377 research outputs found
Electromechanical instability in suspended carbon nanotubes
We have theoretically investigated electromechanical properties of freely
suspended carbon nanotubes when a current is injected into the tubes using a
scanning tunneling microscope. We show that a shuttle-like electromechanical
instability can occur if the bias voltage exceeds a dissipation-dependent
threshold value. An instability results in large amplitude vibrations of the
carbon nanotube bending mode, which modify the current-voltage characteristics
of the system
Impact of van der Waals forces on the classical shuttle instability
The effects of including the van der Waals interaction in the modelling of
the single electron shuttle have been investigated numerically. It is
demonstrated that the relative strength of the vdW-forces and the elastic
restoring forces determine the characteristics of the shuttle instability. In
the case of weak elastic forces and low voltages the grain is trapped close to
one lead, and this trapping can be overcome by Coulomb forces by applying a
bias voltage larger than a threshold voltage . This allows for
grain motion leading to an increase in current by several orders of magnitude
above the transition voltage . Associated with the process is also
hysteresis in the I-V characteristics.Comment: minor revisions, updated references, Article published in Phys. Rev.
B 69, 035309 (2004
Incoherent dynamics of vibrating single-molecule transistors
We study the tunneling conductance of nano-scale quantum ``shuttles'' in
connection with a recent experiment (H. Park et al., Nature, 407, 57 (2000)) in
which a vibrating C^60 molecule was apparently functioning as the island of a
single electron transistor (SET). While our calculation starts from the same
model of previous work (D. Boese and H. Schoeller, Europhys. Lett. 54,
66(2001)) we obtain quantitatively different dynamics. Calculated I-V curves
exhibit most features present in experimental data with a physically reasonable
parameter set, and point to a strong dependence of the oscillator's potential
on the electrostatics of the island region. We propose that in a regime where
the electric field due to the bias voltage itself affects island position, a
"catastrophic" negative differential conductance (NDC) may be realized. This
effect is directly attributable to the magnitude of overlap of final and
initial quantum oscillator states, and as such represents experimental control
over quantum transitions of the oscillator via the macroscopically controllable
bias voltage.Comment: 6 pages, LaTex, 6 figure
Multiscale Modeling of a Nanoelectromechanical Shuttle
In this article, we report a theoretical analysis of a nanoelectromechanical
shuttle based on a multiscale model that combines microscopic electronic
structure data with macroscopic dynamics. The microscopic part utilizes a
(static) density functional description to obtain the energy levels and
orbitals of the shuttling particle together with the forces acting on the
particle. The macroscopic part combines stochastic charge dynamics that
incorporates the microscopically evaluated tunneling rates with a Newtonian
dynamics.
We have applied the multiscale model to describe the shuttling of a single
copper atom between two gold-like jellium electrodes. We find that energy
spectrum and particle surface interaction greatly influence shuttling dynamics;
in the specific example that we studied the shuttling is found to involve only
charge states Q=0 and Q=+e. The system is found to exhibit two quasi-stable
shuttling modes, a fundamental one and an excited one with a larger amplitude
of mechanical motion, with random transitions between them.Comment: 9 pages, 9 figure
a0+(980)-resonance production in pp->dK+Kbar0 reactions close to threshold
The reaction pp->d K+ Kbar0 has been investigated at an excess energy of Q=46
MeV above the (K+ Kbar0) threshold with ANKE at COSY-Juelich. From the detected
coincident dK+ pairs about 1000 events with a missing Kbar0 were identified,
corresponding to a total cross section of sigma(pp->d K+ Kbar0)=(38 +/- 2(stat)
+/- 14(syst)) nb. Invariant-mass and angular distributions have been jointly
analyzed and reveal s-wave dominance between the two kaons, accompanied by a
p-wave between the deuteron and the kaon system. This is interpreted in terms
of a0+(980)-resonance production.Comment: 4 pages, 4 figures; accepted by Phys. Rev. Let
Luminosity determination for the pd reaction at 2.14 GeV with WASA-at-COSY
The luminosity for a WASA-at-COSY experiment involving the pd reaction at
2.14 GeV proton-beam energy is determined by the forward pd elastic scattering,
which yields an average beam-on-target value of
[5.2\pm0.3(stat)\pm0.3(syst)]*10^{30} s^{-1}cm^{-2}. In addition, the forward
pd elastic-scattering angular distribution is obtained with four-momentum
transfer squared -t between 0.16 (GeV/c)^{2} and 0.78 (GeV/c)^{2} at this beam
energy, which is compared with other experimental data and the pd double
scattering model.Comment: 5 pages, 6 figure
Scalar-isovector K K-bar production close to threshold
The reaction pp -> d K^+ K^0-bar has been investigated at excess energies Q =
47.4 and 104.7 MeV above the K^+ K^0-bar threshold at COSY-Juelich. Coincident
dK^+ pairs were detected with the ANKE spectrometer, and events with a missing
K^0-bar invariant-mass subsequently identified. The joint analysis of
invariant-mass and angular distributions reveals s-wave dominance between the
two kaons, in conjunction with a p-wave between the deuteron and the kaon pair,
i.e. K K-bar production via the a_0^+(980) channel. Integration of the
differential distributions yields total cross sections of sigma(pp -> d K^+
K^0-bar) = (38 +/- 2(stat) +/- 14(syst)) nb and 190 +/- 4(stat) +/- 39(syst))
nb for the low and high Q values, respectively.Comment: 7 pages, 6 figures; accepted for publication in Eur.Phys.J.
Dynamics of Cryogenic Jets: Non-Rayleigh Breakup and Onset of Nonaxisymmetric Motions
We report development of generators for periodic, satellite-free fluxes of
mono-disperse drops with diameters down to 10 mikrometers from cryogenic
liquids like H_2, N_2, Ar and Xe (and, as reference fluid, water). While the
breakup of water jets can well be described by Rayleigh's linear theory, we
find jet regimes for H_2 and N_2 which reveal deviations from this behavior.
Thus, Rayleigh's theory is inappropriate for thin jets that exchange energy
and/or mass with the surrounding medium. Moreover, at high evaporation rates,
axial symmetry of the dynamics is lost. When the drops pass into vacuum, frozen
pellets form due to surface evaporation. The narrow width of the pellet flux
paves the way towards various industrial and scientific applications.Comment: 4 pages, 4 figures, 1 table; final version to appear in Phys.Rev.Lett
(minor changes with respect to v1
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