1,266 research outputs found

    Modelling (001) surfaces of II-VI semiconductors

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    First, we present a two-dimensional lattice gas model with anisotropic interactions which explains the experimentally observed transition from a dominant c(2x2) ordering of the CdTe(001) surface to a local (2x1) arrangement of the Cd atoms as an equilibrium phase transition. Its analysis by means of transfer-matrix and Monte Carlo techniques shows that the small energy difference of the competing reconstructions determines to a large extent the nature of the different phases. Then, this lattice gas is extended to a model of a three-dimensional crystal which qualitatively reproduces many of the characteristic features of CdTe which have been observed during sublimation and atomic layer epitaxy.Comment: 5 pages, 3 figure

    Ultrafast far-infrared optics of carbon nanotubes

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    The optical properties of single-wall carbon nanotube sheets in the far-infrared (FIR) spectral range from few THz to several tens of THz have been investigated with terahertz spectroscopy both with static measurements elucidating the absorption mechanism in the FIR and with time-resolved experiments yielding information on the charge carrier dynamics after optical excitation of the nanotubes. We observe an overall depletion of the dominating broad absorption peak at around 4THz when the nanotubes are excited by a short visible laser pulse. This finding excludes particle-plasmon resonances as absorption mechanism and instead shows that interband transitions in tubes with an energy gap of ~10meV govern the far-infrared conductivity. A simple model based on an ensemble of two-level systems naturally explains the weak temperature dependence of the far-infrared conductivity by the tube-to-tube variation of the chemical potential. Furthermore, the time-resolved measurements do not show any evidence of a distinct free-carrier response which is attributed to the photogeneration of strongly bound excitons in the tubes with large energy gaps. The rapid decay of a featureless background with pronounced dichroism is associated with the increased absorption of spatially localized charge carriers before thermalization is completed

    Temperature dependence of ultrafast phonon dynamics in graphite

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    Nonequilibrium optical phonons are generated in graphite following the excitation of electron-hole pairs with a femtosecond laser pulse. Their energy relaxation is probed by means of terahertz pulses. We find that the hot-phonon lifetime increases by a factor of 2 when the sample temperature decreases from 300 to 5 K. These results suggest that the energy relaxation in graphite at room temperature and above is dominated by the anharmonic decay of hot Aâ€Č1phonons at the K point into acoustic phonons with energies of about 10 meV

    The First Training Workshop on Permafrost Research Methods: IMPETUS 2007 : OSL-APECS-PYRN Training Workshop; St. Petersburg, Russia, 29 November to 2 December 2007

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    Fifty young researchers from 14 countries met in St. Petersburg, Russia, to learn about the latest methods used in permafrost research and engineering and to discuss future plans to address climate change issues in permafrost areas. This workshop was an official International Polar Year (IPY) event organized jointly by the Otto Schmidt Laboratory for Polar and Marine Sciences (OSL) in St. Petersburg, the Permafrost Young Researchers Network (PYRN), and the Association of Polar Early Career Scientists (APECS). The workshop provided insights into the latest techniques and methods used in permafrost research in fields as diverse as permafrost modeling, investigations of mountain ice segregation, bubbling from thermokarst lakes, and submarine permafrost detection. It brought together experts to provide young investigators with a multidisciplinary and cross-border perspective on permafrost research, a much needed approach in a discipline marked by strong research history yet strongly entangled within national borders. Presentations and speaker biographies are now available on the conference Web site (http://pyrn.ways.org/activities/pyrn-meetings/2007-saint-petersburg)

    Drawing a pandemic vulnerabilities' map: The SoNAR-global Vulnerabilities Assessment digital and its output

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    This paper describes the process, advantages and limitations of a qualitative methodology for defining and analyzing vulnerabilities during the COVID-19 pandemic. Implemented in Italy in two sites (Rome and outside Rome, in some small-medium sized municipalities in Latium) in 2021, this investigation employed a mixed digital research tool that was also used simultaneously in four other European countries. Its digital nature encompasses both processes of data collection. Among the most salient is that the pandemic catalyzed new vulnerabilities in addition to exacerbating old ones, particularly economic. Many of the vulnerabilities detected, in fact, are linked to previous situations, such as the uncertainties of labor markets, having in COVID-19 to the greatest negative effects on the most precarious workers (non-regular, part-time, and seasonal). The consequences of the pandemic are also reflected in other forms of vulnerability that appear less obvious, having exacerbated social isolation, not only out of fear of contagion, but because of the psychological challenges posed by containment measures themselves. These measures created not mere discomfort, but behavioral changes characterized by anxiety, fearfulness, and disorientation. More generally, this investigation reveals the strong influence of social determinants throughout the COVID-19 pandemic, creating new forms of vulnerability, as the effects of social, economic, and biological risk factors were compounded, in particular, among already marginalized populations

    Effects of pallidal deep brain stimulation and levodopa treatment on reaction-time performance in Parkinson's disease

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    Basal ganglia-thalamocortical circuits play an important role in movement preparation and execution. Tracer, single-cell, and lesion studies in monkeys suggest the existence of topologically segregated motor and nonmotor basal ganglia cortical circuits. In this study we used deep brain stimulation (DBS) of the posteroventrolateral globus pallidus internus (GPi) in patients with Parkinson's disease to elucidate the function of the GPi in human sensorimotor behavior. This question was investigated by comparing the influence of DBS on patients' performance in various reaction-time tasks that differed with respect to cognitive but not motor requirements. As a main result, DBS improved performance on the different tasks independently of the complexity of the involved cognitive processing functions. Furthermore, the observed effects did not depend on the modality of the processed information. These results suggest that the functional state of the posteroventrolateral GPi selectively affects the motor stage in simple sensorimotor acts, because this stage was the only stage involved in all investigated tasks. In addition to DBS, we manipulated the levodopa medication state of the PD patients. In contrast to DBS, levodopa effects on reaction times were less consistent. Levodopa improved reaction times in choice reaction tasks significantly, while affecting reaction times in a simple reaction task to a lesser extent. Error analysis revealed that the medication-dependent reaction-time improvement in the choice reaction tasks was accompanied by an increase in errors, suggesting a shift of the speed-accuracy criteria of the patients. A similar pattern of results was not observed for the DBS effects. Taken together, our data are in agreement with recent findings in monkeys that indicate a topological organization of the GPi in which motor functions are localized in posterolateral regions apart from cognitive regions. Furthermore, our data show a way to uncover the subcortical-cortical circuitry serving human sensorimotor behavior

    Cultivating compliance: governance of North Indian organic basmati smallholders in a global value chain

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    Focusing on a global value chain (GVC) for organic basmati rice, we study how farmers’ practices are governed through product and process standards, organic certification protocols, and contracts with buyer firms. We analyze how farmers’ entry into the GVC reconfigures their agencements (defined as heterogeneous arrangements of human and nonhuman agencies which are associated with each other). These reconfigurations entail the severance of some associations among procedural and material elements of the agencements and the formation of new associations, in order to produce cultivation practices that are accurately described by the GVC’s standards and protocols. Based on ethnography of two farmers in Uttarakhand, North India, we find that the same standards were enacted differently on the two farmers’ fields, producing variable degrees of (selective) compliance with the ‘official’ GVC standards. We argue that the disjuncture between the ‘official’ scripts of the standards and actual cultivation practices must be nurtured to allow farmers’ agencements to align their practices with local sociotechnical relations and farm ecology. Furthermore, we find that compliance and disjuncture were facilitated by many practices and associations that were officially ungoverned by the GVC