2,651 research outputs found
Structural and Magnetic Characteristics of MnAs Nanoclusters Embedded in Be-doped GaAs
We describe a systematic study of the synthesis, microstructure and
magnetization of hybrid ferromagnet-semiconductor nanomaterials comprised of
MnAs nanoclusters embedded in a p-doped GaAs matrix. These samples are created
during the in situ annealing of Be-doped (Ga,Mn)As heterostructures grown by
molecular beam epitaxy. Transmission electron microscopy and magnetometry
studies reveal two distinct classes of nanoclustered samples whose structural
and magnetic properties depend on the Mn content of the initial (Ga,Mn)As
layer. For Mn content in the range 5% - 7.5%, annealing creates a
superparamagnetic material with a uniform distribution of small clusters
(diameter around 6 nm) and with a low blocking temperature (T_B approximately
10 K). While transmission electron microscopy cannot definitively identify the
composition and crystalline phase of these small clusters, our experimental
data suggest that they may be comprised of either zinc-blende MnAs or Mn-rich
regions of (Ga,Mn)As. At higher Mn content (> 8 %), we find that annealing
results in an inhomogeneous distribution of both small clusters as well as much
larger NiAs-phase MnAs clusters (diameter around 25 nm). These samples also
exhibit supermagnetism, albeit with substantially larger magnetic moments and
coercive fields, and blocking temperatures well above room temperature
Using Whole Language Materials In The Adult ESOL Classroom
Many English for Speakers of Other Languages (ESOL) students live in areas in which the English language is not the principal means of communication in everyday life. However, because English is the principal language of the broader society, non-English speakers are motivated to attend ESOL classes in order to participate in the mainstream and to receive society\u27s benefits. The classroom is the principal source of meaningful English input for these students. Opportunities for second language acquisition are limited once the students are outside the classroom. This reality makes the quantity and quality of language presented in the classroom a crucial factor in students success in acquiring English as a new language. This practicum explores 1) the nature of language acquisition and its affect upon the roles of the teacher, student and classroom and 2) the use of whole language materials in the classroom. Alternatives to commercial ESOL materials are suggested in order to aid the English language acquisition process and to enhance student exposure to real language. The adult education center at which this practicum was implemented has been offering a language learning ESOL program based on the use of publisher textbooks. The problem which this study addresses is that these ESOL materials have not provided sufficient English language input and that as a result, students lack contact with English as a whole language. This practicum suggests the means through which students may be exposed to natural and whole language through the development of language acquisition procedures and activities. Implementation was carried out in a beginning ESOL classroom with the use of whole language materials. The level of student success was determined by measuring the increase in student writing and oral language production through classroom and non-classroom activities. The results indicate that the use of whole-language materials in the ESOL classroom promotes language acquisition and language production at a faster rate than the use of commercial ESOL materials in a language learning environment. Appendices include step-by-step procedures 1or using whole-language materials in the ESOL classroom, sample student progress charts, and a student interests survey
Boundary conditions and the entropy bound
The entropy-to-energy bound is examined for a quantum scalar field confined
to a cavity and satisfying Robin condition on the boundary of the cavity. It is
found that near certain points in the space of the parameter defining the
boundary condition the lowest eigenfrequency (while non-zero) becomes
arbitrarily small. Estimating, according to Bekenstein and Schiffer, the ratio
by the -function, , we compute
explicitly and find that it is not bounded near those points that signals
violation of the bound. We interpret our results as imposing certain
constraints on the value of the boundary interaction and estimate the forbidden
region in the parameter space of the boundary conditions.Comment: 16 pages, latex, v2: typos corrected, to appear in Phys.Rev.
Study of Convective Flow Effects in Endwall Casing Treatments in Transonic Compressor Rotors
The unsteady convective flow effects in a transonic compressor rotor with a circumferential-groove casing treatment are investigated in this paper. Experimental results show that the circumferential-groove casing treatment increases the compressor stall margin by almost 50% for the current transonic compressor rotor. Steady flow simulation of the current casing treatment, however, yields only a 15% gain in stall margin. The flow field at near-stall operation is highly unsteady due to several self-induced flow phenomena. These include shock oscillation, vortex shedding at the trailing edge, and interaction between the passage shock and the tip clearance vortex. The primary focus of the current investigation is to assess the effects of flow unsteadiness and unsteady flow convection on the circumferential-groove casing treatment. Unsteady Reynolds-averaged Navier-Stokes (URANS) and Large Eddy Simulation (LES) techniques were applied in addition to steady Reynolds-averaged Navier-Stokes (RANS) to simulate the flow field at near-stall operation and to determine changes in stall margin. The current investigation reveals that unsteady flow effects are as important as steady flow effects on the performance of the circumferential grooves casing treatment in extending the stall margin of the current transonic compressor rotor. The primary unsteady flow mechanism is unsteady flow injection from the grooves into the main flow near the casing. Flows moving into and out of the grooves are caused due to local pressure difference near the grooves. As the pressure field becomes transient due to self-induced flow oscillation, flow injection from the grooves also becomes unsteady. The unsteady flow simulation shows that this unsteady flow injection from the grooves is substantial and contributes significantly to extending the compressor stall margin. Unsteady flows into and out of the grooves have as large a role as steady flows in the circumferential grooves. While the circumferential-groove casing treatment seems to be a steady flow device, unsteady flow effects should be included to accurately assess its performance as the flow is transient at near-stall operation
Advanced Methods for Static and Dynamic Shafting Calculations
This article introduces a computer program developed by Wärtsilä Switzerland which provides a three-dimensional model of the shafting for the calculation of coupled vibrations, alignment and whirling in a ship propulsion plant. Based on the fi nite-element method the program covers both dynamic and static problems. Each node has six degrees of freedom. The following features are included: calculation in frequency range and time domain, linear and nonlinear bearing characteristic, consideration of variation of running gear inertia, and optimization of engine parameters. The mathematical model of the shaft line with all associated parameters and boundary conditions are
represented by calculation results and validated by measurement. An everyday example for the calculation of coupled axial vibrations with the associated measured data is shown in this paper.
Furthermore the infl uence of the variation of torsional inertia is demonstrated and a possibility for reduction of torsional stress in the crankshaft by injection timing optimization is explained. As an
example for the static capabilities the reverse bearing offset calculation based on measured web defl ections, bearing loads and bending moments is demonstrated
Measurements of Nanoscale Domain Wall Flexing in a Ferromagnetic Thin Film
We use the high spatial sensitivity of the anomalous Hall effect in the
ferromagnetic semiconductor Ga1-xMnxAs, combined with the magneto-optical Kerr
effect, to probe the nanoscale elastic flexing behavior of a single magnetic
domain wall in a ferromagnetic thin film. Our technique allows position
sensitive characterization of the pinning site density, which we estimate to be
around 10^14 cm^{-3}. Analysis of single site depinning events and their
temperature dependence yields estimates of pinning site forces (10 pN range) as
well as the thermal deactivation energy. Finally, our data hints at a much
higher intrinsic domain wall mobility for flexing than previously observed in
optically-probed micron scale measurements
Thermodynamic Study of Excitations in a 3D Spin Liquid
In order to characterize thermal excitations in a frustrated spin liquid, we
have examined the magnetothermodynamics of a model geometrically frustrated
magnet. Our data demonstrate a crossover in the nature of the spin excitations
between the spin liquid phase and the high-temperature paramagnetic state. The
temperature dependence of both the specific heat and magnetization in the spin
liquid phase can be fit within a simple model which assumes that the spin
excitations have a gapped quadratic dispersion relation.Comment: 5 figure
Quantum Mechanics, Common Sense and the Black Hole Information Paradox
The purpose of this paper is to analyse, in the light of information theory
and with the arsenal of (elementary) quantum mechanics (EPR correlations,
copying machines, teleportation, mixing produced in sub-systems owing to a
trace operation, etc.) the scenarios available on the market to resolve the
so-called black-hole information paradox. We shall conclude that the only
plausible ones are those where either the unitary evolution of quantum
mechanics is given up, in which information leaks continuously in the course of
black-hole evaporation through non-local processes, or those in which the world
is polluted by an infinite number of meta-stable remnants.Comment: 15 pages, Latex, CERN-TH.6889/9
Spiraling Solitons: a Continuum Model for Dynamical Phyllotaxis and Beyond
A novel, protean, topological soliton has recently been shown to emerge in
systems of repulsive particles in cylindrical geometries, whose statics is
described by the number-theoretical objects of phyllotaxis. Here we present a
minimal and local continuum model that can explain many of the features of the
phyllotactic soliton, such as locked speed, screw shift, energy transport and,
for Wigner crystal on a nanotube, charge transport. The treatment is general
and should apply to other spiraling systems. Unlike e.g. Sine-Gornon-like
systems, our solitons can exist between non-degenerate structure, imply a power
flow through the system, dynamics of the domains it separates; we also predict
pulses, both static and dynamic. Applications include charge transport in
Wigner Crystals on nanotubes or A- to B-DNA transitions.Comment: 8 Pages, 6 Figures, Phys Rev E in pres
Amplitude dependent frequency, desynchronization, and stabilization in noisy metapopulation dynamics
The enigmatic stability of population oscillations within ecological systems
is analyzed. The underlying mechanism is presented in the framework of two
interacting species free to migrate between two spatial patches. It is shown
that that the combined effects of migration and noise cannot account for the
stabilization. The missing ingredient is the dependence of the oscillations'
frequency upon their amplitude; with that, noise-induced differences between
patches are amplified due to the frequency gradient. Migration among
desynchronized regions then stabilizes a "soft" limit cycle in the vicinity of
the homogenous manifold. A simple model of diffusively coupled oscillators
allows the derivation of quantitative results, like the functional dependence
of the desynchronization upon diffusion strength and frequency differences. The
oscillations' amplitude is shown to be (almost) noise independent. The results
are compared with a numerical integration of the marginally stable
Lotka-Volterra equations. An unstable system is extinction-prone for small
noise, but stabilizes at larger noise intensity
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