4,954 research outputs found
Giacobini-Zinner comet: Polarimetric and physical observations
The results of observations of the Giacobini-Zinner comet on 25 and 31 October 1959 are presented. The magnitude of the comet was measured photoelectrically in two spectral regions. The radius is on the order of one kilometer. The photoelectric measurements of comets 1959b and 1957c were used to measure the abundances of the CN and C2 radicals and of solid particles in the heads
Dynamics of a hyperbolic system that applies at the onset of the oscillatory instability
A real hyperbolic system is considered that applies near the onset of the oscillatory instability in large spatial domains. The validity of that system requires that some intermediate scales (large compared with the basic wavelength of the unstable modes but small compared with the size of the system) remain inhibited; that condition is analysed in some detail. The dynamics associated with the hyperbolic system is fully analysed to conclude that it is very simple if the coefficient of the cross-nonlinearity is such that , while the system exhibits increasing complexity (including period-doubling sequences, quasiperiodic transitions, crises) as the bifurcation parameter grows if ; if then the system behaves subcritically. Our results are seen to compare well, both qualitatively and quantitatively, with the experimentally obtained ones for the oscillatory instability of straight rolls in pure Rayleigh - BĂ©nard convection
Unexpected Scaling of the Performance of Carbon Nanotube Transistors
We show that carbon nanotube transistors exhibit scaling that is
qualitatively different than conventional transistors. The performance depends
in an unexpected way on both the thickness and the dielectric constant of the
gate oxide. Experimental measurements and theoretical calculations provide a
consistent understanding of the scaling, which reflects the very different
device physics of a Schottky barrier transistor with a quasi-one-dimensional
channel contacting a sharp edge. A simple analytic model gives explicit scaling
expressions for key device parameters such as subthreshold slope, turn-on
voltage, and transconductance.Comment: 4 pages, 4 figure
Gravitational perturbations of the Schwarzschild spacetime: A practical covariant and gauge-invariant formalism
We present a formalism to study the metric perturbations of the Schwarzschild
spacetime. The formalism is gauge invariant, and it is also covariant under
two-dimensional coordinate transformations that leave the angular coordinates
unchanged. The formalism is applied to the typical problem of calculating the
gravitational waves produced by material sources moving in the Schwarzschild
spacetime. We examine the radiation escaping to future null infinity as well as
the radiation crossing the event horizon. The waveforms, the energy radiated,
and the angular-momentum radiated can all be expressed in terms of two
gauge-invariant scalar functions that satisfy one-dimensional wave equations.
The first is the Zerilli-Moncrief function, which satisfies the Zerilli
equation, and which represents the even-parity sector of the perturbation. The
second is the Cunningham-Price-Moncrief function, which satisfies the
Regge-Wheeler equation, and which represents the odd-parity sector of the
perturbation. The covariant forms of these wave equations are presented here,
complete with covariant source terms that are derived from the stress-energy
tensor of the matter responsible for the perturbation. Our presentation of the
formalism is concluded with a separate examination of the monopole and dipole
components of the metric perturbation.Comment: 21 page
Non-volatile molecular memory elements based on ambipolar nanotube field effect transistors
We have fabricated air-stable n-type, ambipolar carbon nanotube field effect
transistors (CNFETs), and used them in nanoscale memory cells. N-type
transistors are achieved by annealing of nanotubes in hydrogen gas and
contacting them by cobalt electrodes. Scanning gate microscopy reveals that the
bulk response of these devices is similar to gold-contacted p-CNFETs,
confirming that Schottky barrier formation at the contact interface determines
accessibility of electron and hole transport regimes. The transfer
characteristics and Coulomb Blockade (CB) spectroscopy in ambipolar devices
show strongly enhanced gate coupling, most likely due to reduction of defect
density at the silicon/silicon-dioxide interface during hydrogen anneal. The CB
data in the ``on''-state indicates that these CNFETs are nearly ballistic
conductors at high electrostatic doping. Due to their nanoscale capacitance,
CNFETs are extremely sensitive to presence of individual charge around the
channel. We demonstrate that this property can be harnessed to construct data
storage elements that operate at the few-electron level.Comment: 6 pages text, 3 figures and 1 table of content graphic; available as
NanoLetters ASAP article on the we
GePEToS : A Geant4 Monte Carlo simulation package for Positron Emission Tomography
GePEToS is a simulation framework developed over the last few years for
assessing the instrumental performance of future PET scanners. It is based on
Geant4, written in Object-Oriented C++ and runs on Linux platforms. The
validity of GePEToS has been tested on the well-known Siemens ECAT EXACT HR+
camera. The results of two application examples are presented : the design
optimization of a liquid Xe micro-PET camera dedicated to small animal imaging
as well as the evaluation of the effect of a strong axial magnetic field on the
image resolution of a Concorde P4 micro-PET camera.Comment: 5 pages, 12 figures, submitted to IEEE Transactions on Nuclear
Scienc
Anthropic Explanation of the Dark Matter Abundance
I use Bousso's causal diamond measure to make a statistical prediction for
the dark matter abundance, assuming an axion with a large decay constant f_a >>
10^{12} GeV. Using a crude approximation for observer formation, the prediction
agrees well with observation: 30% of observers form in regions with less dark
matter than we observe, while 70% of observers form in regions with more dark
matter. Large values of the dark matter ratio are disfavored by an elementary
effect: increasing the amount of dark matter while holding fixed the baryon to
photon ratio decreases the number of baryons inside one horizon volume. Thus
the prediction is rather insensitive to assumptions about observer formation in
universes with much more dark matter than our own. The key assumption is that
the number of observers per baryon is roughly independent of the dark matter
ratio for ratios near the observed value.Comment: 10 pages; v3: published version, references adde
Influence of single-neutron stripping on near-barrier <sup>6</sup>He+<sup>208</sup>Pb and <sup>8</sup>He+<sup>208</sup>Pb elastic scattering
The influence of single-neutron stripping on the near-barrier elastic scattering angular distributions for the 6,8He+208Pb systems is investigated through coupled reaction channels (CRC) calculations fitting recently published data to explore the differences in the absorptive potential found in the scattering of these two neutron-rich nuclei. The inclusion of the coupling reduces the elastic cross section in the Coulomb-nuclear interference region for 8He scattering, whereas for 6He its major impact is on the large-angle elastic scattering. The real and imaginary dynamic polarization potentials are obtained by inverting the CRC elastic scattering S-matrix elements. These show that the main absorptive features occur between 11 and 12 fm for both projectiles, while the attractive features are separated by about 1 fm, with their main structures occurring at 10.5 fm for 6He and 11.5 fm for 8He
Judicial Application of the Newspaper Preservation Act: Will Congressional Intent Be Relegated to the Back Pages?
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