J. Y. Y. Lin

Design and operation of the wide angular-range chopper spectrometer ARCS at the Spallation Neutron Source

The wide angular-range chopper spectrometer ARCS at the Spallation Neutron Source (SNS) is optimized to provide a high neutron flux at the sample position with a large solid angle of detector coverage. The instrument incorporates modern neutron instrumentation, such as an elliptically focused neutron guide, high speed magnetic bearing choppers, and a massive array of (3)He linear position sensitive detectors. Novel features of the spectrometer include the use of a large gate valve between the sample and detector vacuum chambers and the placement of the detectors within the vacuum, both of which provide a window-free final flight path to minimize background scattering while allowing rapid changing of the sample and sample environment equipment. ARCS views the SNS decoupled ambient temperature water moderator, using neutrons with incident energy typically in the range from 15 to 1500 meV. This range, coupled with the large detector coverage, allows a wide variety of studies of excitations in condensed matter, such as lattice dynamics and magnetism, in both powder and single-crystal samples. Comparisons of early results to both analytical and Monte Carlo simulation of the instrument performance demonstrate that the instrument is operating as expected and its neutronic performance is understood. ARCS is currently in the SNS user program and continues to improve its scientific productivity by incorporating new instrumentation to increase the range of science covered and improve its effectiveness in data collection.

Phonon density of states of LaFeAsO(1-x)Fx.

We have studied the phonon density of states (PDOS) in LaFeAsO(1-x)Fx with inelastic neutron scattering methods. The PDOS of the parent compound (x=0) is very similar to the PDOS of samples optimally doped with fluorine to achieve the maximum Tc (x approximately 0.1). Good agreement is found between the experimental PDOS and first-principles calculations with the exception of a small difference in Fe mode frequencies. The PDOS reported here is not consistent with conventional electron-phonon mediated superconductivity.

Phonon Density of States of LaFeAsO1-xFx

We have studied the phonon density of states (PDOS) in LaFeAsO(1-x)Fx with inelastic neutron scattering methods. The PDOS of the parent compound (x=0) is very similar to the PDOS of samples optimally doped with fluorine to achieve the maximum Tc (x approximately 0.1). Good agreement is found between the experimental PDOS and first-principles calculations with the exception of a small difference in Fe mode frequencies. The PDOS reported here is not consistent with conventional electron-phonon mediated superconductivity.

MCViNE – An object oriented Monte Carlo neutron ray tracing simulation package

MCViNE (Monte-Carlo VIrtual Neutron Experiment) is an open-source Monte Carlo (MC) neutron ray-tracing software for performing computer modeling and simulations that mirror real neutron scattering experiments. We exploited the close similarity between how instrument components are designed and operated and how such components can be modeled in software. For example we used object oriented programming concepts for representing neutron scatterers and detector systems, and recursive algorithms for implementing multiple scattering. Combining these features together in MCViNE allows one to handle sophisticated neutron scattering problems in modern instruments, including, for example, neutron detection by complex detector systems, and single and multiple scattering events in a variety of samples and sample environments. In addition, MCViNE can use simulation components from linear-chain-based MC ray tracing packages which facilitates porting instrument models from those codes. Furthermore it allows for components written solely in Python, which expedites prototyping of new components. These developments have enabled detailed simulations of neutron scattering experiments, with non-trivial samples, for time-of-flight inelastic instruments at the Spallation Neutron Source. Examples of such simulations for powder and single-crystal samples with various scattering kernels, including kernels for phonon and magnon scattering, are presented. With simulations that closely reproduce experimental results, scattering mechanisms can be turned on and off to determine how they contribute to the measured scattering intensities, improving our understanding of the underlying physics.

Light atom quantum oscillations in UC and US

High-energy vibrational scattering in the binary systems UC and US is measured using time-of-flight inelastic neutron scattering. A clear set of well-defined peaks equally separated in energy is observed in UC, corresponding to harmonic oscillations of the light C atoms in a cage of heavy U atoms. The scattering is much weaker in US and only a few oscillator peaks are visible. We show how the difference between the materials can be understood by considering the neutron scattering lengths and masses of the lighter atoms. Monte Carlo ray tracing is used to simulate the scattering, with near quantitative agreement with the data in UC, and some differences with US. The possibility of observing anharmonicity and anisotropy in the potentials of the light atoms is investigated in UC. Overall, the observed data is well accounted for by considering each light atom as a single atom isotropic quantum harmonic oscillator.

AtomSim: web-deployed atomistic dynamics simulator

AtomSim, a collection of interfaces for computational crystallography simulations, has been developed. It uses forcefield-based dynamics through physics engines such as the General Utility Lattice Program, and can be integrated into larger computational frameworks such as the Virtual Neutron Facility for processing its dynamics into scattering functions, dynamical functions etc. It is also available as a Google App Engine-hosted web-deployed interface. Examples of a quartz molecular dynamics run and a hafnium dioxide phonon calculation are presented.

Spatial periodicities of defect environments in 57Fe3A1 studied by Mössbauer powder diffractometry

Abstract Mössbauer powder diffractometry was used to study partially-ordered 57Fe3Al. Multiple diffraction patterns were measured at Doppler velocities across all nuclear resonances in the sample. The superlattice diffractions were analyzed to provide data on the long-range order of Fe atoms having different numbers of Al neighbors. Comparing experimental data to calculations showed that Fe atoms having three Al atoms as first-nearest neighbors (1nn) have simple cubic long-range order, similar to that of Fe atoms with four Al 1nn. The simple cubic periodicity of Fe atoms with three Al 1nn was significantly lower than expected for homogeneous antisite disorder, however. Monte-Carlo simulations and transmission electron microscopy suggest that a significant fraction of aperiodic Fe atoms with three Al 1nn are near antiphase domain boundaries.

Phonon Density of States ofLaFeAsO1−xFx

We have studied the phonon density of states (PDOS) in LaFeAsO(1-x)Fx with inelastic neutron scattering methods. The PDOS of the parent compound (x=0) is very similar to the PDOS of samples optimally doped with fluorine to achieve the maximum Tc (x approximately 0.1). Good agreement is found between the experimental PDOS and first-principles calculations with the exception of a small difference in Fe mode frequencies. The PDOS reported here is not consistent with conventional electron-phonon mediated superconductivity.

Mossbauer diffractometry on chemical sites of 57Fe in Fe3Al

A Mössbauer powder diffractometer was used to measure diffraction patterns from polycrystalline 57Fe3Al. The intensities of Bragg diffractions were measured when the incident γ-rays were tuned to different resonances of the Mössbauer spectrum. The bcc fundamental diffractions were strong, and superlattice diffractions of (1/2 1/2 1/2) and (1 0 0) families were measurable. The 57Fe local environments with 0 Al and 4 Al first-nearest-neighbors had the periodicities expected from their sites in the perfect D03 unit cell, but the long-range periodicity of the 3 Al environment was also measured. This disordered site had a surprisingly high degree of B2 long-range order.