8,233 research outputs found
Mössbauer Spectrometry Study of Thermally-Activated Electronic Processes in Li_xFePO_4
The solid solution phase of Li_xFePO_4 with different Li concentrations, x, was investigated by Mössbauer spectrometry at temperatures between 25 and 210 °C. The Mössbauer spectra show a temperature dependence of their isomer shifts (E_(IS)) and electric quadrupole splittings (E_Q), typical of thermally activated, electronic relaxation processes involving ^(57)Fe ions. The activation energies for the fluctuations of E_Q and E_(IS) for Fe^(3+) are nearly the same, 570 ± 9 meV, suggesting that both originate from charge hopping. For the Fe^(2+) components of the spectra, the fluctuations of E_Q occurred at lower temperatures than the fluctuations of E_(IS), with an activation energy of 512 ± 12 meV for E_Q and one of 551 ± 7 meV for E_(IS). The more facile fluctuations of E_Q for Fe^(2+) are evidence for local motions of neighboring Li^+ ions. It appears that the electron hopping frequency is lower than that of Li^+ ions. The activation energies of relaxation did not have a measurable dependence on the concentration of lithium, x
Phase Diagram of Li_xFePO_4
The phase diagram for LixFePO4 has been determined for different lithium concentrations and temperatures. The two low-temperature phases, heterosite and triphylite, have previously been shown to transform to a disordered solid solution at elevated temperatures. This disordered phase allows for a continuous transition between the heterosite and triphylite phases and is stable at relatively low temperatures. At intermediate temperatures the proposed phase diagram resembles a eutectoid system, with eutectoid point at around x=0.6 and 200°C. Kinetics of mixing and unmixing transformations are reported, including the hysteresis between heating and cooling. The enthalpy of this transition is at least 700 J/mol
Practical considerations in aeroelastic design
The structural design process for large transport aircraft is described. Critical loads must be determined from a large number of load cases within the flight maneuver envelope. The structural design is also constrained by considerations of producibility, reliability, maintainability, durability, and damage tolerance, as well as impact dynamics and multiple constraints due to flutter and aeroelasticity. Aircraft aeroelastic design considerations in three distinct areas of product development (preliminary design, advanced design, and detailed design) are presented and contrasted. The present state of the art is challenged to solve the practical difficulties associated with design, analysis, and redesign within cost and schedule constraints. The current practice consists of largely independent engineering disciplines operating with unorganized data interfaces. The need is then demonstrated for a well-planned computerized aeroelastic structural design optimization system operating with a common interdisciplinary data base. This system must incorporate automated interfaces between modular programs. In each phase of the design process, a common finite-element model for static and dynamic optimization is required to reduce errors due to modeling discrepancies. As the design proceeds from the simple models in preliminary design to the more complex models in advanced and detailed design, a means of retrieving design data from the previous models must be established
Localized matter-waves patterns with attractive interaction in rotating potentials
We consider a two-dimensional (2D) model of a rotating attractive
Bose-Einstein condensate (BEC), trapped in an external potential. First, an
harmonic potential with the critical strength is considered, which generates
quasi-solitons at the lowest Landau level (LLL). We describe a family of the
LLL quasi-solitons using both numerical method and a variational approximation
(VA), which are in good agreement with each other. We demonstrate that kicking
the LLL mode or applying a ramp potential sets it in the Larmor (cyclotron)
motion, that can also be accurately modeled by the VA.Comment: 13 pages, 11 figure
Two point correlations of a trapped interacting Bose gas at finite temperature
We develop a computationally tractable method for calculating correlation
functions of the finite temperature trapped Bose gas that includes the effects
of s-wave interactions. Our approach uses a classical field method to model the
low energy modes and treats the high energy modes using a Hartree-Fock
description. We present results of first and second order correlation
functions, in position and momentum space, for an experimentally realistic
system in the temperature range of to . We also characterize
the spatial coherence length of the system. Our theory should be applicable in
the critical region where experiments are now able to measure first and second
order correlations.Comment: 9 pages, 4 figure
Vortex stability in nearly two-dimensional Bose-Einstein condensates with attraction
We perform accurate investigation of stability of localized vortices in an
effectively two-dimensional ("pancake-shaped") trapped BEC with negative
scattering length. The analysis combines computation of the stability
eigenvalues and direct simulations. The states with vorticity S=1 are stable in
a third of their existence region, , where is
the number of atoms, and is the corresponding collapse
threshold. Stable vortices easily self-trap from arbitrary initial
configurations with embedded vorticity. In an adjacent interval, , the unstable vortex
periodically splits in two fragments and recombines. At , the fragments do not recombine, as each one collapses by
itself. The results are compared with those in the full 3D Gross-Pitaevskii
equation. In a moderately anisotropic 3D configuration, with the aspect ratio
, the stability interval of the S=1 vortices occupies
of their existence region, hence the 2D limit provides for a reasonable
approximation in this case. For the isotropic 3D configuration, the stability
interval expands to 65% of the existence domain. Overall, the vorticity
heightens the actual collapse threshold by a factor of up to 2. All vortices
with are unstable.Comment: 21 pages, 8 figures, to appear in Physical Review
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Speech production in children with Down's syndrome: The effects of reading, naming and imitation
People with DS are known to have difficulties with expressive language, and often have difficulties with intelligibility. They often have stronger visual than verbal short-term memory skills and, therefore, reading has often been suggested as an intervention for speech and language in this population. However, there is as yet no firm evidence that reading can improve speech outcomes. This study aimed to compare reading, picture naming and repetition for the same 10 words, to identify if the speech of eight children with DS (aged 11-14 years) was more accurate, consistent and intelligible when reading. Results show that children were slightly, yet significantly, more accurate and intelligible when they read words compared with when they produced those words in naming or imitation conditions although the reduction in inconsistency was non-significant. The results of this small-scale study provide tentative support for previous claims about the benefits of reading for children with DS. The mechanisms behind a facilitatory effect of reading are considered, and directions are identified for future research
Discrete surface solitons in two dimensions
We investigate fundamental localized modes in 2D lattices with an edge
(surface). Interaction with the edge expands the stability area for ordinary
solitons, and induces a difference between perpendicular and parallel dipoles;
on the contrary, lattice vortices cannot exist too close to the border.
Furthermore, we show analytically and numerically that the edge stabilizes a
novel wave species, which is entirely unstable in the uniform lattice, namely,
a "horseshoe" soliton, consisting of 3 sites. Unstable horseshoes transform
themselves into a pair of ordinary solitons.Comment: 6 pages, 4 composite figure
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