4,855 research outputs found
The goldstone real-time connected element interferometer
Connected element interferometry (CEI) is a technique of observing a celestial radio source at two spatially separated antennas and then interfering the received signals to extract the relative phase of the signal at the two antennas. The high precision of the resulting phase delay data type can provide an accurate determination of the angular position of the radio source relative to the baseline vector between the two stations. This article describes a recently developed connected element interferometer on a 21-km baseline between two antennas at the Deep Space Network's Goldstone, California, tracking complex. Fiber-optic links are used to transmit the data to a common site for processing. The system incorporates a real-time correlator to process these data in real time. The architecture of the system is described, and observational data are presented to characterize the potential performance of such a system. The real-time processing capability offers potential advantages in terms of increased reliability and improved delivery of navigational data for time-critical operations. Angular accuracies of 50-100 nrad are achievable on this baseline
Modelisation of transition and noble metal vicinal surfaces: energetics, vibrations and stability
The energetics of transition and noble metal (Rh, Pd, Cu) vicinal surfaces,
i.e., surface energy, step energy, kink energy and electronic interactions
between steps, is studied at 0K from electronic structure calculations in the
tight-binding approximation using a {\it s, p} and {\it d} valence orbital
basis set. Then, the surface phonon spectra of copper are investigated in the
harmonic approximation with the help of a semi-empirical inter-atomic
potential. This allows to derive the contribution of phonons at finite
temperatures to the step free energy and to the interactions between steps. The
last part is devoted to the stability of vicinal surfaces relative to faceting
with special attention to the domain of orientations (100)-(111).
Semi-empirical potentials are shown to be not realistic enough to give a
reliable answer to this problem. The results derived from electronic structure
calculations predict a variety of behaviors and, in particular, a possible
faceting into two other vicinal orientations. Finally, temperature effects are
discussed. Comparisons are made with other theoretical works and available
experiments
Dynamics of fullerene coalescence
Fullerene coalescence experimentally found in fullerene-embedded single-wall
nanotubes under electron-beam irradiation or heat treatment is simulated by
minimizing the classical action for many atom systems. The dynamical trajectory
for forming a (5,5) C nanocapsule from two C fullerene molecules
consists of thermal motions around potential basins and ten successive
Stone-Wales-type bond rotations after the initial cage-opening process for
which energy cost is about 8 eV. Dynamical paths for forming large-diameter
nanocapsules with (10,0), (6,6), and (12,0) chiral indexes have more bond
rotations than 25 with the transition barriers in a range of 10--12 eV.Comment: 4 pages, 2 figures, 1 supplementary movie at
http://dielc.kaist.ac.kr/yonghyun/coal.mpeg. To be published in Physical
Review Letter
Atomistic Simulations of Nanotube Fracture
The fracture of carbon nanotubes is studied by atomistic simulations. The
fracture behavior is found to be almost independent of the separation energy
and to depend primarily on the inflection point in the interatomic potential.
The rangle of fracture strians compares well with experimental results, but
predicted range of fracture stresses is marketly higher than observed. Various
plausible small-scale defects do not suffice to bring the failure stresses into
agreement with available experimental results. As in the experiments, the
fracture of carbon nanotubes is predicted to be brittle. The results show
moderate dependence of fracture strength on chirality.Comment: 12 pages, PDF, submitted to Phy. Rev.
Coil Formation in Multishell Carbon Nanotubes: Competition between Curvature Elasticity and Interlayer Adhesion
To study the shape formation process of carbon nanotubes, a string equation
describing the possible existing shapes of the axis-curve of multishell carbon
tubes (MCTs) is obtained in the continuum limit by minimizing the shape energy,
that is the difference between the MCT energy and the energy of the
carbonaceous mesophase (CM). It is shown that there exists a threshold relation
of the outmost and inmost radii, that gives a parameter regime in which a
straight MCT will be bent or twisted. Among the deformed shapes, the regular
coiled MCTs are shown being one of the solutions of the string equation. In
particular,the optimal ratio of pitch and radius for such a coil is
found to be equal to , which is in good agreement with recent
observation of coil formation in MCTs by Zhang et al.Comment: RevTeX, no figure, 12 pages, to appear in Phys. Rev. Let
Effects of finite curvature on soliton dynamics in a chain of nonlinear oscillators
We consider a curved chain of nonlinear oscillators and show that the
interplay of curvature and nonlinearity leads to a number of qualitative
effects. In particular, the energy of nonlinear localized excitations centered
on the bending decreases when curvature increases, i.e. bending manifests
itself as a trap for excitations. Moreover, the potential of this trap is
double-well, thus leading to a symmetry breaking phenomenon: a symmetric
stationary state may become unstable and transform into an energetically
favorable asymmetric stationary state. The essentials of symmetry breaking are
examined analytically for a simplified model. We also demonstrate a threshold
character of the scattering process, i.e. transmission, trapping, or reflection
of the moving nonlinear excitation passing through the bending.Comment: 13 pages (LaTeX) with 10 figures (EPS
Pressure-Induced Interlinking of Carbon Nanotubes
We predict new forms of carbon consisting of one and two dimensional networks
of interlinked single wall carbon nanotubes, some of which are energetically
more stable than van der Waals packing of the nanotubes on a hexagonal lattice.
These interlinked nanotubes are further transformed with higher applied
external pressures to more dense and complicated stable structures, in which
curvature-induced carbon sp re-hybridizations are formed. We also discuss
the energetics of the bond formation between nanotubes and the electronic
properties of these predicted novel structures.Comment: 4 pages, 4 postscript figures; To be appear in PR
Chromospheric polarimetry through multi-line observations of the 850 nm spectral region III: Chromospheric jets driven by twisted magnetic fields
We investigate the diagnostic potential of the spectral lines at 850 nm for
understanding the magnetism of the lower atmosphere. For that purpose, we use a
newly developed 3D simulation of a chromospheric jet to check the sensitivity
of the spectral lines to this phenomenon as well as our ability to infer the
atmospheric information through spectropolarimetric inversions of noisy
synthetic data. We start comparing the benefits of inverting the entire
spectrum at 850 nm versus only the Ca II 8542 A spectral line. We found a
better match of the input atmosphere for the former case, mainly at lower
heights. However, the results at higher layers were not accurate. After several
tests, we determined that we need to weight more the chromospheric lines than
the photospheric ones in the computation of the goodness of the fit. The new
inversion configuration allows us to obtain better fits and consequently more
accurate physical parameters. Therefore, to extract the most from multi-line
inversions, a proper set of weights needs to be estimated. Besides that, we
conclude again that the lines at 850 nm, or a similar arrangement with Ca II
8542 A plus Zeeman sensitive photospheric lines, poses the best observing
configuration for examining the thermal and magnetic properties of the lower
solar atmosphere.Comment: 14 pages, 11 figure
On the Dynamical Ferromagnetic, Quantum Hall, and Relativistic Effects on the Carbon Nanotubes Nucleation and Growth Mechanism
The mechanism of carbon nanotube (CNT) nucleation and growth has been a
mystery for over 15 years. Prior models have attempted the extension of older
classical transport mechanisms. In July 2000, a more detailed and accurate
nonclassical, relativistic mechanism was formulated considering the detailed
dynamics of the electronics of spin and orbital rehybridization between the
carbon and catalyst via novel mesoscopic phenomena and quantum dynamics.
Ferromagnetic carbon was demonstrated. Here, quantum (Hall) effects and
relativistic effects of intense many body spin-orbital interactions for novel
orbital rehybridization dynamics (Little Effect) are proposed in this new
dynamical magnetic mechanism. This dynamic ferromagnetic mechanism is proven by
imposing dynamic and static magnetic fields during CNT syntheses and observing
the different influence of these external magnetic environments on the
catalyzing spin currents and spin waves and the resulting CNT formation
Trapping cold atoms near carbon nanotubes: thermal spin flips and Casimir-Polder potential
We investigate the possibility to trap ultracold atoms near the outside of a
metallic carbon nanotube (CN) which we imagine to use as a miniaturized
current-carrying wire. We calculate atomic spin flip lifetimes and compare the
strength of the Casimir-Polder potential with the magnetic trapping potential.
Our analysis indicates that the Casimir-Polder force is the dominant loss
mechanism and we compute the minimum distance to the carbon nanotube at which
an atom can be trapped.Comment: 8 pages, 3 figure
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