2,088 research outputs found
Low temperature acoustic properties of amorphous silica and the Tunneling Model
Internal friction and speed of sound of a-SiO(2) was measured above 6 mK
using a torsional oscillator at 90 kHz, controlling for thermal decoupling,
non-linear effects, and clamping losses. Strain amplitudes e(A) = 10^{-8} mark
the transition between the linear and non-linear regime. In the linear regime,
excellent agreement with the Tunneling Model was observed for both the internal
friction and speed of sound, with a cut-off energy of E(min) = 6.6 mK. In the
non-linear regime, two different behaviors were observed. Above 10 mK the
behavior was typical for non-linear harmonic oscillators, while below 10 mK a
different behavior was found. Its origin is not understood.Comment: 1 tex file, 6 figure
Local Properties of the Potential Energy Landscape of a Model Glass: Understanding the Low Temperature Anomalies
Though the existence of two-level systems (TLS) is widely accepted to explain
low temperature anomalies in the sound absorption, heat capacity, thermal
conductivity and other quantities, an exact description of their microscopic
nature is still lacking. We performed computer simulations for a binary
Lennard-Jones system, using a newly developed algorithm to locate double-well
potentials (DWP) and thus two-level systems on a systematic basis. We show that
the intrinsic limitations of computer simulations like finite time and finite
size problems do not hamper this analysis. We discuss how the DWP are embedded
in the total potential energy landscape. It turns out that most DWP are
connected to the dynamics of the smaller particles and that these DWP are
rather localized. However, DWP related to the larger particles are more
collective
Path integral Monte Carlo simulation of helium at negative pressures
Path integral Monte Carlo (PIMC) simulations of liquid helium at negative
pressure have been carried out for a temperature range from the critical
temperature to below the superfluid transition. We have calculated the
temperature dependence of the spinodal line as well as the pressure dependence
of the isothermal sound velocity in the region of the spinodal. We discuss the
slope of the superfluid transition line and the shape of the dispersion curve
at negative pressures.Comment: 6 pages, 7 figures, submitted to Physical Review B Revised: new
reference, replaced figure
In vitro techniques for the assessment of neurotoxicity.
Risk assessment is a process often divided into the following steps: a) hazard identification, b) dose-response assessment, c) exposure assessment, and d) risk characterization. Regulatory toxicity studies usually are aimed at providing data for the first two steps. Human case reports, environmental research, and in vitro studies may also be used to identify or to further characterize a toxic hazard. In this report the strengths and limitations of in vitro techniques are discussed in light of their usefulness to identify neurotoxic hazards, as well as for the subsequent dose-response assessment. Because of the complexity of the nervous system, multiple functions of individual cells, and our limited knowledge of biochemical processes involved in neurotoxicity, it is not known how well any in vitro system would recapitulate the in vivo system. Thus, it would be difficult to design an in vitro test battery to replace in vivo test systems. In vitro systems are well suited to the study of biological processes in a more isolated context and have been most successfully used to elucidate mechanisms of toxicity, identify target cells of neurotoxicity, and delineate the development and intricate cellular changes induced by neurotoxicants. Both biochemical and morphological end points can be used, but many of the end points used can be altered by pharmacological actions as well as toxicity. Therefore, for many of these end points it is difficult or impossible to set a criterion that allows one to differentiate between a pharmacological and a neurotoxic effect. For the process of risk assessment such a discrimination is central. Therefore, end points used to determine potential neurotoxicity of a compound have to be carefully selected and evaluated with respect to their potential to discriminate between an adverse neurotoxic effect and a pharmacologic effect. It is obvious that for in vitro neurotoxicity studies the primary end points that can be used are those affected through specific mechanisms of neurotoxicity. For example, in vitro systems may be useful for certain structurally defined compounds and mechanisms of toxicity, such as organophosphorus compounds and delayed neuropathy, for which target cells and the biochemical processes involved in the neurotoxicity are well known. For other compounds and the different types of neurotoxicity, a mechanism of toxicity needs to be identified first. Once identified, by either in vivo or in vitro methods, a system can be developed to detect and to evaluate predictive ability for the type of in vivo neurotoxicity produced. Therefore, in vitro tests have their greatest potential in providing information on basic mechanistic processes in order to refine specific experimental questions to be addressed in the whole animal
Cavitation of Electrons Bubbles in Liquid Helium Below saturation Pressure
We have used a Hartree-type electron-helium potential together with a density
functional description of liquid He and He to study the explosion of
electron bubbles submitted to a negative pressure. The critical pressure at
which bubbles explode has been determined as a function of temperature. It has
been found that this critical pressure is very close to the pressure at which
liquid helium becomes globally unstable in the presence of electrons. It is
shown that at high temperatures the capillary model overestimates the critical
pressures. We have checked that a commonly used and rather simple
electron-helium interaction yields results very similar to those obtained using
the more accurate Hartree-type interaction. We have estimated that the
crossover temperature for thermal to quantum nucleation of electron bubbles is
very low, of the order of 6 mK for He.Comment: 22 pages, 9 figure
The effect of pressure on statics, dynamics and stability of multielectron bubbles
The effect of pressure and negative pressure on the modes of oscillation of a
multi-electron bubble in liquid helium is calculated. Already at low pressures
of the order of 10-100 mbar, these effects are found to significantly modify
the frequencies of oscillation of the bubble. Stabilization of the bubble is
shown to occur in the presence of a small negative pressure, which expands the
bubble radius. Above a threshold negative pressure, the bubble is unstable.Comment: 4 pages, 2 figures, accepted for publication in Physical Review
Letter
From a certain point of view: sensory phenomenological envisionings of running space and place
The precise ways in which we go about the mundane, repetitive, social actions of everyday life are central concerns of ethnographers and theorists working within the traditions of the sociology of the mundane and sociological phenomenology. In this article, we utilize insights derived from sociological phenomenology and the newly developing field of sensory sociology to investigate a particular, mundane, and embodied social practice, that of training for distance running in specific places: our favored running routes. For, despite a growing body of ethnographic studies of particular sports, little analytic attention has been devoted to the actual, concrete practices of “doing” or “producing” sporting activity, particularly from a sensory ethnographic perspective. Drawing upon data from a 2-year joint autoethnographic research project, here we explore the visual dimension, focusing upon three key themes in relation to our runners’ visualization of, respectively, (1) hazardous places, (2) performance places, (3) the time–space–place nexus
Grasping the phenomenology of sporting bodies
The last two decades have witnessed a vast expansion in research and writing on the sociology of the body and on issues of embodiment. Indeed, both sociology in general and the sociology of sport specifically have well heeded the long-standing and vociferous calls ‘to bring the body back in’ to social theory. It seems particularly curious therefore that the sociology of sport has to-date addressed this primarily at a certain abstract, theoretical level, with relatively few accounts to be found that are truly grounded in the corporeal realities of the lived sporting body; a ‘carnal sociology’ of sport, to borrow Crossley’s (1995) expression. To portray and understand more fully this kind of embodied perspective, it is argued, demands engaging with the phenomenology of the body, and this article seeks to contribute to a small but growing literature providing this particular form of ‘embodied’ analysis of the body in sport. Here we identify some useful intellectual resources for developing a phenomenology of sporting experience, specifically its sensory elements, and also subsequently examine the potential for its evocative portrayal and effective analysis via different kinds of textual forms.
Key words: phenomenology; sociology of the sporting body; embodiment; the sense
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