Injection, Mirka Deza, Francine Battaglia, Theodore J. Heindel

Mechanical Engineering Conference Presentations, Papers, and Proceedings

Fluidized beds are used to gasify materials such as coal or biomass in the production of producer gas. Modeling these reactors using computational fluid dynamics is advantageous when performing parametric studies for design and scale-up. While two-dimensional simulations are easier to perform than three-dimensional simulations, they may not capture the proper physics. This paper compares two- and three-dimensional simulations with experiments for a reactor geometry with side port air injection. The side port is located within the bed region so that the injected air can help promote mixing. Of interest in this study is validating the hydrodynamics of fluidizing biomass ...


Repeatability Of Gas Holdup In A Fluidized Bed Using X-Ray Computed Tomography, Joshua B. Drake, Theodore J. Heindel Aug 2009

Repeatability Of Gas Holdup In A Fluidized Bed Using X-Ray Computed Tomography, Joshua B. Drake, Theodore J. Heindel

Mechanical Engineering Conference Presentations, Papers, and Proceedings

Characterizing the hydrodynamics in fluidized beds is important to many processes from producing biofuels to coating pharmaceuticals. X-ray computed tomography (CT) can quantify local time-averaged phase fractions in multiphase systems that are highly dynamic, like fluidized beds. This paper describes the calibration methods used to produced CT images of a 15.24 cm diameter fluidized bed, how in-house software used these CTs to calculate gas holdup, and how well multiple CTs of a dynamic fluidized bed produced repeatable results while varying bed material and superficial gas velocities. It was concluded there is a very high degree of repeatability using the ...


X-Ray Particle Tracking Velocimetry In Fluidized Beds, Joshua B. Drake, Lie Tang, Theodore J. Heindel Aug 2009

X-Ray Particle Tracking Velocimetry In Fluidized Beds, Joshua B. Drake, Lie Tang, Theodore J. Heindel

Mechanical Engineering Conference Presentations, Papers, and Proceedings

Fluidized beds are commonly found in the chemical and energy processing industries because of their low pressure drop, uniform temperature distribution, and high heat transfer rates. For example, in biomass gasification, biomass particles are injected into a heated bubbling bed of inert material (typically refractory sand) that volatilizes to form a flammable gas. However, the movement of the biomass particle through the bubbling bed is difficult to quantify because the systems are opaque. This paper describes X-ray particle tracking velocimetry (XPTV) applied to fluidized beds, where X-ray flow visualization is used to track the location of a single fabricated tracer ...


Surface Geometry Of C60 On Ag(111), H. I. Li, K. Pussi, K. J. Hanna, Lin-Lin Wang, Duane D. Johnson, H.-P. Cheng, H. Shin, S. Curtarolo, W. Moritz, J. A. Smerdon, R. Mcgrath, R. D. Diehl Jul 2009

Surface Geometry Of C60 On Ag(111), H. I. Li, K. Pussi, K. J. Hanna, Lin-Lin Wang, Duane D. Johnson, H.-P. Cheng, H. Shin, S. Curtarolo, W. Moritz, J. A. Smerdon, R. Mcgrath, R. D. Diehl

Duane D. Johnson

The geometry of adsorbed C60 influences its collective properties. We report the first dynamical low-energy electron diffraction study to determine the geometry of a C60 monolayer, Ag(111)−(23√×23√)30°−C60, and related density functional theory calculations. The stable monolayer has C60 molecules in vacancies that result from the displacement of surface atoms. C60 bonds with hexagons down, with their mirror planes parallel to that of the substrate. The results indicate that vacancy structures are the rule rather than the exception for C60 monolayers on close-packed metal surfaces.


Solvation Dynamics In Ionic Fluids: An Extended Debye–Hückel Dielectric Continuum Model, Xueyu Song Jul 2009

Solvation Dynamics In Ionic Fluids: An Extended Debye–Hückel Dielectric Continuum Model, Xueyu Song

Chemistry Publications

Motivated by our recent proposition on the possibility of using dielectric continuum models to interpret experimental measurements of solvation dynamics in room temperature ionic liquids [J. Phys. Chem. A110, 8623 (2006)], some detailed simulation studies are performed to test the validity of our proposition. From these simulation studies, it seems to be justified that an extended Debye–Hückel continuum model can be used to understand the solvation dynamics of ionic fluids. The theoretical underpinning of such an extended Debye–Hückel model is presented from the general dispersion relation in electrodynamics. The connection with the static extension from the dressed ion ...


Gluon-Spin Contribution To The Proton Spin From The Double-Helicity Asymmetry In Inclusive Pi(0) Production In Polarized P Plus P Collisions At Root S=200 Gev, Andrew Adare, John C. Hill, Todd Kempel, John G. Lajoie, Alexandre Lebedev, Craig Ogilvie, H. Pei, Marzia Rosati, Alexey Yu. Semenov, Carla Vale, Feng Wei, Et Al., Phenix Collaboration Jul 2009

Gluon-Spin Contribution To The Proton Spin From The Double-Helicity Asymmetry In Inclusive Pi(0) Production In Polarized P Plus P Collisions At Root S=200 Gev, Andrew Adare, John C. Hill, Todd Kempel, John G. Lajoie, Alexandre Lebedev, Craig Ogilvie, H. Pei, Marzia Rosati, Alexey Yu. Semenov, Carla Vale, Feng Wei, Et Al., Phenix Collaboration

Physics and Astronomy Publications

The double helicity asymmetry in neutral pion production for p(T) = 1 to 12 GeV/c was measured with the PHENIX experiment to access the gluon-spin contribution, Delta G, to the proton spin. Measured asymmetries are consistent with zero, and at a theory scale of mu 2 = 4 GeV2 a next to leading order QCD analysis gives Delta G([0.02,0.3]) = 0.2, with a constraint of -0.7 < Delta G([0.02,0.3]) < 0.5 at Delta chi(2) = 9 (similar to 3 sigma) for the sampled gluon momentum fraction (x) range, 0.02 to 0.3. The results are obtained using predictions for the measured asymmetries generated from four representative fits to polarized deep inelastic scattering data. We also consider the dependence of the Delta G constraint on the choice of the theoretical scale, a dominant uncertainty in these predictions.


Lattice Dynamics In Magnetic Superelastic Ni-Mn-In Alloys: Neutron Scattering And Ultrasonic Experiments, Xavier Moya, David González-Alonso, Lluís Mañosa, Antoni Planes, V. O. Garlea, Thomas A. Lograsso, Deborah L. Schlagel, Jerel L. Zarestky, Seda Aksoy, Mehmed Acet Jun 2009

Lattice Dynamics In Magnetic Superelastic Ni-Mn-In Alloys: Neutron Scattering And Ultrasonic Experiments, Xavier Moya, David González-Alonso, Lluís Mañosa, Antoni Planes, V. O. Garlea, Thomas A. Lograsso, Deborah L. Schlagel, Jerel L. Zarestky, Seda Aksoy, Mehmed Acet

Ames Laboratory Publications

Neutron scattering and ultrasonic methods have been used to study the lattice dynamics of two single crystals of Ni-Mn-In Heusler alloys close to Ni50Mn34In16 magnetic superelastic composition. The paper reports on the experimental determination of the low-lying phonon-dispersion curves and the elastic constants for this alloy system. We found that the frequencies of the TA2 branch are relatively low and it exhibits a small dip anomaly at a wave number ξ0≈1/3, which softens with decreasing temperature. Associated with the softening of this phonon, we also observed the softening of the shear ...


Jump In Specific Heat At The Superconducting Transition Temperature In Ba(Fe1−Xcox)2as2 And Ba(Fe1−Xnix)2as2 Single Crystals, Sergey L. Bud'ko, Ni Ni, Paul C. Canfield Jun 2009

Jump In Specific Heat At The Superconducting Transition Temperature In Ba(Fe1−Xcox)2as2 And Ba(Fe1−Xnix)2as2 Single Crystals, Sergey L. Bud'ko, Ni Ni, Paul C. Canfield

Ames Laboratory Publications

We present detailed heat capacity measurements for Ba(Fe1−xCox)2As2 and Ba(Fe1−xNix)2As2 single crystals in the vicinity of the superconducting transitions. The specific-heat jump at the superconducting transition temperature (Tc), ΔCp/Tc, changes by a factor of ∼10 across these series. The ΔCp/Tc vs Tc data of this work [together with the literature data for Ba(Fe0.939Co0.061)2As2, (Ba0.55K0.45)Fe2As2, and (Ba0.6K0.4)Fe2As2] scale well to a single log-log plot over two orders of magnitude in ΔCp/Tc and over about an order of magnitude in Tc, giving ΔCp ...


In Situ High Energy X-Ray Synchrotron Diffraction Study Of The Synthesis And Stoichiometry Of Lafeaso And Lafeaso1−Xfy, R. William Mccallum, J.-Q. Yan, Gustav E. Rustan, E. D. Mun, Yogesh Singh, S. Das, R. Nath, Sergey L. Bud'ko, Kevin W. Dennis, David C. Johnston, Paul C. Canfield, Matthew J. Kramer, Andreas Kreyssig, Thomas A. Lograsso, Alan I. Goldman Jun 2009

In Situ High Energy X-Ray Synchrotron Diffraction Study Of The Synthesis And Stoichiometry Of Lafeaso And Lafeaso1−Xfy, R. William Mccallum, J.-Q. Yan, Gustav E. Rustan, E. D. Mun, Yogesh Singh, S. Das, R. Nath, Sergey L. Bud'ko, Kevin W. Dennis, David C. Johnston, Paul C. Canfield, Matthew J. Kramer, Andreas Kreyssig, Thomas A. Lograsso, Alan I. Goldman

Ames Laboratory Publications

The reaction path for the synthesis of LaFeAsO and LaFeAsO1−xFy by solid state reaction was studied by in situ high temperature x-ray diffraction technique and differential thermal analysis in the temperature interval 100 °C ⩽ T ⩽ 1150 °C. Starting with LaAs, Fe2O3, Fe, and LaF3 as precursors, the results show that the synthesis is characterized by three temperature intervals: (1) Below 500 °C the sequential reduction of Fe2O3 and Fe3O4takes place through the oxidization of LaAs. Below 400 °C, Fe2O3 is reduced to ...


Aperiodic And Modulated Pb Thin Films On Fivefold Icosahedral Al-Cu-Fe And Al(111): Tailoring The Structure Of Pb, Th. Deniozou, J. Ledieu, V. Fournée, D. M. Wu, Thomas A. Lograsso, H. I. Li, R. D. Diehl Jun 2009

Aperiodic And Modulated Pb Thin Films On Fivefold Icosahedral Al-Cu-Fe And Al(111): Tailoring The Structure Of Pb, Th. Deniozou, J. Ledieu, V. Fournée, D. M. Wu, Thomas A. Lograsso, H. I. Li, R. D. Diehl

Ames Laboratory Publications

We report on the growth of Pb thin films deposited either on the Al-rich fivefold surface of the icosahedral Al-Cu-Fe quasicrystal or on the (111) surface of fcc Al. On the quasicrystalline substrate, the diffusion length of Pb adatoms is short due to heterogeneous nucleation that enforces a quasiperiodic structure in the monolayer. On the Al(111) substrate, the mobility of Pb adatoms is high and the interaction with the substrate is flatter, leading to the formation of a (√31×√31)R8.95° higher-order commensurate structure. This moiré structure propagates up to the highest coverages investigated.


Quantitative Prediction Of Twinning Stress In Fcc Alloys: Application To Cu-Al, Sandeep A. Kibey, Lin-Lin Wang, J. B. Liu, H. T. Johnson, H. Sehitoglu, Duane D. Johnson Jun 2009

Quantitative Prediction Of Twinning Stress In Fcc Alloys: Application To Cu-Al, Sandeep A. Kibey, Lin-Lin Wang, J. B. Liu, H. T. Johnson, H. Sehitoglu, Duane D. Johnson

Duane D. Johnson

Twinning is one of most prevalent deformation mechanisms in materials. Having established a quantitative theory to predict onset twinning stress τcrit in fcc elemental metals from their generalized planar-fault-energy (GPFE) surface, we exemplify its use in alloys where the Suzuki effect (i.e., solute energetically favors residing at and near planar faults) is operative; specifically, we apply it in Cu-xAl (x is 0, 5, and 8.3 at. %) in comparison with experimental data. We compute the GPFE via density-functional theory, and we predict the solute dependence of the GPFE and τcrit, in agreement with measured values. We show that τcrit ...


Properties Of Rre2al10 (R=Y, Gd–Lu) Crystals, Athena S. Sefat, Sergey L. Bud'ko, Paul C. Canfield May 2009

Properties Of Rre2al10 (R=Y, Gd–Lu) Crystals, Athena S. Sefat, Sergey L. Bud'ko, Paul C. Canfield

Ames Laboratory Publications

Large single crystals of rare-earth rhenium aluminide RRe2Al10, with R=Y, and Gd–Lu were grown out of an Al-rich solution. Single crystal x-ray diffraction data confirmed the orthorhombic Cmcm structure for all members: R=Gd-Dy with TbRe2Al10-structure type (formula unit per cell Z=8); R=Y, and Ho–Lu with LuRe2Al10-structure type (Z=12). There is no evidence of a localized 3d electron moment in R=Y and Lu; R=Yb is nonmagnetic down to 1.8 K, but develops an enhanced electronic specific heat of ∼95 mJ mol−1 K−2. Ordering temperatures range from ferromagnetic order in ...