476 research outputs found
Scaling Law in Carbon Nanotube Electromechanical Devices
We report a method for probing electromechanical properties of multiwalled
carbon nanotubes(CNTs). This method is based on AFM measurements on a doubly
clamped suspended CNT electrostatically deflected by a gate electrode. We
measure the maximum deflection as a function of the applied gate voltage. Data
from different CNTs scale into an universal curve within the experimental
accuracy, in agreement with a continuum model prediction. This method and the
general validity of the scaling law constitute a very useful tool for designing
actuators and in general conducting nanowire-based NEMS.Comment: 12 pages, 4 figures. To be published in Phys. Rev. Let
Magnetic field reversals in an experimental turbulent dynamo
We report the first experimental observation of reversals of a dynamo field
generated in a laboratory experiment based on a turbulent flow of liquid
sodium. The magnetic field randomly switches between two symmetric solutions B
and -B. We observe a hierarchy of time scales similar to the Earth's magnetic
field: the duration of the steady phases is widely distributed, but is always
much longer than the time needed to switch polarity. In addition to reversals
we report excursions. Both coincide with minima of the mechanical power driving
the flow. Small changes in the flow driving parameters also reveal a large
variety of dynamo regimes.Comment: 5 pages, 4 figure
Algas planctônicas do Rio Amazonas e alguns tributários (Pará, Brasil).
Este trabalho faz parte do projeto HiBam, o qual visa a compreensĂŁo dos processos biogeoquĂmicos e fĂsicos de transferĂŞncia de massas d?água do rio Amazonas para a várzea do Curuai (PA). Objetivo deste estudo Ă© apresentar a composição fi toplanctĂ´nica de um trecho do rio Amazonas (alguns tributários) e informações ecolĂłgicas. As coletas foram realizadas em junho/2002 (águas altas) em 33 estações, sendo 6 no rio Amazonas, 15 no rio Solimões e as demais nos rios Javari, Jandiatuba, Içá, JutaĂ, Juruá, Trombetas, Gurupatuba e ParanĂŁ no AranapĂş. As algas foram amostradas em subsuperfĂcie e fi xadas com lugol acĂ©tico. Foram identificadas 147 espĂ©cies, sendo destaque Cyanophyceae (23% do total), seguido de Cholophyceae (22%). Cyanogranis ferruginea, Merismopedia tenuissima, M. insignis, Pseudanabaena catenata, P. mucicola, Synechococcus captatus e Chroococcus minor foram abundantes nos rios Solimões, Juruá, Içá, JutaĂ e no ParanĂŁ do AranapĂş, prĂłximo ao Solimões. Entre as Chlorophyceae a espĂ©cie mais freqĂĽente foi Chlorella minutissima, com alta densidade nos rios Solimões, JutaĂ e Trombetas. As densidades mais baixas ocorreram nas estações do rio Amazonas
Probing quantum and classical turbulence analogy through global bifurcations in a von K\'arm\'an liquid Helium experiment
We report measurements of the dissipation in the Superfluid Helium high
REynold number von Karman flow (SHREK) experiment for different forcing
conditions, through a regime of global hysteretic bifurcation. Our
macroscopical measurements indicate no noticeable difference between the
classical fluid and the superfluid regimes, thereby providing evidence of the
same dissipative anomaly and response to asymmetry in fluid and superfluid
regime. %In the latter case, A detailed study of the variations of the
hysteretic cycle with Reynolds number supports the idea that (i) the stability
of the bifurcated states of classical turbulence in this closed flow is partly
governed by the dissipative scales and (ii) the normal and the superfluid
component at these temperatures (1.6K) are locked down to the dissipative
length scale.Comment: 5 pages, 5 figure
Generation of magnetic field by dynamo action in a turbulent flow of liquid sodium
We report the observation of dynamo action in the VKS experiment, i.e., the
generation of magnetic field by a strongly turbulent swirling flow of liquid
sodium. Both mean and fluctuating parts of the field are studied. The dynamo
threshold corresponds to a magnetic Reynolds number Rm \sim 30. A mean magnetic
field of order 40 G is observed 30% above threshold at the flow lateral
boundary. The rms fluctuations are larger than the corresponding mean value for
two of the components. The scaling of the mean square magnetic field is
compared to a prediction previously made for high Reynolds number flows.Comment: 4 pages, 5 figure
Path lengths in turbulence
By tracking tracer particles at high speeds and for long times, we study the
geometric statistics of Lagrangian trajectories in an intensely turbulent
laboratory flow. In particular, we consider the distinction between the
displacement of particles from their initial positions and the total distance
they travel. The difference of these two quantities shows power-law scaling in
the inertial range. By comparing them with simulations of a chaotic but
non-turbulent flow and a Lagrangian Stochastic model, we suggest that our
results are a signature of turbulence.Comment: accepted for publication in Journal of Statistical Physic
First-principles study of As interstitials in GaAs: Convergence, relaxation, and formation energy
Convergence of density-functional supercell calculations for defect formation
energies, charge transition levels, localized defect state properties, and
defect atomic structure and relaxation is investigated using the arsenic split
interstitial in GaAs as an example. Supercells containing up to 217 atoms and a
variety of {\bf k}-space sampling schemes are considered. It is shown that a
good description of the localized defect state dispersion and charge state
transition levels requires at least a 217-atom supercell, although the defect
structure and atomic relaxations can be well converged in a 65-atom cell.
Formation energies are calculated for the As split interstitial, Ga vacancy,
and As antisite defects in GaAs, taking into account the dependence upon
chemical potential and Fermi energy. It is found that equilibrium
concentrations of As interstitials will be much lower than equilibrium
concentrations of As antisites in As-rich, -type or semi-insulating GaAs.Comment: 10 pages, 5 figure
MHD in von Kármán swirling flows, development and first run of the sodium experiment
URL: http://www-spht.cea.fr/articles/s01/004 MHD dans les écoulements de von Kármán | Collaboration VKSNATO Science Series II 26, 35-50 (2001). NATO Advanced Research Workshop, Dynamo and Dynamics, A Mathematical ChallengeWe describe the motivations, development and first run of the Von Kármán Sodium (VKS) experiment built to study high Reynolds number magnetohydrodynamics and applications to the dynamo effect. The flow is optimized using water experiments at scale 1/2 and kinematic dynamo simulations. In VKS run1, induction measurements are made in the presence of an externally applied field. Results are reported concerning the geometry of the induced field and its fluctuations in time
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