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Test of the consistency of various linearized semiclassical initial value time correlation functions in application to inelastic neutron scattering from liquid para-hydrogen

Description: The linearized approximation to the semiclassical initial value representation (LSC-IVR) is used to calculate time correlation functions relevant to the incoherent dynamic structure factor for inelastic neutron scattering from liquid para-hydrogen at 14 K. Various time correlations functions were used which, if evaluated exactly, would give identical results, but they do not because the LSC-IVR is approximate. Some of the correlation functions involve only linear operators, and others involve non-linear operators. The consistency of the results obtained with the various time correlation functions thus provides a useful test of the accuracy of the LSC-IVR approximation and its ability to treat correlation functions involving both linear and nonlinear operators in realistic anharmonic systems. The good agreement of the results obtained from different correlation functions, their excellent behavior in the spectral moment tests based on the exact moment constraints, and their semi-quantitative agreement with the inelastic neutron scattering experimental data all suggest that the LSC-IVR is indeed a good short-time approximation for quantum mechanical correlation functions.
Date: March 15, 2008
Creator: Miller, William; Liu, Jian & Miller, William H.
Partner: UNT Libraries Government Documents Department

Stochastic Hard-Sphere Dynamics for Hydrodynamics of Non-Ideal Fluids

Description: A novel stochastic fluid model is proposed with a nonideal structure factor consistent with compressibility, and adjustable transport coefficients. This stochastic hard-sphere dynamics (SHSD) algorithm is a modification of the direct simulation Monte Carlo algorithm and has several computational advantages over event-driven hard-sphere molecular dynamics. Surprisingly, SHSD results in an equation of state and a pair correlation function identical to that of a deterministic Hamiltonian system of penetrable spheres interacting with linear core pair potentials. The fluctuating hydrodynamic behavior of the SHSD fluid is verified for the Brownian motion of a nanoparticle suspended in a compressible solvent.
Date: February 26, 2008
Creator: Donev, A; Alder, B J & Garcia, A L
Partner: UNT Libraries Government Documents Department

Spin dynamics of Li Doped La[sub 2]CuO[sub 4]with a disordered ground state

Description: Only 3% hole doping by Li is sufficient to suppress the long-range antiferromagnetic order in LazCu04. Spin dynamics in the disordered state was investigated by measuring the dynamic magnetic structure factor, S(q,w), as a function of temperature, using cold neutron spectroscopy, on La2Cuo.96Lio.0404a nd La{approx}Cuo.gLio.104s ingle crystals. The details of observed spin dynamics of the quantum disordered spin liquid were compared to the theoretical results of the nonlinear u model for a two dimensional Heisenberg antiferromagnet, which predicts a phase crossover upon cooling from a quantum critical regime of a quantum spin liquid to its quantum paramagnetic regime.
Date: January 1, 2002
Creator: Chen, Y.; Bao, W. (Wei); Sarrao, John L.,; Park, S.-H. L. (Park, S.-H. Lee) & Lorenzo, J. E.
Partner: UNT Libraries Government Documents Department

Heterogeneity, growing lenght scales, and universality in thedynamicsof kinetically constrained lattice gases in two dimensions

Description: We study dynamical heterogeneity and growing dynamical length scales in two kinetically constrained models, namely, the one- and two-vacancy assisted triangular lattice gases. One of the models is a strong glassformer and the other is a fragile glassformer. Both exhibit heterogeneous dynamics with broadly distributed timescales as seen in the distribution of persistence times. We show that the Stokes-Einstein relation is violated, to a greater degree in the fragile glassformer, and show how this violation is related to dynamic heterogeneity. Finally, we extract dynamical length scales from structure factors of mobile particles and show, quantitatively, the growth of this lengthscale as density increases. Our results indicate that the one-vacancy assisted lattice gas is in the same universality class as the one-spin facilitated Fredrickson-Andersen model, while the two-vacancy assisted model is in a different universality class. Our findings support the idea that the dynamics of supercooled liquids can be understood from the existence of a zero temperature dynamic critical point.
Date: June 26, 2005
Creator: Pan, Albert P.; Garrahan, Juan P. & Chandler, David
Partner: UNT Libraries Government Documents Department

Molecular engineering of polymer alloys: A final report of results obtained on CRADA No. 1078

Description: This report summarizes the technical progress made in the past three years on CRADA No. 1078, Molecular Engineering of Polymer Alloys. The thrust of this CRADA was to start with the basic ideas of PRISM theory and develop it to the point where it could be applied to modeling of polymer alloys. In this program, BIOSYM, Sandia and the University of Illinois worked jointly to develop the theoretical techniques and numerical formalisms necessary to implement the theoretical ideas into commercial software aimed at molecular engineering of polymer alloys. This CRADA focused on developing the techniques required to make the transition from theory to practice. These techniques were then used by BIOSYM to incorporate PRISM theory and other new developments into their commercial software.
Date: December 1, 1995
Creator: Curro, J.G.; Schweizer, K.S. & Honeycutt, J.D.
Partner: UNT Libraries Government Documents Department

Structure factor measurement in TiAl and silicon

Description: It proved impossible to obtain results with sufficient precision for TiAl reproducibly (<0.1%) from samples of different thicknesses. In order to ascertain the origin of this inconsistency, this study concentrates on the development of the experimental and theoretical tools required for such refinement and measurements for Si have been performed.
Date: March 1, 1996
Creator: Swaminathan, S.; Wiezorek, J.M.; Jones, I.P.; Zaluzec, N.J.; Fraser & Maher, D.M.
Partner: UNT Libraries Government Documents Department

Structure of charged polymer chains in confined geometry.

Description: The intra- and interchain structure of sodium poly(styrenesulphonate) when free and when confined in contrast matched porous Vycor has been investigated by SANS. When confined, a peak is observed whose intensity increases with molecular weight and the 1/q scattering region is extended compared to the bulk. We infer that the chains are sufficiently extended, under the influence of confinement, to highlight the large scale disordered structure of Vycor. The asymptotic behavior of the observed interchain structure factor is = 1/q{sup 2} and = 1/q for free and confined chains respectively.
Date: December 15, 2000
Creator: Gilbert, E. P.; Auvray, L. & Lal, J.
Partner: UNT Libraries Government Documents Department

Interaction of polymer with clays.

Description: Normally synthetic well defined monodisperse discotic laponite clays are known to form a gel phase at mass concentrations as low as a few percent in distilled water. Hydrosoluble polymer polyethylene oxide was added to this intriguing clay system, it was observed that it either prevents gelation or slows it down extremely depending on the polymer weight, concentration or the laponite concentration. Small Angle Neutron scattering (SANS) was used to study these systems because only by isotopic labelling can the structure of the adsorbed polymer layers be determined. The contrast variation technique is specifically used to determine separately the different partial structure factors of the clay and polymer. In this way the signal of the adsorbed chains is separated from the signal of the free chains.
Date: July 2, 1999
Creator: Auvray, L. & Lal, J.
Partner: UNT Libraries Government Documents Department

ELEMENTARY APPROACH TO SELF-ASSEMBLY AND ELASTIC PROPERTIES OF RANDOM COPOLYMERS

Description: The authors have mapped the physics of a system of random copolymers onto a time-dependent density functional-type field theory using techniques of functional integration. Time in the theory is merely a label for the location of a given monomer along the extent of a flexible chain. We derive heuristically within this approach a non-local constraint which prevents segments on chains in the system from straying too far from each other, and leads to self-assembly. The structure factor is then computed in a straightforward fashion. The long wave-length limit of the structure factor is used to obtain the elastic modulus of the network. It is shown that there is a surprising competition between the degree of micro-phase separation and the elastic moduli of the system.
Date: October 1, 2000
Creator: CHITANVIS, S. M.
Partner: UNT Libraries Government Documents Department

Determination of structure factors of copper by convergent beam electron diffraction

Description: There is interest in determining structure factors from convergent beam electron diffraction (CBED) patterns; goal is to retrieve crystal structure by inverting a CBED pattern. In this paper, the zone-axis pattern fitting technique of Bird and Saunders is used to obtain structure factors for copper.
Date: April 1, 1993
Creator: Mansfield, J.; Saunders, M.; Bird, D.; Burgess, G. & Zaluzec, N.
Partner: UNT Libraries Government Documents Department

On the experimental determination of low order structure factors in TiAl by energy filtered convergent beam electron diffraction

Description: It has been claimed that effective Peierls stresses and mobilities of some dislocations in TiAl are influenced by anisotropy of bonding charge densities. This paper describes experiments conducted to verify these calculations. The experiments involved recording convergent beam electron diffraction patterns from a number of reflections, with the sample oriented at the Bragg condition. The rocking curves were matched. The deformation electron charge density distribution map for the (001) section is in good agreement with theory. 8 refs., 2 figs.
Date: April 1, 1993
Creator: Swaminathan, S.; Fraser, H. L.; Jones, I. P.; Zaluzec, N. J. & Maher, D. M.
Partner: UNT Libraries Government Documents Department

Electrical Conductivity Calculations from the Purgatorio Code

Description: The Purgatorio code [Wilson et al., JQSRT 99, 658-679 (2006)] is a new implementation of the Inferno model describing a spherically symmetric average atom embedded in a uniform plasma. Bound and continuum electrons are treated using a fully relativistic quantum mechanical description, giving the electron-thermal contribution to the equation of state (EOS). The free-electron density of states can also be used to calculate scattering cross sections for electron transport. Using the extended Ziman formulation, electrical conductivities are then obtained by convolving these transport cross sections with externally-imposed ion-ion structure factors.
Date: January 9, 2006
Creator: Hansen, S B; Isaacs, W A; Sterne, P A; Wilson, B G; Sonnad, V & Young, D A
Partner: UNT Libraries Government Documents Department

A Thermodynamically-Consistent Non-Ideal Stochastic Hard-Sphere Fluid

Description: A grid-free variant of the Direct Simulation Monte Carlo (DSMC) method is proposed, named the Isotropic DSMC (I-DSMC) method, that is suitable for simulating collision-dominated dense fluid flows. The I-DSMC algorithm eliminates all grid artifacts from the traditional DSMC algorithm and is Galilean invariant and microscopically isotropic. The stochastic collision rules in I-DSMC are modified to introduce a non-ideal structure factor that gives consistent compressibility, as first proposed in [Phys. Rev. Lett. 101:075902 (2008)]. The resulting Stochastic Hard Sphere Dynamics (SHSD) fluid is empirically shown to be thermodynamically identical to a deterministic Hamiltonian system of penetrable spheres interacting with a linear core pair potential, well-described by the hypernetted chain (HNC) approximation. We develop a kinetic theory for the SHSD fluid to obtain estimates for the transport coefficients that are in excellent agreement with particle simulations over a wide range of densities and collision rates. The fluctuating hydrodynamic behavior of the SHSD fluid is verified by comparing its dynamic structure factor against theory based on the Landau-Lifshitz Navier-Stokes equations. We also study the Brownian motion of a nano-particle suspended in an SHSD fluid and find a long-time power-law tail in its velocity autocorrelation function consistent with hydrodynamic theory and molecular dynamics calculations.
Date: August 3, 2009
Creator: Donev, A; Alder, B J & Garcia, A L
Partner: UNT Libraries Government Documents Department

Comparative study of Ti and Ni clusters from first principles

Description: Icosahedral clusters in Ti and Ni are studied with first-principles density functional calculations. We find significant distortion on the Ti icosahedron caused by the strong interaction between surface atoms on the icosahedron but not between the center atom and surface atoms, whereas no such distortion is observed on Ni clusters. In addition, distortion becomes more severe when atoms are added to the Ti13 cluster resulting in short bonds. Such distorted icosahedra having short bonds are essentially to explain the structure factor of Ti liquid obtained in experiment.
Date: August 20, 2007
Creator: Lee, B & Lee, G W
Partner: UNT Libraries Government Documents Department

Inelastic x-ray scattering as a probe of electronic excitations. Solid and liquid metals

Description: Use of inelastic x-ray scattering to study excitations in condensed matter systems is discussed with emphasis on complementarity with inelastic neutron scattering. In particular, studies of electronic excitations in simple metals are detailed. A long standing controversy on the role of crystalline long range order on electron dynamics at intermediate momentum transfers is resolved by comparison of response functions obtained in liquid an solid phase of Li, Na, and Al. They show that the overall shape of the response function is unchanged on melting, implying that it is not determined by the long range order of the ion cores. Conversely, orientationally independent fine structure disappears on melting and therefore must be attributed to band structure-induced transitions, not many-body effects as had previously been argued. At small q, the plasmon lifetime and dispersion remain unchanged on melting. Other experiments are also briefly reviewed.
Date: December 31, 1996
Creator: Hill, J.P.
Partner: UNT Libraries Government Documents Department

Interaction of polymer with discotic clay particles.

Description: Normally synthetic well defined monodisperse discotic laponite clays are known to form a gel phase at mass concentrations as low as a few percent in distilled water. Hydrosoluble polymer polyethylene oxide was added to this intriguing clay system, it was observed that it either prevents gelation or slows it down extremely depending on the polymer weight, concentration or the laponite concentration. Small Angle Neutron scattering (SANS) was used to study these systems because only by isotopic labeling can the structure of the adsorbed polymer layers be determined. The contrast variation technique is specifically used to determine separately the different partial structure factors of the clay and polymer. In this way the signal of the adsorbed chains is separated from the signal of the free chains in the dilute regime. Attempts have also been made to characterize the structure in the concentrated regime of laponite with polymer.
Date: August 4, 1999
Creator: Auvray, L. & Lal, J.
Partner: UNT Libraries Government Documents Department

Extended-range order in glasses

Description: A new type of order is identified in complex glasses, characterized by diffraction peaks at values of the wave vector below those typical of intermediate-range order. Combined neutron and anomalous x-ray diffraction studies of one glass exhibiting this behavior, vitreous rubidium germanate, indicate it to be associated with chemical ordering of the two cations with respect to each other.
Date: March 1, 1994
Creator: Ellison, A. J. G.; Price, D. L.; Saboungi, M. L.; Egami, T.; Hu, Rui-Zhong & Howells, W. S.
Partner: UNT Libraries Government Documents Department

Light scattering investigation of phase separation in a micelle system

Description: We report a real-time, two-dimensional light scattering study of the evolution of structure in a two component nonionic micelle system during phase separation via spinodal decomposition. Our principal finding is that domain growth proceeds much slower than the cube root of time prediction for simple binary fluids. In fact, the growth kinetics can be empirically described as a stretched exponential approach to a pinned domain size. Although the kinetics are not yet understood, this anomalous behavior may be due to the ability of the spherical micelles to reorganize into more complex structures. The domain structure also shows some anomalies. Although at short times the expected structure factor for a critical quench is observed, at long times the structure factor crosses over to the off-critical form. However, in all cases the average scattered intensity is proportional to the cube of the domain size. These findings are discussed in comparison to standard theories of and experimental work on binary fluids.
Date: December 31, 1993
Creator: Wilcoxon, J. P.; Martin, J. E. & Odinek, J.
Partner: UNT Libraries Government Documents Department

Structure of the modulated magnetic phase of Mn{sub 3}Sn

Description: The triangular antiferromagnetic order that occurs in hexagonal Mn{sub 3}Sn below 420 K becomes modulated below 270 K in what is generally described as a helical modulation propagating along the c axis with a period of about 10 or 12 c{sub 0}. However, such a modulation is inconsistent with a recently observed energy gap in the magnetic excitation spectra for this phase. We have reexamined the magnetic structure of this modulated phase by neutron diffraction and find two distinct modulations, both propagating along the c axis and both with periods that are commensurate with the lattice below about 120 K. The satellite reflections associated with these two modulations have quite different intensities and temperature dependences suggesting coexistent modulated phases. For the dominant 11 c{sub 0} phase, we observe a 3rd harmonic with an intensity that is 4% of the 1st harmonic but find no 5th or 7th harmonic to a sensitivity of about 10{sup {minus}4}. The presence of this 3rd harmonic shows a significant distortion of the helical modulation wave. The observed harmonic intensity distribution was compared with magnetic structure factor calculations for configurational models with moment directions bunched around the easy axes in the basal plane. The results of this comparison are given in this paper.
Date: November 1, 1993
Creator: Cable, J. W.; Wakabayashi, N. & Radhakrishna, P.
Partner: UNT Libraries Government Documents Department

Structure and Dynamics of Biological Systems: Integration of Neutron Scattering with Computer Simulation

Description: The combination of molecular dynamics simulation and neutron scattering techniques has emerged as a highly synergistic approach to elucidate the atomistic details of the structure, dynamics and functions of biological systems. Simulation models can be tested by calculating neutron scattering structure factors and comparing the results directly with experiments. If the scattering profiles agree the simulations can be used to provide a detailed decomposition and interpretation of the experiments, and if not, the models can be rationally adjusted. Comparison with neutron experiment can be made at the level of the scattering functions or, less directly, of structural and dynamical quantities derived from them. Here, we examine the combination of simulation and experiment in the interpretation of SANS and inelastic scattering experiments on the structure and dynamics of proteins and other biopolymers.
Date: January 1, 2011
Creator: Smith, Jeremy C.; Krishnan, Marimuthu; Petridis, Loukas & Smolin, Nikolai
Partner: UNT Libraries Government Documents Department

Experimental and computational study of the liquid-solid transition in tin.

Description: An experimental technique was developed to perform isentropic compression of heated liquid tin samples at the Z Accelerator, and multiple such experiments were performed to investigate solidification under rapid compression. Preliminary analyses, using two different methods, of data from experiments with high uncertainty in sample thickness suggest that solidification can begin to occur during isentropic compression on time scales of less than 100 ns. Repeatability of this result has not been confirmed due to technical issues on the subsequent experiments performed. First-principles molecular-dynamics calculations based on density-functional theory showed good agreement with experimentally-determined structure factors for liquid tin, and were used to investigate the equation of state and develop a novel interatomic pseudo-potential for liquid tin and its high-pressure solid phase. Empirical-potential molecular-dynamics calculations, using the new potential, gave results for the solid-liquid interface velocity, which was found to vary linearly with difference in free energy between the solid and liquid phases, as well as the liquidus, the maximum over-pressurization, and the solid-liquid interfacial energy. These data will prove useful in future modeling of solidification kinetics for liquid tin.
Date: October 1, 2005
Creator: Foiles, Stephen Martin & Davis, Jean-Paul
Partner: UNT Libraries Government Documents Department