J. R. D. Copley

DAVE: A Comprehensive Software Suite for the Reduction, Visualization, and Analysis of Low Energy Neutron Spectroscopic Data

National user facilities such as the NIST Center for Neutron Research (NCNR) require a significant base of software to treat the data produced by their specialized measurement instruments. There is no universally accepted and used data treatment package for the reduction, visualization, and analysis of inelastic neutron scattering data. However, we believe that the software development approach adopted at the NCNR has some key characteristics that have resulted in a successful software package called DAVE (the Data Analysis and Visualization Environment). It is developed using a high level scientific programming language, and it has been widely adopted in the United States and abroad. In this paper we describe the development approach, elements of the DAVE software suite, its usage and impact, and future directions and opportunities for development.

Identifying the Specific Nanostructures Responsible for the High Thermoelectric Performance of (Bi,Sb)2Te3 Nanocomposites

Herein, we report the synthesis of multiscale nanostructured p-type (Bi,Sb)(2)Te(3) bulk materials by melt-spinning single elements of Bi, Sb, and Te followed by a spark plasma sintering process. The samples that were most optimized with the resulting composition (Bi(0.48)Sb(1.52)Te(3)) and specific nanostructures showed an increase of approximately 50% or more in the figure of merit, ZT, over that of the commercial bulk material between 280 and 475 K, making it suitable for commercial applications related to both power generation and refrigeration. The results of high-resolution electron microscopy and small angle and inelastic neutron scattering along with corresponding thermoelectric property measurements corroborate that the 10-20 nm nanocrystalline domains with coherent boundaries are the key constituent that accounts for the resulting exceptionally low lattice thermal conductivity and significant improvement of ZT.

Potential energy, relaxation, vibrational dynamics and the boson peak, of hyperquenched glasses

We describe a combination of laboratory and simulation studies that give quantitative information on the energy landscape for glass-forming liquids. Both types of study focus on the idea of suddenly extracting the thermal energy, so that the system obtained for subsequent study has the structure, and hence potential energy, of a liquid at a much higher temperature than the normal glass temperature Tg .O ne t ype of study gives information on the energy that can be trapped in experimental glasses by hyperquenching, relative to the normal glass, and on the magnitude of barriers separating basins of attraction on the landscape. Stepwise annealing studies also give information on the matter of energy heterogeneity and the question of ‘nanogranularity’ in liquids near Tg .T heother type of study gives information on the vibrational properties of as ystem c onfined to a given basin, and particularly on how that vibrational structure changes with the state of configurational excitation of the liquid. A feature in the low frequency (‘boson peak’) region of the density of vibrational states of the normal glass becomes much stronger in the hyperquenched glass. Qualitatively similar observations are made on heating fragile glass-formers into th es upercooled and stable liquid states. The vibrational dynamics findings are supported and elucidated by constant pressure molecular dynamics/normal mode MD/NM simulations/analysis of the densities of states of different inherent structures of a model fragile liquid (orthoterphenyl (OTP) in the Lewis– Wahnstrom approximation). These show that, when the temperature is raised at constant pressure, the total density of states changes in a manner that can be well represented by a two-Gaussian ‘excitation acros st hecentroid’, leaving a thir da nd major Gaussian component unchanging. The low frequency Gaussian component, which grows with increasing temperature, has a constant peak

Gapped itinerant spin excitations account for missing entropy in the hidden-order state of URu 2 Si 2

Gapped itinerant spin excitations account for missing entropy in the hidden-order state of URu 2 Si 2

Neutron scattering studies of C60 and its compounds

Abstract We describe neutron scattering studies of the structure and dynamics of C60-fullerite. Above the order-disorder phase transition temperature, Tc, diffraction and quasielastic scattering measurements indicate that the molecules are constantly reorienting while their centers remain on a face-centered cubic lattice. Below Tc the molecules are orientationally ordered with four molecules per unit cell. Recent diffraction results suggest that any given molecule occupies one of two possible orientations with unequal probability, and inelastic scattering measurements show that the molecules librate about their equilibrium orientations. The librational amplitude becomes large as the phase transition is approached. Pulsed source measurements yield direct information regarding the structure of the C60 molecule and also support the low temperature two-orientation model. High energy inelastic studies of undoped and alkali metal-doped buckminsterfullerene provide detailed information about intramolecular vibrational frequencies and point to the important role of specific modes in promoting superconductivity at low temperatures.

Nanomagnetic droplets and implications to orbital ordering in La1-xSrxCoO3

Inelastic cold-neutron scattering on LaCoO3 provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic correlations that are dynamic follow the activation to the excited state, identified as the intermediate S = 1 spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in La(1-x)Sr(x)CoO3 , the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.

Translational and rotational dynamics of water in mesoporous silica materials: MCM-41-S and MCM-48-S

We investigated the translational and rotational dynamics of water molecules in mesoporous silica materials MCM-41-S and MCM-48-S using the incoherent quasielastic neutron scattering technique. The range of wave vector transfers Q covered in the measurements was from 0.27 to 1.93 A−1 broad enough to detect both the translational and rotational contributions to the scattering. We used the relaxing-cage models for both translational and rotational motions which we developed earlier, to analyze the QENS spectra and investigated water dynamics in a supercooled range from 250 to 280 K. The results show a marked slowing down of both the translational and rotational relaxation times, and an increasing effect of confinement on the translational motion, as the temperature is lowered.

A comparison of united atom, explicit atom, and coarse-grained simulation models for poly(ethylene oxide).

We compare static and dynamic properties obtained from three levels of modeling for molecular dynamics simulation of poly(ethylene oxide) (PEO). Neutron scattering data are used as a test of each models accuracy. The three simulation models are an explicit atom (EA) model (all the hydrogens are taken into account explicitly), a united atom (UA) model (CH(2) and CH(3) groups are considered as a single unit), and a coarse-grained (CG) model (six united atoms are taken as one bead). All three models accurately describe the PEO static structure factor as measured by neutron diffraction. Dynamics are assessed by comparison to neutron time of flight data, which follow self-motion of protons. Hydrogen atom motion from the EA model and carbon/oxygen atom motion from the UA model closely follow the experimental hydrogen motion, while hydrogen atoms reinserted in the UA model are too fast. The EA and UA models provide a good description of the orientation properties of C-H vectors measured by nuclear magnetic resonance experiments. Although dynamic observables in the CG model are in excellent agreement with their united atom counterparts, they cannot be compared to neutron data because the time after which the CG model is valid is greater than the neutron decay times.

Rotational dynamics and orientational melting of C60: A neutron scattering study

Well‐defined librational excitations have been observed at energies of 2–3 meV in the low temperature ordered phase of solid C60. These relatively high energies imply a stiff orientational potential below the transition. The sharpness of the peaks indicates that this potential does not depend strongly on the axis of the angular displacement. The modes soften and broaden as the temperature approaches that of the orientational melting transition which occurs when the librational amplitude is a considerable fraction of nearest‐neighbor interatomic angles.

Vibrational Density of States of Hydration Water at Biomolecular Sites: Hydrophobicity Promotes Low Density Amorphous Ice Behavior

Inelastic neutron scattering experiments and molecular dynamics simulations have been used to investigate the low frequency modes, in the region between 0 and 100 meV, of hydration water in selected hydrophilic and hydrophobic biomolecules. The results show changes in the plasticity of the hydrogen-bond network of hydration water molecules depending on the biomolecular site. At 200 K, the measured low frequency density of states of hydration water molecules of hydrophilic peptides is remarkably similar to that of high density amorphous ice, whereas, for hydrophobic biomolecules, it is comparable to that of low density amorphous ice behavior. In both hydrophilic and hydrophobic biomolecules, the high frequency modes show a blue shift of the libration mode as compared to the room temperature data. These results can be related to the density of water molecules around the biological interface, suggesting that the apparent local density of water is larger in a hydrophilic environment.