371 research outputs found
Scalable parallel generation of partitioned, unstructured meshes
In this paper we are concerned with the parallel generation of unstructured meshes for use in the finite element solution of computational dynamics problems on parallel distributed memory computers
The impact of broadband in schools
The report reviews evidence for the impact of broadband in English schools, exploring; Variations in provision in level of broadband connectivity; Links between the level of broadband activity and nationally accessible performance data; Aspects of broadband connectivity and the school environment that contribute to better outcomes for pupils and teachers; Academic and motivational benefits associated with educational uses of this technology
Helicoidal magnetic order in a clean copper oxide spin chain compound
We report susceptibility, specific heat, and neutron diffraction measurements
on NaCuO, a spin-1/2 chain compound isostructural to LiCuO,
which has been extensively investigated. Below 13 K, we find a long-range
ordered, incommensurate magnetic helix state with a propagation vector similar
to that of LiCuO. In contrast to the Li analogue, substitutional
disorder is negligible in NaCuO. We can thus rule out that the helix is
induced by impurities, as was claimed on the basis of prior work on
LiCuO. A spin Hamiltonian with frustrated longer-range exchange
interactions provides a good description of both the ordered state and the
paramagnetic susceptibility.Comment: 4 pages, 4 figures Improved Fig.1 and 4. Minor rephrasing. Reference
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Analysis of and workarounds for element reversal for a finite element-based algorithm for warping triangular and tetrahedral meshes
We consider an algorithm called FEMWARP for warping triangular and
tetrahedral finite element meshes that computes the warping using the finite
element method itself. The algorithm takes as input a two- or three-dimensional
domain defined by a boundary mesh (segments in one dimension or triangles in
two dimensions) that has a volume mesh (triangles in two dimensions or
tetrahedra in three dimensions) in its interior. It also takes as input a
prescribed movement of the boundary mesh. It computes as output updated
positions of the vertices of the volume mesh. The first step of the algorithm
is to determine from the initial mesh a set of local weights for each interior
vertex that describes each interior vertex in terms of the positions of its
neighbors. These weights are computed using a finite element stiffness matrix.
After a boundary transformation is applied, a linear system of equations based
upon the weights is solved to determine the final positions of the interior
vertices. The FEMWARP algorithm has been considered in the previous literature
(e.g., in a 2001 paper by Baker). FEMWARP has been succesful in computing
deformed meshes for certain applications. However, sometimes FEMWARP reverses
elements; this is our main concern in this paper. We analyze the causes for
this undesirable behavior and propose several techniques to make the method
more robust against reversals. The most successful of the proposed methods
includes combining FEMWARP with an optimization-based untangler.Comment: Revision of earlier version of paper. Submitted for publication in
BIT Numerical Mathematics on 27 April 2010. Accepted for publication on 7
September 2010. Published online on 9 October 2010. The final publication is
available at http://www.springerlink.co
Electronic correlations in FeGa3 and the effect of hole doping on its magnetic properties
We investigate signatures of electronic correlations in the narrow-gap semiconductor FeGa 3 by means of electrical resistivity and thermodynamic measurements performed on single crystals of FeGa 3 , Fe 1−x Mn x Ga 3 , and FeGa 3−y Zn y , complemented by a study of the 4d analog material RuGa 3 . We find that the inclusion of sizable amounts of Mn and Zn dopants into FeGa 3 does not induce an insulator-to-metal transition. Our study indicates that both substitution of Zn onto the Ga site and replacement of Fe by Mn introduces states into the semiconducting gap that remain localized even at highest doping levels. Most importantly, using neutron powder diffraction measurements, we establish that FeGa 3 orders magnetically above room temperature in a complex structure, which is almost unaffected by the doping with Mn and Zn. Using realistic many-body calculations within the framework of dynamical mean field theory (DMFT), we argue that while the iron atoms in FeGa 3 are dominantly in an S=1 state, there are strong charge and spin fluctuations on short-time scales, which are independent of temperature. Further, the low magnitude of local contributions to the spin susceptibility advocates an itinerant mechanism for the spin response in FeGa 3 . Our joint experimental and theoretical investigations classify FeGa 3 as a correlated band insulator with only small dynamical correlation effects, in which nonlocal exchange interactions are responsible for the spin gap of 0.4 eV and the antiferromagnetic order. We show that hole doping of FeGa 3 leads, within DMFT, to a notable strengthening of many-body renormalizations
Thermal Casimir Force between Magnetic Materials
We investigate the Casimir pressure between two parallel plates made of
magnetic materials at nonzero temperature. It is shown that for real
magnetodielectric materials only the magnetic properties of ferromagnets can
influence the Casimir pressure. This influence is accomplished through the
contribution of the zero-frequency term of the Lifshitz formula. The
possibility of the Casimir repulsion through the vacuum gap is analyzed
depending on the model used for the description of the dielectric properties of
the metal plates.Comment: 9 pages, 3 figures. Contribution to the Proceedings of QFEXT09,
Norman, OK, September 21-25, 200
Computational Efficiency Of The Aerosol Scheme In The Met Office Unified Model
A new data structuring has been implemented which improves the performance of the aerosol subsystem in the UK Met Office Unified Model. Smaller amounts of atmospheric data, in the arrangement of segments of atmospheric columns, are handed to the aerosol sub-processes. The number of columns that are in a segment can be changed at runtime and is tuned to the hardware and science in operation. This revision alone has halved the time spent in some of the aerosol sections for the case under investigation. The new arrangement allows simpler implementation of OpenMP around the whole of the aerosol subsystem and gives close to ideal speed up. Applying a dynamic schedule or retaining a simpler static schedule for the OpenMP parallel loop are shown to differ related to the number of threads
Magnetic, Thermal, and Transport Properties of Layered Arsenides BaRu2As2 and SrRu2As2
The magnetic, thermal and transport properties of polycrystalline BaRu2As2
and SrRu2As2 samples with the ThCr2Si2 structure were investigated by means of
magnetic susceptibility chi(T), electrical resistivity rho(T), and heat
capacity Cp(T) measurements. The temperature (T) dependence of rho indicates
metallic character for both compounds with residual resistivity ratios rho(310
K)/rho(2 K) of 17 and 5 for the Ba and Sr compounds, respectively. The Cp(T)
results reveal a low-T Sommerfeld coefficient gamma = 4.9(1) and 4.1(1) mJ/mol
K^2 and Debye temperature \Theta_D = 271(7) K and 271(4) K for the Ba and Sr
compounds, respectively. The chi (T) was found to be diamagnetic with a small
absolute value for both compounds. No transitions were found for BaRu2As2 above
1.8 K. The chi(T) data for SrRu2As2 exhibit a cusp at \sim 200 K, possibly an
indication of a structural and/or magnetic transition. We discuss the
properties of BaRu2As2 and SrRu2As2 in the context of other ThCr2Si2-type and
ZrCuSiAs-type transition metal pnictides.Comment: 6 pages, 6 figures; v2: additional discussion of the relationship
with FeAs-type materials and the importance of Stoner enhancement of the
susceptibilit
Coded apertures with scatter and partial attenuation for high-energy high-resolution imaging
Laser-plasma x-ray sources have garnered interest from various communities due to their ability to generate high photon-energies from a small source size. The passive imaging of high energy x-rays and neutrons is also a useful diagnostic in laser-driven fusion capsules as well as laboratory astrophysics experiments which aim to study small samples of transient electron-positron plasmas. Here we study a coded aperture with scatter and partial attenuation included, which we call a 'CASPA', and compare them to the more common method of pinhole imaging. As well as discussing the well-known increased throughput of coded apertures, we also show that the decoding algorithm relaxes the need for a thick substrate. We simulate a 511 keV x-ray source through ray-tracing and Geant4 simulations to show how incomplete attenuation of the source by the mask may be less detrimental to imaging using a CASPA than to using a standard pinhole system
Doping effects in the coupled, two-leg spin ladder BiCu2PO6
We report preparation, x-ray diffraction, magnetic susceptibility chi(T) and
heat capacity Cp(T) measurements on the undoped samples as also samples with
Zn-doped (S = 0) at Cu site, Ni doped (S = 1) at Cu site, and Ca-doped (holes)
at Bi site in the coupled two-leg spin ladder system BiCu2PO6. While, Zn shows
complete solid solubility, Ni could be doped to about 20% and Ca to about 15%.
Magnetization and heat capacity data in the undoped compound point towards the
existence of frustration effects. In all the samples, the chi(T) at low
temperature increases with doping content. The Zn-induced susceptibility is
smaller than that due to effective S=1/2 moments possibly due to frustrating
next-nearest-neighbor interactions along the leg. For Zn content x > 0.01,
chi(T) deviates from the Curie-law at low temperatures. The magnetic specific
heat data Cm(T) for the Zn-doped samples show weak anomalies at low temperature
in agreement with chi(T) behavior. The anomalies are suggestive of spin
freezing at low-T. In contrast, prominent effects are observed in chi(T) and
Cm(T) on Ni-doped samples. The zero-field-cooled (ZFC) and field-cooled (FC)
chi(T) data are different from each other at low temperature unlike that for Zn
doped samples, clearly indicating a transition to a spin-glass like phase. No
anomalies were found in Ca- or Pb-doped samples.Comment: 16 pages, 9 figures, Submitted to J. Phy. Cond. Matte
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