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 $\mu= 1.3\times 10^5 cm^2/V\cdot s)$ 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

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 ($T$), whereas that corresponding to the latter (designated as the first channel) vanishes for $T\to 0$. 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

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

Appeals to semiotic registers in ethno-metapragmatic accounts of variation

Discussions of folklinguistic accounts of language use are frequently focused on dismissing them because of their limitations. As a result, not a lot is written regarding how such accounts are done and how they ‘work’. This article examines how folklinguistic evaluations are achieved in interaction, particularly through appeals to semiotic registers (Agha 2007). It describes how in explaining their beliefs regarding linguistic variation, speakers frequently produce voicings with varying transparency. These rely on understandings of the social world and bring large collections of linguistic resources into play. They offer rich insights if analytic attention is given to their details because even when evaluating a single variant, whole ways of speaking, and even being, may be utilized. The paper explores in turn how analysis reveals the inseparability of variants, understandings of context and audience, the relationship between linguistic forms and social types, and the performance of social types via the evaluation of semiotic resources. In each section, discussion is grounded in extracts from interviews on Australian English with speakers of this variety of English. Cumulatively they show the primacy of semiotic registers in ethno-metapragmatic accounts.N/

Communicative competence and institutional affiliation: interactional processes of identity construction by immigrant students in Catalonia

The growing presence of children of immigrant families in the public school system in the bilingual region of Catalonia provides us with an opportunity to study how young multilingual and multicultural speakers construct their social competencies and their identity within the specific context of a gate-keeping social institution such as the school. The study reported in this paper approaches language learning as a process of socialisation that involves not only learning how to make sense of linguistic signs but also learning how to enact different social roles in particular institutions. The analysis focuses on the interactional profiles of two immigrant students in two types of communicative activities that are representative of the school context: responding to questions from an adult and cooperating with a peer in the resolution of a learning task. By shifting the focus of analysis from a decontextualised notion of communicative competence to the notion of 'institutionally affiliated communicative competence' and concentrating on issues such as the (1) the relationship between knowledge and participation, (2) language choice inside and outside school and (3) definitions of correctness in language use, the study reveals how the two students construct a highly 'affiliated' identit

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

Borders traversed, boundaries erected: Creating discursive identities and language communities in the Village of Tewa

Today the Village of Tewa, First Mesa of the Hopi Reservation in Northern Arizona experiences unprecedented linguistic diversity and change due to language shift to English. Despite a wide range of speaker fluency, the now emblematic Tewa language that their ancestors transported from the Rio Grande Valley almost 325 years ago, is widely valorized within the community. However Language factions have emerged andtheir debates and contestations focus on legitimate language learning and the proper maintenance of their emblematic language. Boundary creation and crossing are featuresof discourses that rationalize possible forms of language revitalization and construct communities across temporal barriers. The theoretical implications of these discourseson both local and theoretical notions of language/speech community are explored. © 2014 Elsevier Ltd. All rights reserved