1,398 research outputs found
Ultracold homonuclear and heteronuclear collisions in metastable helium
Scattering and ionizing cross sections and rates are calculated for ultracold
collisions between metastable helium atoms using a fully quantum-mechanical
close-coupled formalism. Homonuclear collisions of the bosonic HeHe and fermionic HeHe systems, and
heteronuclear collisions of the mixed HeHe system,
are investigated over a temperature range 1 K to 1 K. Carefully
constructed Born-Oppenheimer molecular potentials are used to describe the
electrostatic interaction between the colliding atoms, and complex optical
potentials used to represent loss through ionization from the
states. Magnetic spin-dipole mediated transitions from the
state are included and results reported for spin-polarized and unpolarized
systems. Comparisons are made with experimental results, previous
semi-classical models, and a perturbed single channel model.Comment: 14 pages, 9 figure
Single crystal tubes of beta alumina
Edge-defined, film-fed growth process allows both tubular shapes and single crystallinity to be achieved. Beta alumina in single crystal form makes possible membranes with improved conductivities. Single crystal membranes also eliminate problems associated with electrical short circuiting of membrane due to possible sodium metal diffusion
Generalised balance equations for charged particle transport via localised and delocalised states: Mobility, generalised Einstein relations and fractional transport
A generalised phase-space kinetic Boltzmann equation for highly
non-equilibrium charged particle transport via localised and delocalised states
is used to develop continuity, momentum and energy balance equations,
accounting explicitly for scattering, trapping/detrapping and recombination
loss processes. Analytic expressions detail the effect of these microscopic
processes on the mobility and diffusivity. Generalised Einstein relations (GER)
are developed that enable the anisotropic nature of diffusion to be determined
in terms of the measured field-dependence of the mobility. Interesting
phenomena such as negative differential conductivity and recombination
heating/cooling are shown to arise from recombination loss processes and the
localised and delocalised nature of transport. Fractional transport emerges
naturally within this framework through the appropriate choice of divergent
mean waiting time distributions for localised states, and fractional
generalisations of the GER and mobility are presented. Signature impacts on
time-of-flight current transients of recombination loss processes via both
localised and delocalised states are presented.Comment: 21 pages, 4 figure
Electron scattering and transport in liquid argon
The transport of excess electrons in liquid argon driven out of equilibrium
by an applied electric field is revisited using a multi-term solution of
Boltzmann's equation together with ab initio liquid phase cross-sections
calculated using the Dirac-Fock scattering equations. The calculation of liquid
phase cross-sections extends previous treatments to consider multipole
polarisabilities and a non-local treatment of exchange while the accuracy of
the electron-argon potential is validated through comparison of the calculated
gas phase cross-section with experiment. The results presented highlight the
inadequacy of local treatments of exchange that are commonly used in liquid and
cluster phase cross-section calculations. The multi-term Boltzmann equation
framework accounting for coherent scattering enables the inclusion of the full
anisotropy in the differential cross-section arising from the interaction and
the structure factor, without an a priori assumption of quasi-isotropy in the
velocity distribution function. The model, which contains no free parameters
and accounts for both coherent scattering and liquid phase screening effects,
was found to reproduce well the experimental drift velocities and
characteristic energies.Comment: 32 pages, 16 figures; minor corrections, added 1 figur
Photoassociation spectra and the validity of the dipole approximation for weakly bound dimers
Photoassociation (PA) of ultracold metastable helium to the 2s2p manifold is
theoretically investigated using a non-perturbative close-coupled treatment in
which the laser coupling is evaluated without assuming the dipole
approximation. The results are compared with our previous study [Cocks and
Whittingham, Phys. Rev. A 80, 023417 (2009)] that makes use of the dipole
approximation. The approximation is found to strongly affect the PA spectra
because the photoassociated levels are weakly bound, and a similar impact is
predicted to occur in other systems of a weakly bound nature. The inclusion or
not of the approximation does not affect the resonance positions or widths,
however significant differences are observed in the background of the spectra
and the maximum laser intensity at which resonances are discernable. Couplings
not satisfying the dipole selection rule |J-1| <= J' <= |J+1| do not lead to
observable resonances.Comment: 5 pages, 2 figures; Minor textual revision
Third-order transport coefficients for localised and delocalised charged-particle transport
We derive third order transport coefficients of skewness for a phase-space
kinetic model that considers the processes of scattering collisions, trapping,
detrapping and recombination losses. The resulting expression for the skewness
tensor provides an extension to Fick's law which is in turn applied to yield a
corresponding generalised advection-diffusion-skewness equation. A physical
interpretation of trap-induced skewness is presented and used to describe an
observed negative skewness due to traps. A relationship between skewness,
diffusion, mobility and temperature is formed by analogy with Einstein's
relation. Fractional transport is explored and its effects on the flux
transport coefficients are also outlined.Comment: 11 pages, 3 figure
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The impact of impaired semantic knowledge on spontaneous iconic gesture production
Background: Previous research has found that people with aphasia produce more spontaneous iconic gesture than control participants, especially during word-finding difficulties. There is some evidence that impaired semantic knowledge impacts on the diversity of gestural handshapes, as well as the frequency of gesture production. However, no previous research has explored how impaired semantic knowledge impacts on the frequency and type of iconic gestures produced during fluent speech compared with those produced during word-finding difficulties.
Aims: To explore the impact of impaired semantic knowledge on the frequency and type of iconic gestures produced during fluent speech and those produced during word-finding difficulties.
Methods & Procedures: A group of 29 participants with aphasia and 29 control participants were video recorded describing a cartoon they had just watched. All iconic gestures were tagged and coded as either āmannerā, āpath onlyā, āshape outlineā or āotherā. These gestures were then separated into either those occurring during fluent speech or those occurring during a word-finding difficulty. The relationships between semantic knowledge and gesture frequency and form were then investigated in the two different conditions.
Outcomes & Results: As expected, the participants with aphasia produced a higher frequency of iconic gestures than the control participants, but when the iconic gestures produced during word-finding difficulties were removed from the analysis, the frequency of iconic gesture was not significantly different between the groups. While there was not a significant relationship between the frequency of iconic gestures produced during fluent speech and semantic knowledge, there was a significant positive correlation between semantic knowledge and the proportion of word-finding difficulties that contained gesture. There was also a significant positive correlation between the speakersā semantic knowledge and the proportion of gestures that were produced during fluent speech that were classified as āmannerā. Finally while not significant, there was a positive trend between semantic knowledge of objects and the production of āshape outlineā gestures during word-finding difficulties for objects.
Conclusions: The results indicate that impaired semantic knowledge in aphasia impacts on both the iconic gestures produced during fluent speech and those produced during word-finding difficulties but in different ways. These results shed new light on the relationship between impaired language and iconic co-speech gesture production and also suggest that analysis of iconic gesture may be a useful addition to clinical assessment
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The languageāgesture connection: Evidence from aphasia
A significant body of evidence from cross-linguistic and developmental studies converges to suggest that co-speech iconic gesture mirrors language. This paper aims to identify whether gesture reflects impaired spoken language in a similar way. Twenty-nine people with aphasia (PWA) and 29 neurologically healthy control participants (NHPs) produced a narrative discourse, retelling the story of a cartoon video. Gesture and language were analysed in terms of semantic content and structure for two key motion events. The aphasic data showed an influence on gesture from lexical choices but no corresponding clausal influence. Both the groups produced gesture that matched the semantics of the spoken language and gesture that did not, although there was one particular gestureālanguage mismatch (semantically ālightā verbs paired with semantically richer gesture) that typified the PWA narratives. These results indicate that gesture is both closely related to spoken language impairment and compensatory
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