17,843 research outputs found
Chirality Dependence of the -Momentum Dark Excitons in Carbon Nanotubes
Using a collection of twelve semiconducting carbon nanotube samples, each
highly enriched in a single chirality, we study the chirality dependence of the
-momentum dark singlet exciton using phonon sideband optical spectroscopy.
Measurements of bright absorptive and emissive sidebands of this finite
momentum exciton identify its energy as 20 - 38 meV above the bright singlet
exciton, a separation that exhibits systematic dependencies on tube diameter,
family, and semiconducting type. We present calculations that explain
how chiral angle dependence in this energy separation relates to the Coulomb
exchange interaction, and elaborate the dominance of the phonon
sidebands over the zone-center phonon sidebands over a wide range of
chiralities. The Kataura plot arising from these data is qualitatively well
described by theory, but the energy separation between the sidebands shows a
larger chiral dependence than predicted. This latter observation may indicate a
larger dispersion for the associated phonon near the point than expected
from finite distance force modeling.Comment: 24 pages, 12 figures, 1 table; slight title change, Figures 1 and 11
added, reference added, presentation improved throughout documen
Vibration signature analysis of multistage gear transmission
An analysis is presented for multistage multimesh gear transmission systems. The analysis predicts the overall system dynamics and the transmissibility to the gear box or the enclosed structure. The modal synthesis approach of the analysis treats the uncoupled lateral/torsional model characteristics of each stage or component independently. The vibration signature analysis evaluates the global dynamics coupling in the system. The method synthesizes the interaction of each modal component or stage with the nonlinear gear mesh dynamics and the modal support geometry characteristics. The analysis simulates transient and steady state vibration events to determine the resulting torque variations, speeds, changes, rotor imbalances, and support gear box motion excitations. A vibration signature analysis examines the overall dynamic characteristics of the system, and the individual model component responses. The gear box vibration analysis also examines the spectral characteristics of the support system
Ultra narrow AuPd and Al wires
In this letter we discuss a novel and versatile template technique aimed to
the fabrication of sub-10 nm wide wires. Using this technique, we have
successfully measured AuPd wires, 12 nm wide and as long as 20 m. Even
materials that form a strong superficial oxide, and thus not suited to be used
in combination with other techniques, can be successfully employed. In
particular we have measured Al wires, with lateral width smaller or comparable
to 10 nm, and length exceeding 10 m.Comment: 4 pages, 4 figures. Pubblished in APL 86, 172501 (2005). Added
erratum and revised Fig.
Coefficient of performance at maximum figure of merit and its bounds for low-dissipation Carnot-like refrigerators
The figure of merit for refrigerators performing finite-time Carnot-like
cycles between two reservoirs at temperature and () is
optimized. It is found that the coefficient of performance at maximum figure of
merit is bounded between 0 and for the
low-dissipation refrigerators, where is the
Carnot coefficient of performance for reversible refrigerators. These bounds
can be reached for extremely asymmetric low-dissipation cases when the ratio
between the dissipation constants of the processes in contact with the cold and
hot reservoirs approaches to zero or infinity, respectively. The observed
coefficients of performance for real refrigerators are located in the region
between the lower and upper bounds, which is in good agreement with our
theoretical estimation.Comment: 5 journal pages, 3 figure
Finite Size Scaling of Domain Chaos
Numerical studies of the domain chaos state in a model of rotating
Rayleigh-Benard convection suggest that finite size effects may account for the
discrepancy between experimentally measured values of the correlation length
and the predicted divergence near onset
Front Stability in Mean Field Models of Diffusion Limited Growth
We present calculations of the stability of planar fronts in two mean field
models of diffusion limited growth. The steady state solution for the front can
exist for a continuous family of velocities, we show that the selected velocity
is given by marginal stability theory. We find that naive mean field theory has
no instability to transverse perturbations, while a threshold mean field theory
has such a Mullins-Sekerka instability. These results place on firm theoretical
ground the observed lack of the dendritic morphology in naive mean field theory
and its presence in threshold models. The existence of a Mullins-Sekerka
instability is related to the behavior of the mean field theories in the
zero-undercooling limit.Comment: 26 pp. revtex, 7 uuencoded ps figures. submitted to PR
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