96 research outputs found
Electron localization in sound absorption oscillations in the quantum Hall effect regime
The absorption coefficient for surface acoustic waves in a piezoelectric
insulator in contact with a GaAs/AlGaAs heterostructure (with two-dimensional
electron mobility at T=4.2K) via a small
gap has been investigated experimentally as a function of the frequency of the
wave, the width of the vacuum gap, the magnetic field, and the temperature. The
magnetic field and frequency dependencies of the high-frequency conductivity
(in the region 30-210 MHz) are calculated and analyzed. The experimental
results can be explained if it assumed that there exists a fluctuation
potential in which current carrier localization occurs. The absorption of the
surface acoustic waves in an interaction with two-dimensional electrons
localized in the energy "tails" of Landau levels is discussed.Comment: RevTeX 6 pages+6 EPS pic
Surface acoustic wave attenuation by a two-dimensional electron gas in a strong magnetic field
The propagation of a surface acoustic wave (SAW) on GaAs/AlGaAs
heterostructures is studied in the case where the two-dimensional electron gas
(2DEG) is subject to a strong magnetic field and a smooth random potential with
correlation length Lambda and amplitude Delta. The electron wave functions are
described in a quasiclassical picture using results of percolation theory for
two-dimensional systems. In accordance with the experimental situation, Lambda
is assumed to be much smaller than the sound wavelength 2*pi/q. This restricts
the absorption of surface phonons at a filling factor \bar{\nu} approx 1/2 to
electrons occupying extended trajectories of fractal structure. Both
piezoelectric and deformation potential interactions of surface acoustic
phonons with electrons are considered and the corresponding interaction
vertices are derived. These vertices are found to differ from those valid for
three-dimensional bulk phonon systems with respect to the phonon wave vector
dependence. We derive the appropriate dielectric function varepsilon(omega,q)
to describe the effect of screening on the electron-phonon coupling. In the low
temperature, high frequency regime T << Delta (omega_q*Lambda
/v_D)^{alpha/2/nu}, where omega_q is the SAW frequency and v_D is the electron
drift velocity, both the attenuation coefficient Gamma and varepsilon(omega,q)
are independent of temperature. The classical percolation indices give
alpha/2/nu=3/7. The width of the region where a strong absorption of the SAW
occurs is found to be given by the scaling law |Delta \bar{\nu}| approx
(omega_q*Lambda/v_D)^{alpha/2/nu}. The dependence of the electron-phonon
coupling and the screening due to the 2DEG on the filling factor leads to a
double-peak structure for Gamma(\bar{\nu}).Comment: 17 pages, 3 Postscript figures, minor changes mad
Sound and Heat Absorption by a 2D Electron Gas in an Odd-Integer Quantized-Hall Regime
The absorption of bulk acoustic phonons in a two-dimensional (2D) GaAs/AlGaAs
heterostructure is studied (in the clean limit) where the 2D electron-gas
(2DEG), being in an odd-integer quantum-Hall state, is in fact a spin
dielectric. Of the two channels of phonon absorption associated with excitation
of spin waves, one, which is due to the spin-orbit (SO) coupling of electrons,
involves a change of the spin state of the system and the other does not. We
show that the phonon-absorption rate corresponding to the former channel (in
the paper designated as the second absorption channel) is finite at zero
temperature (), whereas that corresponding to the latter (designated as the
first channel) vanishes for . The long-wavelength limit, being the
special case of the first absorption channel, corresponds to sound (bulk and
surface) attenuation by the 2DEG. At the same time, the ballistic phonon
propagation and heat absorption are determined by both channels. The 2DEG
overheat and the attendant spin-state change are found under the conditions of
permanent nonequilibrium phonon pumping.Comment: 26 pages, 2 figure
To what extent does a regional dialect and accent impact on the development of reading and writing skills?
The issue of whether a regional accent and/or dialect impact(s) on the development of literacy skills remains current in the UK. For decades the issue has dogged debate about education outcomes, portable skills and employability. The article summarizes research on the topic using systematic review methodology. A scoping review was undertaken with the research question ‘To what extent does a regional dialect and accent impact on the development of reading and writing skills?’. The review covers research relevant to the teaching of 5-16 year olds in England, but also draws on research within Europe, the USA, Australia and the Caribbean. The results suggest that curricula have marginalized language variation; that the impact of regional accent and dialect on writing is relatively minor; that young people are adept at style-shifting between standard and non-standard forms; and that inappropriate pedagogical responses to regional variation can have detrimental effects on children’s educational achievement
Magnetic resonance imaging in children: common problems and possible solutions for lung and airways imaging
Pediatric chest MRI is challenging. High-resolution
scans of the lungs and airways are compromised by long imaging
times, low lung proton density and motion. Low signal
is a problem of normal lung. Lung abnormalities commonly
cause increased signal intenstities. Among the most important
factors for a successful MRI is patient cooperation, so the long
acquisition times make patient preparation crucial. Children
usually have problems with long breath-holds and with the
concept of quiet breathing. Young children are even more
challenging because of higher cardiac and respiratory rates
giving motion blurring. For these reasons, CT has often been
preferred over MRI for chest pediatric imaging. Despite its
drawbacks, MRI also has advantages over CT, which justifies
its further development and clinical use. The most important
advantage is the absence of ionizing radiation, which allows
frequent scanning for short- and long-term follow-up studie
A PERFORMANCE NARRATIVA DE UMA BLOGUEIRA: "TORNANDO-SE PRETA EM UM SEGUNDO NASCIMENTO"
A web 2.0 propicia aos sujeitos sociais a possibilidade de contar suas histórias assim como de vê-las discutidas em novas formas de interação. Este artigo almeja apresentar os posicionamentos interacionais que constroem a performance narrativa de co-construção de raça de uma mulher negra no blog "Eu, Mulher Preta". O estudo se ampara nos aportes teóricos dos novos letramentos digitais, na concepção de raça proposta pelas Teorias Queer e na teorização de narrativa como performance. Para analisar a narrativa da blogueira como performance, o quadro analÃtico se ancora no construto de posicionamento interacional e nas pistas que marcam tal posicionamento na encenação da performance. Os resultados indicam que a narradora se posiciona interacionalmente como mulher preta. Identificamos, porém, um posicionamento interacional anterior ao renascimento como negra: o de mulher "branc[a] meio suj[a]". Observamos ainda que tais posicionamentos refletem duas performances discursivas conflitantes, uma que se aproxima e valoriza a negritude e outra que se distancia de sua origem. Esta investigação, baseando-se nas Teorias Queer, possibilita, também, tratar a questão racial como um traço performativo, colocando-a ao lado de gênero e sexualidade
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