17,972 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
Dynamics of Multistage Gear Transmission with Effects of Gearbox Vibrations
A comprehensive approach is presented in analyzing the dynamic behavior of multistage gear transmission systems with the effects of gearbox induced vibrations and mass imbalances of the rotor. The modal method, with undamped frequencies and planar mode shapes, is used to reduce the degrees of freedom of the gear system for time-transient dynamic analysis. Both the lateral and torsional vibration modes of each rotor-bearing-gear stage as well as the interstage vibrational characteristics are coupled together through localized gear mesh tooth interactions. In addition, gearbox vibrations are also coupled to the rotor-bearing-gear system dynamics through bearing support forces between the rotor and the gearbox. Transient and steady state dynamics of lateral and torsional vibrations of the geared system are examined in both time and frequency domains to develop interpretations of the overall modal dynamic characteristics under various operating conditions. A typical three-stage geared system is used as an example. Effects of mass imbalance and gearbox vibrations on the system dynamic behavior are presented in terms of modal excitation functions for both lateral and torsional vibrations. Operational characteristics and conclusions are drawn from the results presented
Observation of an in-plane magnetic-field-driven phase transition in a quantum Hall system with SU(4) symmetry
In condensed matter physics, the study of electronic states with SU(N)
symmetry has attracted considerable and growing attention in recent years, as
systems with such a symmetry can often have a spontaneous symmetry-breaking
effect giving rise to a novel ground state. For example, pseudospin quantum
Hall ferromagnet of broken SU(2) symmetry has been realized by bringing two
Landau levels close to degeneracy in a bilayer quantum Hall system. In the past
several years, the exploration of collective states in other multi-component
quantum Hall systems has emerged. Here we show the conventional pseudospin
quantum Hall ferromagnetic states with broken SU(2) symmetry collapsed rapidly
into an unexpected state with broken SU(4) symmetry, by in-plane magnetic field
in a two-subband GaAs/AlGaAs two-dimensional electron system at filling factor
around . Within a narrow tilting range angle of 0.5 degrees, the
activation energy increases as much as 12 K. While the origin of this puzzling
observation remains to be exploited, we discuss the possibility of a
long-sought pairing state of electrons with a four-fold degeneracy.Comment: 13 pages, 4 figure
Modal analysis of multistage gear systems coupled with gearbox vibrations
An analytical procedure to simulate vibrations in gear transmission systems is presented. This procedure couples the dynamics of the rotor-bearing gear system with the vibration in the gear box structure. The model synthesis method is used in solving the overall dynamics of the system, and a variable time-stepping integration scheme is used in evaluating the global transient vibration of the system. Locally each gear stage is modeled as a multimass rotor-bearing system using a discrete model. The modal characteristics are calculated using the matrix-transfer technique. The gearbox structure is represented by a finite element models, and modal parameters are solved by using NASTRAN. The rotor-gear stages are coupled through nonlinear compliance in the gear mesh while the gearbox structure is coupled through the bearing supports of the rotor system. Transient and steady state vibrations of the coupled system are examined in both time and frequency domains. A typical three-geared system is used as an example for demonstration of the developed procedure
Molecular Motor of Double-Walled Carbon Nanotube Driven by Temperature Variation
An elegant formula for coordinates of carbon atoms in a unit cell of a
single-walled nanotube (SWNT) is presented and a new molecular motor of
double-walled carbon nanotube whose inner tube is a long (8,4) SWNT and outer
tube a short (14,8) SWNT is constructed. The interaction between inner an outer
tubes is analytically derived by summing the Lennard-Jones potentials between
atoms in inner and outer tubes. It is proved that the molecular motor in a
thermal bath exhibits a directional motion with the temperature variation of
the bath.Comment: 9 pages, 4 figures, revtex
Description of Hf in the constrained relativistic mean field theory
The properties of the ground state of Hf and the isomeric state
Hf are studied within the adiabatic and diabatic constrained
relativistic mean field (RMF) approaches. The RMF calculations reproduce well
the binding energy and the deformation for the ground state of Hf.
Using the ground state single-particle eigenvalues obtained in the present
calculation, the lowest excitation configuration with is found to
be
. Its excitation energy calculated by the
RMF theory with time-odd fields taken into account is equal to 2.801 MeV, i.e.,
close to the Hf experimental excitation energy 2.446 MeV. The
self-consistent procedure accounting for the time-odd component of the meson
fields is the most important aspect of the present calculation.Comment: 12 pages(preprint), 2 figures, 1 tabl
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
2D and 3D cubic monocrystalline and polycrystalline materials: their stability and mechanical properties
We consider 2- and 3-dimensional cubic monocrystalline and polycrystalline
materials. Expressions for Young's and shear moduli and Poisson's ratio are
expressed in terms of eigenvalues of the stiffness tensor. Such a form is well
suited for studying properties of these mechanical characteristics on sides of
the stability triangles. For crystalline high-symmetry directions lines of
vanishing Poisson's ratio are found. These lines demarcate regions of the
stability triangle into areas of various auxeticity properties. The simplest
model of polycrystalline 2D and 3D cubic materials is considered. In
polycrystalline phases the region of complete auxetics is larger than for
monocrystalline materials.Comment: 9 pages, 3 figures, in proceedings of the Tenth International School
on Theoretical Physics, Symmetry and Structural Properties of Condensed
Matter, Myczkowce 200
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