12,590 research outputs found
Installed performance of air-augmented nozzles based on analytical determination of internal ejector characteristics
Procedures for matching intake and ejector pumping characteristics of air-augmented nozzle
Muonium as a shallow center in GaN
A paramagnetic muonium (Mu) state with an extremely small hyperfine parameter
was observed for the first time in single-crystalline GaN below 25 K. It has a
highly anisotropic hyperfine structure with axial symmetry along the [0001]
direction, suggesting that it is located either at a nitrogen-antibonding or a
bond-centered site oriented parallel to the c-axis. Its small ionization energy
(=< 14 meV) and small hyperfine parameter (--10^{-4} times the vacuum value)
indicate that muonium in one of its possible sites produces a shallow state,
raising the possibility that the analogous hydrogen center could be a source of
n-type conductivity in as-grown GaN.Comment: 4 figures, to be published in Phys. Rev. Letter
Multiplpe Choice Minority Game With Different Publicly Known Histories
In the standard Minority Game, players use historical minority choices as the
sole public information to pick one out of the two alternatives. However,
publishing historical minority choices is not the only way to present global
system information to players when more than two alternatives are available.
Thus, it is instructive to study the dynamics and cooperative behaviors of this
extended game as a function of the global information provided. We numerically
find that although the system dynamics depends on the kind of public
information given to the players, the degree of cooperation follows the same
trend as that of the standard Minority Game. We also explain most of our
findings by the crowd-anticrowd theory.Comment: Extensively revised, to appear in New J Phys, 7 pages with 4 figure
Analysis of free turbulent shear flows by numerical methods
Studies are described in which the effort was essentially directed to classes of problems where the phenomenologically interpreted effective transport coefficients could be absorbed by, and subsequently extracted from (by comparison with experimental data), appropriate coordinate transformations. The transformed system of differential equations could then be solved without further specifications or assumptions by numerical integration procedures. An attempt was made to delineate different regimes for which specific eddy viscosity models could be formulated. In particular, this would account for the carryover of turbulence from attached boundary layers, the transitory adjustment, and the asymptotic behavior of initially disturbed mixing regions. Such models were subsequently used in seeking solutions for the prescribed two-dimensional test cases, yielding a better insight into overall aspects of the exchange mechanisms
Phonon Emission from a 2D Electron Gas: Evidence of Transition to the Hydrodynamic Regime
Using as a thermometer the temperature dependent magneto-transport of a
two-dimensional electron gas, we find that effective temperature scales with
current as , where in the {\it Shubnikov
de-Haas} regime, and in both the {\it integer and fractional}
quantum Hall effect. This implies the phonon energy emission rate changes from
the expected to . We explain this, as well as the
dramatic enhancement in phonon emission efficiency using a hydrodynamic model.Comment: 4 pages, 2 Postscript figures uuencoded with TeX file uses psfig
macro. Submitted to Phys. Rev. Let
Nitrogen incorporation and trace element analysis of nanocrystalline diamond thin films by secondary ion mass spectrometry
Nitrogen has been successfully incorporated into nanocrystalline diamond films produced by a CH4/N-2 microwave plasma-enhanced chemical vapor deposition method. High mass resolution secondary ion mass spectrometry (SIMS) characterization shows that the density of the incorporated nitrogen, monitored via CN-, can be as high as 10(21) atoms/cm(3) depending on the ratio of CH4 to N-2 in the reactant gas and on the substrate temperature used for the film preparation. SIMS depth profiles demonstrate that the incorporated nitrogen is uniform within the diamond films (about 1.5 mu m thick) except at the surfaces and at the interface between film and substrate. Furthermore, the SIMS analyses reveal that alkali elements such as Na, K, and Li appear to be contaminants at the surface of nanocrystalline diamond films produced using a CH4/Ar or CH4/H-2 discharge, but are bulk impurities only for the films prepared using a CH4/N-2 plasma. These alkali impurities can play an important role in electronic properties such as electron field emissions of nanocrystalline diamond thin films
Threshold effects in excited charmed baryon decays
Motivated by recent results on charmed baryons from CLEO and FOCUS, we
reexamine the couplings of the orbitally excited charmed baryons. Due to its
proximity to the [Sigma_c pi] threshold, the strong decays of the
Lambda_c(2593) are sensitive to finite width effects. This distorts the shape
of the invariant mass spectrum in Lambda_{c1}-> Lambda_c pi^+pi^- from a simple
Breit-Wigner resonance, which has implications for the experimental extraction
of the Lambda_c(2593) mass and couplings. We perform a fit to unpublished CLEO
data which gives M(Lambda_c(2593)) - M(Lambda_c) = 305.6 +- 0.3 MeV and h2^2 =
0.24^{+0.23}_{-0.11}, with h2 the Lambda_{c1}-> Sigma_c pi strong coupling in
the chiral Lagrangian. We also comment on the new orbitally excited states
recently observed by CLEO.Comment: 9 pages, 3 figure
Noise-assisted spike propagation in myelinated neurons
We consider noise-assisted spike propagation in myelinated axons within a
multi-compartment stochastic Hodgkin-Huxley model. The noise originates from a
finite number of ion channels in each node of Ranvier. For the subthreshold
internodal electric coupling, we show that (i) intrinsic noise removes the
sharply defined threshold for spike propagation from node to node, and (ii)
there exists an optimum number of ion channels which allows for the most
efficient signal propagation and it corresponds to the actual physiological
values.Comment: 8 pages, 12 figures, accepted for publication in Phys. Rev.
Theory of terahertz electric oscillations by supercooled superconductors
We predict that below T_c a regime of negative differential conductivity
(NDC) can be reached. The superconductor should be supercooled to T<T_c in the
normal phase under DC voltage. In such a nonequilibrium situation the NDC of
the superconductor is created by the excess conductivity of the fluctuation
Cooper pairs. We propose NDC of supercooled superconductors to be used as an
active medium for generation of electric oscillations. Such generators can be
used in the superconducting electronics as a new type THz source of radiation.
Oscillations can be modulated by the change of the bias voltage, electrostatic
doping by a gate electrode when the superconductor is the channel of a field
effect transistor, or by light. When small amplitude oscillations are
stabilized near the critical temperature T_c the generator can be used as a
bolometer. The essential for the applications NDC is predicted by the solution
of the Boltzmann kinetic equation for the metastable in the normal phase Cooper
pairs. Boltzmann equation for fluctuation Cooper pairs is a result of
state-of-the-art application of the microscopic theory of superconductivity.
Our theoretical conclusions are based on some approximations like time
dependent Ginzburg-Landau theory, but nevertheless can reliably predict
appearance of NDC. The maximal frequency at which superconductors can operate
as generators is determined by the critical temperature \hbar omega_max ~ k_B
T_c. For high-T_c superconductors this maximal frequency falls well inside the
terahertz range. Technical conditions to avoid nucleation of the
superconducting phase are briefly discussed. We suggest that nanostructured
high-T_c superconductors patterned in a single chip can give the best technical
performance of the proposed oscillator.Comment: 7 page
Twisted and Nontwisted Bifurcations Induced by Diffusion
We discuss a diffusively perturbed predator-prey system. Freedman and
Wolkowicz showed that the corresponding ODE can have a periodic solution that
bifurcates from a homoclinic loop. When the diffusion coefficients are large,
this solution represents a stable, spatially homogeneous time-periodic solution
of the PDE. We show that when the diffusion coefficients become small, the
spatially homogeneous periodic solution becomes unstable and bifurcates into
spatially nonhomogeneous periodic solutions.
The nature of the bifurcation is determined by the twistedness of an
equilibrium/homoclinic bifurcation that occurs as the diffusion coefficients
decrease. In the nontwisted case two spatially nonhomogeneous simple periodic
solutions of equal period are generated, while in the twisted case a unique
spatially nonhomogeneous double periodic solution is generated through
period-doubling.
Key Words: Reaction-diffusion equations; predator-prey systems; homoclinic
bifurcations; periodic solutions.Comment: 42 pages in a tar.gz file. Use ``latex2e twisted.tex'' on the tex
files. Hard copy of figures available on request from
[email protected]
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