1,053 research outputs found
Application of optimality criteria in structural synthesis
The rational use of optimality criteria was investigated for a class of structural synthesis problems where materials, configuration and applied load conditions are specified, and the minimum weight design is to be determined. The potential of hybrid methods of structural optimization for dealing with relatively large design problems involving practical complexity was explored. The reduced basis concept in design space was used to decrease the number of generalized design variables dealt with by the mathematical programming algorithm. Optimality criteria methods for obtaining design vectors associated with displacement, system buckling and natural frequency constraints are presented. A stress ratio method was used to generate a basis design vector representing the stress constraints. The finite element displacement method was used as the basic structural analysis tool. Results for several examples of truss systems subject to stress, displacement and minimum size constraints are presented. An assessment of these results indicates the effectiveness of the hybrid method developed
Dynamics of an Acoustic Polaron in One-Dimensional Electron-Lattice System
The dynamical behavior of an acoustic polaron in typical non-degenerate
conjugated polymer, polydiacetylene, is numerically studied by using
Su-Schrieffer-Heeger's model for the one dimensional electron-lattice system.
It is confirmed that the velocity of a polaron accelerated by a constant
electric field shows a saturation to a velocity close to the sound velocity of
the system, and that the width of a moving polaron decreases as a monotonic
function of the velocity tending to zero at the saturation velocity. The
effective mass of a polaron is estimated to be about one hundred times as heavy
as the bare electron mass. Furthermore the linear mode analysis in the presence
of a polaron is carried out, leading to the conclusion that there is only one
localized mode, i.e. the translational mode. This is confirmed also from the
phase shift of extended modes. There is no localized mode corresponding to the
amplitude mode in the case of the soliton in polyacetylene. Nevertheless the
width of a moving polaron shows small oscillations in time. This is found to be
related to the lowest odd symmetry extended mode and to be due to the finite
size effect.Comment: 12 pages, latex, 9 figures (postscript figures abailble on request to
[email protected]) to be published in J. Phys. Soc. Jpn. vol.65
(1996) No.
Nonlinear optical response in doped conjugated polymers
Exciton effects on conjugated polymers are investigated in soliton lattice
states. We use the Su-Schrieffer-Heeger model with long-range Coulomb
interactions. The Hartree-Fock (HF) approximation and the single-excitation
configuration- interaction (single-CI) method are used to obtain optical
absorption spectra. The third-harmonic generation (THG) at off-resonant
frequencies is calculated as functions of the soliton concentration and the
chain length of the polymer. The magnitude of the THG at the 10 percent doping
increases by the factor about 10^2 from that of the neutral system. This is
owing to the accumulation of the oscillator strengths at the lowest exciton
with increasing the soliton concentration. The increase by the order two is
common for several choices of Coulomb interaction strengths.Comment: Accepted for publication in J. Phys.: Condens. Matte
Origin of intrinsic dark count in superconducting nanowire single-photon detectors
The origin of the decoherence in superconducting nanowire single-photon
detectors, the so-called dark count, was investigated. We measured the
direct-current characteristics and bias-current dependencies of the dark count
rate in a wide range of temperatures from 0.5 K to 4 K, and analyzed the
results by theoretical models of thermal fluctuations of vortices. Our results
indicate that the current-assisted unbinding of vortex-antivortex pairs is the
dominant origin of the dark count.Comment: 10 pages, 2 figure
Phase Change Observed in Ultrathin Ba0.5Sr0.5TiO3 Films by in-situ Resonant Photoemission Spectroscopy
Epitaxial Ba0.5Sr0.5TiO3 thin films were prepared on Nb-doped SrTiO3
(100)substrates by the pulsed laser deposition technique, and were studied by
measuring the Ti 2p - 3d resonant photoemission spectra in the valence-band
region as a function of film thickness, both at room temperature and low
temperature. Our results demonstrated an abrupt variation in the spectral
structures between 2.8 nm (~7 monolayers) and 2.0 nm (~5 monolayers)
Ba0.5Sr0.5TiO3 films, suggesting that there exists a critical thickness for
phase change in the range of 2.0 nm to 2.8 nm. This may be ascribed mainly to
the intrinsic size effects.Comment: 13 pages, 4 figure
Frustrated quantum-spin system on a triangle coupled with lattice vibrations - Correspondence to Longuet-Higgins et al.'s Jahn-Teller model -
We investigate the quantum three spin model
of spin on a triangle, in which spins are coupled with
lattice-vibrational modes through the exchange interaction depending on
distances between spin sites. The present model corresponds to the dynamic
Jahn-Teller system proposed by Longuet-Higgins {\it et al.},
Proc.R.Soc.A.{\bf 244},1(1958). This correspondence is revealed by using the
transformation to Nakamura-Bishop's bases proposed in Phys.Rev.Lett.{\bf
54},861(1985). Furthermore, we elucidate the relationship between the behavior
of a chiral order parameter and
that of the electronic orbital angular momentum in vibronic model: The regular oscillatory behavior of the expectation value
. The increase of the additional
anharmonicity(chaoticity) is found to yield a rapidly decaying irregular
oscillation of
Numerical study of the lattice vacancy effects on the single-channel electron transport of graphite ribbons
Lattice vacancy effects on electrical conductance of nanographite ribbon are
investigated by means of the Landauer approach using a tight binding model. In
the low-energy regime ribbons with zigzag boundary provide a single conducting
channel whose origin is connected with the presence of edge states. It is found
that the chemical potential dependence of conductance strongly depends on the
difference () of the number of removed A and B sublattice sites. The
large lattice vacancy with shows zero-conductance dips
in the single-channel region, however, the large lattice vacancy with
has no dip structure in this region. The connection between this
conductance rule and the Longuet-Higgins conjecture is also discussed
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