35,039 research outputs found
Dynamic characteristics and processing of fillers in polyurethane elastomers for vibration damping applications
Polyurethane elastomers have the potential of being used to reduce vibrational noise in many engineering applications. The performance of the elastomer is directly related to matching the nature of the mechanical loss characteristics to the frequency and temperature dependence of the source of the vibration. Materials with a broad frequency response and good mechanical properties are desirable for situations were load bearing and isolation becomes an issue. Because automobile, and other related vehicles operate over a broad temperature range, it is desirable for the damping characteristics of the elastomer to ideally be independent of temperature and frequency. In practice, this is not possible and the creation of materials with a broad spectrum response is desirable. In this paper, the effects of various fillers on the breadth and temperature dependence of the vibration damping characteristics of a filled and crosslinked polyurethane elastomer are explored. The fillers studied are wollastonite, barium sulphate and talc. These materials have different shapes, sizes and surface chemistry and undergo different types of interaction with the matrix. The vibration damping characteristics were further varied by the use of a crosslinking agent. Data presented on the rheological characteristics indicate the strength of the filler-polyol interactions. Dielectric relaxation and dynamic mechanical thermal analysis demonstrate the way in which changes in the type of filler, concentration and amount of crosslinker lead to changes in the location and breadth of the energy dissipation process in these elastomers. The vibration damping characteristics of a selected material are presented to demonstrate the potential of these materials
Study of intermixing in a GaAs/AlGaAs quantum-well structure using doped spin-on silica layers
The effect of two different dopants, P and Ga, in spin-on glass (SOG) films on impurity-free vacancy disordering (IFVD) in GaAs/AlGaAs quantum-well structures has been investigated. It is observed that by varying the annealing and baking temperatures, P-doped SOG films created a similar amount of intermixing as the undoped SOG films. This is different from the results of other studies of P-doped SiO₂ and is ascribed to the low doping concentration of P, indicating that the doping concentration of P in the SiO₂ layer is one of the key parameters that may control intermixing. On the other hand, for all the samples encapsulated with Ga-doped SOG layers, significant suppression of the intermixing was observed, making them very promising candidates with which to achieve the selective-area defect engineering that is required for any successful application of IFVD.One of the authors (H.H.T.) acknowledges a fellowship
awarded to him by the Australian Research Council
Simulating Gyrokinetic Microinstabilities in Stellarator Geometry with GS2
The nonlinear gyrokinetic code GS2 has been extended to treat
non-axisymmetric stellarator geometry. Electromagnetic perturbations and
multiple trapped particle regions are allowed. Here, linear, collisionless,
electrostatic simulations of the quasi-axisymmetric, three-field period
National Compact Stellarator Experiment (NCSX) design QAS3-C82 have been
successfully benchmarked against the eigenvalue code FULL. Quantitatively, the
linear stability calculations of GS2 and FULL agree to within ~10%.Comment: Submitted to Physics of Plasmas. 9 pages, 14 figure
Lattice study on scattering with moving wall source
The s-wave pion-kaon () scattering lengths at zero momentum are
calculated in lattice QCD with sufficiently light quarks and strange
quark at its physical value by the finite size formula. The light quark masses
correspond to GeV. In the "Asqtad" improved staggered
fermion formulation, we measure the four-point correlators for both
isospin and 3/2 channels, and analyze the lattice simulation data at
the next-to-leading order in the continuum three-flavor chiral perturbation
theory, which enables us a simultaneous extrapolation of scattering
lengths at physical point. We adopt a technique with the moving wall sources
without gauge fixing to obtain the substantiable accuracy, moreover, for channel, we employ the variational method to isolate the contamination
from the excited states. Extrapolating to the physical point yields the
scattering lengths as and for and 1/2 channels, respectively. Our simulation results
for scattering lengths are in agreement with the experimental reports
and theoretical predictions, and can be comparable with other lattice
simulations. These simulations are carried out with MILC flavor
gauge configurations at lattice spacing fm.Comment: Use variational method to analyze I=1/2 channel for isolating the
contaminatio
Effects of the Lattice Discreteness on a Soliton in the Su-Schrieffer-Heeger Model
In this paper we analytically study the effects of the lattice discreteness
on the electron band in the SSH model. We propose a modified version of the TLM
model which is derived from the SSH model using a continuum approximation. When
a soliton is induced in the electron-lattice system, the electron scattering
states both at the bottom of the valence band and the top of the conduction
band are attracted to the soliton. This attractive force induces weakly
localized electronic states at the band edges. Using the modified version of
the TLM model, we have succeeded in obtaining analytical solutions of the
weakly localized states and the extended states near the bottom of the valence
band and the top of the conduction band. This band structure does not modify
the order parameters. Our result coincides well with numerical simulation
works.Comment: to be appear in J.Phys.Soc.Jpn. Figures should be requested to the
author. They will be sent by the conventional airmai
Nonlinearity-assisted quantum tunneling in a matter-wave interferometer
We investigate the {\em nonlinearity-assisted quantum tunneling} and
formation of nonlinear collective excitations in a matter-wave interferometer,
which is realised by the adiabatic transformation of a double-well potential
into a single-well harmonic trap. In contrast to the linear quantum tunneling
induced by the crossing (or avoided crossing) of neighbouring energy levels,
the quantum tunneling between different nonlinear eigenstates is assisted by
the nonlinear mean-field interaction. When the barrier between the wells
decreases, the mean-field interaction aids quantum tunneling between the ground
and excited nonlinear eigenstates. The resulting {\em non-adiabatic evolution}
depends on the input states. The tunneling process leads to the generation of
dark solitons, and the number of the generated dark solitons is highly
sensitive to the matter-wave nonlinearity. The results of the numerical
simulations of the matter-wave dynamics are successfully interpreted with a
coupled-mode theory for multiple nonlinear eigenstates.Comment: 11 pages, 6 figures, accept for publication in J. Phys.
Ground state of a polydisperse electrorheological solid: Beyond the dipole approximation
The ground state of an electrorheological (ER) fluid has been studied based
on our recently proposed dipole-induced dipole (DID) model. We obtained an
analytic expression of the interaction between chains of particles which are of
the same or different dielectric constants. The effects of dielectric constants
on the structure formation in monodisperse and polydisperse electrorheological
fluids are studied in a wide range of dielectric contrasts between the
particles and the base fluid. Our results showed that the established
body-centered tetragonal ground state in monodisperse ER fluids may become
unstable due to a polydispersity in the particle dielectric constants. While
our results agree with that of the fully multipole theory, the DID model is
much simpler, which offers a basis for computer simulations in polydisperse ER
fluids.Comment: Accepted for publications by Phys. Rev.
Stability of Unconventional Superconductivity on Surfaces of Topological Insulators
Superconductivity on the surface of topological insulators is known to be
anisotropic and unconventional in that the symmetry is the mixture of s-wave
and nodeless p-wave component. In contrast to Anderson's theorem for the
insensitivity of the s-wave superconducting critical temperature to the
nonmagnetic (time-reversal symmetric (TRS)) impurities, anisotropic
superconductors including nodeless p-wave one are in general fragile even with
small concentration of the TRS impurities. By employing the Abrikosov-Gor'kov
theory, we clarify that this type of unconventional superconductivity emergent
on the surface state of the strong topological insulators robustly survive
against TRS impurities
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