21,183 research outputs found
Spin glass behavior in an interacting gamma-Fe2O3 nanoparticle system
In this paper we investigate the superspin glass behavior of a concentrated
assembly of interacting maghemite nanoparticles and compare it to that of
canonical atomic spin glass systems. ac versus temperature and frequency
measurements show evidence of a superspin glass transition taking place at low
temperature. In order to fully characterize the superspin glass phase, the
aging behavior of both the thermo-remanent magnetization (TRM) and ac
susceptibility has been investigated. It is shown that the scaling laws obeyed
by superspin glasses and atomic spin glasses are essentially the same, after
subtraction of a superparamagnetic contribution from the superspin glass
response functions. Finally, we discuss a possible origin of this
superparamagnetic contribution in terms of dilute spin glass models
On the Identification of Symmetric Quadrature Rules for Finite Element Methods
In this paper we describe a methodology for the identification of symmetric
quadrature rules inside of quadrilaterals, triangles, tetrahedra, prisms,
pyramids, and hexahedra. The methodology is free from manual intervention and
is capable of identifying an ensemble of rules with a given strength and a
given number of points. We also present polyquad which is an implementation of
our methodology. Using polyquad we proceed to derive a complete set of
symmetric rules on the aforementioned domains. All rules possess purely
positive weights and have all points inside the domain. Many of the rules
appear to be new, and an improvement over those tabulated in the literature.Comment: 17 pages, 6 figures, 1 tabl
A Spatial Analysis of Farm Payment Recipients Using the FSA 1614 Dataset
We report results from preliminary analysis of the recently constructed dataset from the Farm Service Agency, FSA 1614. FSA 1614 provides the location of the farm and the farm payment recipient for all Title I payments. This makes it possible to analyze the spatial dispersion between landowner and farm more precisely than previously possible. A discussion of what research questions could be informed through the use of this data is provided. We find that a significant percentage of payments are sent to individuals that are likely to be absentee landowners, although this value is much smaller when looking at the total value of payments. These national results are compared to four corn belt states.Agricultural Finance,
Quantitative computer simulations of extraterrestrial processing operations
The automation of a small, solid propellant mixer was studied. Temperature control is under investigation. A numerical simulation of the system is under development and will be tested using different control options. Control system hardware is currently being put into place. The construction of mathematical models and simulation techniques for understanding various engineering processes is also studied. Computer graphics packages were utilized for better visualization of the simulation results. The mechanical mixing of propellants is examined. Simulation of the mixing process is being done to study how one can control for chaotic behavior to meet specified mixing requirements. An experimental mixing chamber is also being built. It will allow visual tracking of particles under mixing. The experimental unit will be used to test ideas from chaos theory, as well as to verify simulation results. This project has applications to extraterrestrial propellant quality and reliability
Memory interference effects in spin glasses
When a spin glass is cooled down, a memory of the cooling process is
imprinted in the spin structure. This memory can be disclosed in a continuous
heating measurement of the ac-susceptibility. E.g., if a continuous cooling
process is intermittently halted during a certain aging time at one or two
intermediate temperatures, the trace of the previous stop(s) is recovered when
the sample is continuously re-heated [1]. However, heating the sample above the
aging temperature, but keeping it below Tg, erases the memory of the thermal
history at lower temperatures. We also show that a memory imprinted at a higher
temperature can be erased by waiting a long enough time at a lower temperature.
Predictions from two complementary spin glass descriptions, a hierarchical
phase space model and a real space droplet picture are contested with these
memory phenomena and interference effects.
[1] K. Jonason, E. Vincent, J. Hammann, J. P. Bouchaud and P. Nordblad, Phys.
Rev. Lett. 31, 3243 (1998).Comment: 7 pages, 1 LaTex file + 5 figures in EPS Revised version of June 17,
1999 (minor changes), to appear in EPJ B around November 9
Heterogeneous Computing on Mixed Unstructured Grids with PyFR
PyFR is an open-source high-order accurate computational fluid dynamics
solver for mixed unstructured grids that can target a range of hardware
platforms from a single codebase. In this paper we demonstrate the ability of
PyFR to perform high-order accurate unsteady simulations of flow on mixed
unstructured grids using heterogeneous multi-node hardware. Specifically, after
benchmarking single-node performance for various platforms, PyFR v0.2.2 is used
to undertake simulations of unsteady flow over a circular cylinder at Reynolds
number 3 900 using a mixed unstructured grid of prismatic and tetrahedral
elements on a desktop workstation containing an Intel Xeon E5-2697 v2 CPU, an
NVIDIA Tesla K40c GPU, and an AMD FirePro W9100 GPU. Both the performance and
accuracy of PyFR are assessed. PyFR v0.2.2 is freely available under a 3-Clause
New Style BSD license (see www.pyfr.org).Comment: 21 pages, 9 figures, 6 table
An integrated database with system optimization and design features
A customized, mission-specific relational database package was developed to allow researchers working on the Mars oxygen manufacturing plant to enter physical description, engineering, and connectivity data through a uniform, graphical interface and to store the data in formats compatible with other software also developed as part of the project. These latter components include an optimization program to maximize or minimize various criteria as the system evolves into its final design; programs to simulate the behavior of various parts of the plant in Martian conditions; an animation program which, in different modes, provides visual feedback to designers and researchers about the location of and temperature distribution among components as well as heat, mass, and data flow through the plant as it operates in different scenarios; and a control program to investigate the stability and response of the system under different disturbance conditions. All components of the system are interconnected so that changes entered through one component are reflected in the others
Imaging a boson star at the Galactic center
Millimeter very long baseline interferometry will soon produce accurate
images of the closest surroundings of the supermassive compact object at the
center of the Galaxy, Sgr A*. These images may reveal the existence of a
central faint region, the so-called shadow, which is often interpreted as the
observable consequence of the event horizon of a black hole. In this paper, we
compute images of an accretion torus around Sgr A* assuming this compact object
is a boson star, i.e. an alternative to black holes within general relativity,
with no event horizon and no hard surface. We show that very relativistic
rotating boson stars produce images extremely similar to Kerr black holes,
showing in particular shadow-like and photon-ring-like structures. This result
highlights the extreme difficulty of unambiguously telling the existence of an
event horizon from strong-field images.Comment: 21 pages, 9 figures, accepted in CQG; main difference wrt previous
version is the last paragraph of the conclusio
- …