7,183 research outputs found
Large Magnetic Susceptibility Anisotropy of Metallic Carbon Nanotubes
Through magnetic linear dichroism spectroscopy, the magnetic susceptibility
anisotropy of metallic single-walled carbon nanotubes has been extracted and
found to be 2-4 times greater than values for semiconducting single-walled
carbon nanotubes. This large anisotropy is consistent with our calculations and
can be understood in terms of large orbital paramagnetism of electrons in
metallic nanotubes arising from the Aharonov-Bohm-phase-induced gap opening in
a parallel field. We also compare our values with previous work for
semiconducting nanotubes, which confirm a break from the prediction that the
magnetic susceptibility anisotropy increases linearly with the diameter.Comment: 4 pages, 4 figure
Existence of an upper limit on the density of excitons in carbon nanotubes by diffusion-limited exciton-exciton annihilation: Experiment and theory
Through an investigation of photoemission properties of highly-photoexcited
single-walled carbon nanotubes, we demonstrate that there is an upper limit on
the achievable excitonic density. As the intensity of optical excitation
increases, all photoluminescence emission peaks arising from different
chirality single-walled carbon nanotubes showed clear saturation in intensity.
Each peak exhibited a saturation value that was independent of the excitation
wavelength, indicating that there is an upper limit on the excitonic density
for each nanotube species. We propose that this saturation behavior is a result
of efficient exciton-exciton annihilation through which excitons decay
non-radiatively. In order to explain the experimental results and obtain
excitonic densities in the saturation regime, we have developed a model, taking
into account the generation, diffusion-limited exciton-exciton annihilation,
and spontaneous decays of one-dimensional excitons. Using the model, we were
able to reproduce the experimentally obtained saturation curves under certain
approximations, from which the excitonic densities were estimated. The validity
of the model was confirmed through comparison with Monte Carlo simulations.
Finally, we show that the conventional rate equation for exciton-exciton
annihilation without taking into account exciton diffusion fails to fit the
experimentally observed saturation behavior, especially at high excitonic
densities.Comment: 5 figures, 1 tabl
The ATLAS trigger menu for early data-taking
The ATLAS trigger system is based on three levels of event selection that
select the physics of interest from an initial bunch-crossing rate of 40 MHz.
During nominal LHC operations at a luminosity of 10^34 cm^-2 s^-1, decisions
must be taken every 25 ns with each bunch crossing containing about 23
interactions. The selections in the three trigger levels must provide
sufficient rejection to reduce the rate down to 200 Hz, compatible with the
offline computing power and storage capacity. The LHC is expected to begin
operations in summer 2008 with a peak luminosity of 10^31 cm^-2 s^-1 with far
fewer bunches than nominal running, but quickly ramp up to higher luminosities.
Hence, we need to deploy trigger selections that can adapt to the changing beam
conditions preserving the interesting physics and detector requirements that
may vary with these conditions.
We present the status of the preparation of the trigger menu for the early
data-taking showing how we plan to deploy the trigger system from the first
collision to the nominal luminosity. We also show expected rates and physics
performance obtained from simulated data.Comment: Poster presentation at the Hadron Collider Physics Symposium
(HCP2008), Galena, Illinois, USA, May 27-31, 2008; 5 pages, LaTeX, 2 eps
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