13,011 research outputs found
Entanglement and the nonlinear elastic behavior of forests of coiled carbon nanotubes
Helical or coiled nanostructures have been object of intense experimental and
theoretical studies due to their special electronic and mechanical properties.
Recently, it was experimentally reported that the dynamical response of
foamlike forest of coiled carbon nanotubes under mechanical impact exhibits a
nonlinear, non-Hertzian behavior, with no trace of plastic deformation. The
physical origin of this unusual behavior is not yet fully understood. In this
work, based on analytical models, we show that the entanglement among
neighboring coils in the superior part of the forest surface must be taken into
account for a full description of the strongly nonlinear behavior of the impact
response of a drop-ball onto a forest of coiled carbon nanotubes.Comment: 4 pages, 3 figure
Effect of the frequency chirp on laser wakefield acceleration
The role of laser frequency chirps in the laser wakefield accelerator is
examined. We show that in the linear regime, the evolution of the laser pulse
length is affected by the frequency chirp, and that positive (negative) chirp
compresses (stretches) the laser pulse, thereby increasing (decreasing) the
peak vector potential and wakefield amplitude. In the blowout regime, the
frequency chirp can be used to fine tune the localized etching rates at the
front of the laser. In our simulations, chirped laser pulses can lead to 15%
higher self-trapped electrons, and 10% higher peak energies as compare to the
transform-limited pulse. Chirps may be used to control the phase velocity of
the wake, and to relax the self-guiding conditions at the front of the laser.
Our predictions are confirmed by multi-dimensional particle-in-cell simulations
with OSIRIS.Comment: Submitted in NJP, 4 figure
Coherent photon assisted cotunneling in a Coulomb blockade device
We study cotunneling in a double junction Coulomb blockade device under the
influence of time dependent potentials. It is shown that the ac-bias leads to
photon assisted cotunneling which in some cases may dominate the transport. We
derive a general non-perturbative expression for the tunneling current in the
presence of oscillating potentials and give a perturbative expression for the
photon assisted cotunneling current.Comment: Replaced with a longer paper which includes a non-perturbative
calculation. 13 pages with 1 figure. To be published in Physical Review
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