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

MCViNE (Monte-Carlo VIrtual Neutron Experiment) is a versatile Monte Carlo (MC) neutron ray-tracing program that provides researchers with tools for performing computer modeling and simulations that mirror real neutron scattering experiments. By adopting modern software engineering practices such as using composite and visitor design patterns for representing and accessing neutron scatterers, and using recursive algorithms for multiple scattering, MCViNE is flexible enough to handle sophisticated neutron scattering problems 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 take advantage of simulation components in linear-chain-based MC ray tracing packages widely used in instrument design and optimization, as well as NumPy-based components that make prototypes useful and easy to develop. These developments have enabled us to carry out detailed simulations of neutron scattering experiments with non-trivial samples in 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.Comment: 34 pages, 14 figure

Lattice dynamics reveals a local symmetry breaking in the emergent dipole phase of PbTe

Local symmetry breaking in complex materials is emerging as an important contributor to materials properties but is inherently difficult to study. Here we follow up an earlier structural observation of such a local symmetry broken phase in the technologically important compound PbTe with a study of the lattice dynamics using inelastic neutron scattering (INS). We show that the lattice dynamics are responsive to the local symmetry broken phase, giving key insights in the behavior of PbTe, but also revealing INS as a powerful tool for studying local structure. The new result is the observation of the unexpected appearance on warming of a new zone center phonon branch in PbTe. In a harmonic solid the number of phonon branches is strictly determined by the contents and symmetry of the unit cell. The appearance of the new mode indicates a crossover to a dynamic lower symmetry structure with increasing temperature. No structural transition is seen crystallographically but the appearance of the new mode in inelastic neutron scattering coincides with the observation of local Pb off-centering dipoles observed in the local structure. The observation resembles relaxor ferroelectricity but since there are no inhomogeneous dopants in pure PbTe this anomalous behavior is an intrinsic response of the system. We call such an appearance of dipoles out of a non-dipolar ground-state "emphanisis" meaning the appearance out of nothing. It cannot be explained within the framework of conventional phase transition theories such as soft-mode theory and challenges our basic understanding of the physics of materials

Phonon Lifetime Investigation of Anharmonicity and Thermal Conductivity of UO₂ by Neutron Scattering and Theory

Inelastic neutron scattering measurements of individual phonon lifetimes and dispersion at 295 and 1200 K have been used to probe anharmonicity and thermal conductivity in UO2. They show that longitudinal optic phonon modes carry the largest amount of heat, in contrast to past simulations and that the total conductivity demonstrates a quantitative correspondence between microscopic and macroscopic phonon physics. We have further performed first-principles simulations for UO2 showing semiquantitative agreement with phonon lifetimes at 295 K, but larger anharmonicity than measured at 1200 K

Spin-dynamics of the low-dimensional magnet (CH3)2NH2CuCl3

Dimethylammonium copper (II) chloride (also known as DMACuCl3 or MCCL) is a low dimensional S=1/2 quantum spin system proposed to be an alternating ferro-antiferromagnetic chain with similar magnitude ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions. Subsequently, it was shown that the existing bulk measurements could be adequately modeled by considering DMACuCl3 as independent AFM and FM dimer spin pairs. We present here new inelastic neutron scattering measurements of the spin-excitations in single crystals of DMACuCl3. These results show significant quasi-one-dimensional coupling, however the magnetic excitations do not propagate along the expected direction. We observe a band of excitations with a gap of 0.95 meV and a bandwidth of 0.82 meV.Comment: 3 pages, 2 figures included in text, submitted to proceedings of International Conference on Neutron Scattering, December 200

Frustration-induced diffusive scattering anomaly and dimension change in FeGe2\rm FeGe_2

Magnetic frustration, arising from the competition of exchange interactions, has received great attention because of its relevance to exotic quantum phenomena in materials. In the current work, we report an unusual checkerboard-shaped scattering anomaly in $\rm FeGe_2$, far from the known incommensurate magnetic satellite peaks, for the first time by inelastic neutron scattering. More surprisingly, such phenomenon appears as spin dynamics at low temperature, but it becomes prominent above N\'eel transition as elastic scattering. A new model Hamiltonian that includes an intraplane next-nearest neighbor was proposed and attributes such anomaly to the near-perfect magnetic frustration and the emergence of unexpected two-dimensional magnetic order in the quasi-one-dimensional $\rm FeGe_2$.Comment: 24 pages, 10 figure

Early results and lessons learned from a multicenter, randomized, double-blind trial of bone marrow aspirate concentrate in critical limb ischemia

ObjectivesDespite advances in endovascular therapies, critical limb ischemia (CLI) continues to be associated with high morbidity and mortality. Patients without direct revascularization options have the worst outcomes. We sought to explore the feasibility of conducting a definitive trial of a bone marrow-derived cellular therapy for CLI in this “no option” population.MethodsA pilot, multicenter, prospective, randomized, double-blind, placebo-controlled trial for “no option” CLI patients was performed. The therapy consisted of bone marrow aspirate concentrate (BMAC), prepared using a point of service centrifugation technique and injected percutaneously in 40 injections to the affected limb. Patients were randomized to BMAC or sham injections (dilute blood). We are reporting the 12-week data.ResultsForty-eight patients were enrolled. The mean age was 69.5 years (range, 42-93 years). Males predominated (68%). Diabetes was present in 50%. Tissue loss (Rutherford 5) was present in 30 patients (62.5%), and 18 (37.5%) had rest pain without tissue loss (Rutherford 4). Patients were deemed unsuitable for conventional revascularization based on multiple prior failed revascularization efforts (24 [50%]), poor distal targets (43 [89.6%]), and medical risk (six [12.5%]). Thirty-four patients were treated with BMAC and 14 with sham injections. There were no adverse events attributed to the injections. Renal function was not affected. Effective blinding was confirmed; blinding index of 61% to 85%. Subjective and objective outcome measures were effectively obtained with the exception of treadmill walking times, which could only be obtained at baseline and follow-up in 15 of 48 subjects. This pilot study was not powered to demonstrate statistical significance but did demonstrate favorable trends for BMAC versus control in major amputations (17.6% vs 28.6%), improved pain (44% vs 25%), improved ankle brachial index (ABI; 32.4% vs 7.1%), improved Rutherford classification (35.3% vs 14.3%), and quality-of-life scoring better for BMAC in six of eight domains.ConclusionsIn this multicenter, randomized, double-blind, placebo-controlled trial of autologous bone marrow cell therapy for CLI, the therapy was well tolerated without significant adverse events. The BMAC group demonstrated trends toward improvement in amputation, pain, quality of life, Rutherford classification, and ABI when compared with controls. This pilot allowed us to identify several areas for improvement for future trials and CLI studies. These recommendations include elimination of treadmill testing, stratification by Rutherford class, and more liberal inclusion of patients with renal insufficiency. Our strongest recommendation is that CLI studies that include Rutherford 4 patients should incorporate a composite endpoint reflecting pain and quality of life