4,605 research outputs found
Antenna evaluation study for the shuttle multispectral radar, phase 1
Critical parameters of the shuttle multispectral radar antenna (SMRA) which most affect antenna performance were identified. A preliminary methematical model is presented for describing SMRA performance under the influence of various physical and environmental factors which might degrade performance. Because user groups have not agreed on optimum frequencies best suited for the broadest range of application, the study incorporates frequencies ranging from 1.2 to 14.5 GHz, as well as a consideration of incidence angles from near nadir to nearly 50 deg
Spin Dynamics of a Canted Antiferromagnet in a Magnetic Field
The spin dynamics of a canted antiferromagnet with a quadratic spin-wave
dispersion near \vq =0 is shown to possess a unique signature. When the
anisotropy gap is negligible, the spin-wave stiffness \dsw (\vq, B) =
(\omega_{\vq}-B)/q^2 depends on whether the limit of zero field or zero
wavevector is taken first. Consequently, \dsw is a strong function of
magnetic field at a fixed wavevector. Even in the presence of a sizeable
anisotropy gap, the field dependence of both \dsw and the gap energy
distinguishes a canted antiferromagnet from a phase-separated mixture
containing both ferromagnetic and antiferromagnetic regions.Comment: 10 pages, 3 figure
POTENTIAL EFFECTS OF SUBSIDIZED LIVESTOCK INSURANCE ON LIVESTOCK PRODUCTION
Recent legislation has cleared the way for subsidized livestock price insurance. Such programs could increase production. Expected feeder cattle prices with and without subsidized insurance will be analyzed using E-V and Stochastic Dominance. Results will highlight the potential effects of the program on marketing risk and production decisions.Livestock Production/Industries, Risk and Uncertainty,
Magnetic susceptibility of a CuO2 plane in the La2CuO4 system: I. RPA treatment of the Dzyaloshinskii-Moriya Interactions
Motivated by recent experiments on undoped La2CuO4, which found pronounced
temperature-dependent anisotropies in the low-field magnetic susceptibility, we
have investigated a two-dimensional square lattice of S=1/2 spins that interact
via Heisenberg exchange plus the symmetric and anti-symmetric
Dzyaloshinskii-Moriya anisotropies. We describe the transition to a state with
long-ranged order, and find the spin-wave excitations, with a mean-field
theory, linear spin-wave analysis, and using Tyablikov's RPA decoupling scheme.
We find the different components of the susceptibility within all of these
approximations, both below and above the N'eel temperature, and obtain evidence
of strong quantum fluctuations and spin-wave interactions in a broad
temperature region near the transition.Comment: 20 pages, 2 column format, 22 figure
Compositional stability of FePt nanoparticles on SiO2/Si during annealing
The loss of Fe due to oxidation or diffusion into the substrate can prevent the successful preparation of well-ordered, stoichiometric, FePt nanoparticles. In this work we report the composition changes during annealing observed for small ( \u3c 10 nm) FePt nanoparticles on thermally grown SiO2 layers on Si wafer substrates. Additionally, we describe the use of a controlled reducing gas mixture, Ar+H-2+H2O, to reduce the loss of Fe
Antenna evaluation study for the shuttle multispectral radar, phase 2
The results of the second phase of the Antenna Evaluation Study for the Shuttle Imaging Radar are presented. The objectives of Phase II were (1) to complete the specifications for the subarray test panels, (2) to begin a study of the effects of electrical and mechanical tolerance variations on overall SIRA performance, (3) to initiate the development of a mathematical model which adequately described the array performance and (4) to begin the development of a comprehensive computer program which will eventually simulate the performance characteristics of the antenna in a spaceborne environment. Items (2), (3), and (4) were begun in Phase I (ahead of schedule), and because of this, it has been possible to accelerate the Phase II modeling/simulation objectives to the point where simulations of expected mechanical/electrical errors have already been produced
Monte Carlo simulation with time step quantification in terms of Langevin dynamics
For the description of thermally activated dynamics in systems of classical
magnetic moments numerical methods are desirable. We consider a simple model
for isolated magnetic particles in a uniform field with an oblique angle to the
easy axis of the particles. For this model, a comparison of the Monte Carlo
method with Langevin dynamics yields new insight in the interpretation of the
Monte Carlo process, leading to the implementation of a new algorithm where the
Monte Carlo step is time-quantified. The numeric results for the characteristic
time of the magnetisation reversal are in excellent agreement with asymptotic
solutions which itself are in agreement with the exact numerical results
obtained from the Fokker-Planck equation for the Neel-Brown model.Comment: 5 pages, Revtex, 4 Figures include
SYMPOSIUM ON RECENT DEVELOPMENTS IN DATA ANALYSIS
The purpose of this symposium is to present recent developments in biomechanical data analyses in two areas. First, current methods used in a dynamical systems approach will be described. Second, two statistical approaches, Principal Components Analysis and Functional Data Analysis, will be presented. The emphasis in this symposium will be on how to use each of these recent analysis techniques
Accurate Results from Perturbation Theory for Strongly Frustrated Heisenberg Spin Clusters
We investigate the use of perturbation theory in finite sized frustrated spin
systems by calculating the effect of quantum fluctuations on coherent states
derived from the classical ground state. We first calculate the ground and
first excited state wavefunctions as a function of applied field for a 12-site
system and compare with the results of exact diagonalization. We then apply the
technique to a 20-site system with the same three fold site coordination as the
12-site system. Frustration results in asymptotically convergent series for
both systems which are summed with Pad\'e approximants.
We find that at zero magnetic field the different connectivity of the two
systems leads to a triplet first excited state in the 12-site system and a
singlet first excited state in the 20-site system, while the ground state is a
singlet for both. We also show how the analytic structure of the Pad\'e
approximants at evolves in the complex plane at
the values of the applied field where the ground state switches between spin
sectors and how this is connected with the non-trivial dependence of the
number on the strength of quantum fluctuations. We discuss the origin
of this difference in the energy spectra and in the analytic structures. We
also characterize the ground and first excited states according to the values
of the various spin correlation functions.Comment: Final version, accepted for publication in Physical review
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