3,692 research outputs found
Acoustic waves: should they be propagated forward in time, or forward in space?
The evolution of acoustic waves can be evaluated in two ways: either as a
temporal, or a spatial propagation. Propagating in space provides the
considerable advantage of being able to handle dispersion and propagation
across interfaces with remarkable efficiency; but propagating in time is more
physical and gives correctly behaved reflections and scattering without effort.
Which should be chosen in a given situation, and what compromises might have to
be made? Here the natural behaviors of each choice of propagation are compared
and contrasted for an ordinary second order wave equation, the time-dependent
diffusion wave equation, an elastic rod wave equation, and the Stokes'/ van
Wijngaarden's equations, each case illuminating a characteristic feature of the
technique. Either choice of propagation axis enables a partitioning the wave
equation that gives rise to a directional factorization based on a natural
"reference" dispersion relation. The resulting exact coupled bidirectional
equations then reduce to a single unidirectional first-order wave equation
using a simple "slow evolution" assumption that minimizes effect of subsequent
approximations, while allowing a direct term-to-term comparison between exact
and approximate theories.Comment: 12 pages, v2 correcte
New Approach to Nonlinear Dynamics of Fullerenes and Fullerites
New type of nonlinear (anharmonic) excitations -- bushes of vibrational modes
-- in physical systems with point or space symmetry are discussed. All infrared
active and Raman active bushes for C60 fulerene are found by means of special
group-theoretical methods.Comment: LaTeX, 8 pages, to be published in Fizika Tverdogo Tela, 200
Surface polaritons on left-handed cylinders: A complex angular momentum analysis
We consider the scattering of electromagnetic waves by a left-handed cylinder
-- i.e., by a cylinder fabricated from a left-handed material -- in the
framework of complex angular momentum techniques. We discuss both the TE and TM
theories. We emphasize more particularly the resonant aspects of the problem
linked to the existence of surface polaritons. We prove that the long-lived
resonant modes can be classified into distinct families, each family being
generated by one surface polariton propagating close to the cylinder surface
and we physically describe all the surface polaritons by providing, for each
one, its dispersion relation and its damping. This can be realized by noting
that each surface polariton corresponds to a particular Regge pole of the
matrix of the cylinder. Moreover, for both polarizations, we find that there
exists a particular surface polariton which corresponds, in the large-radius
limit, to the surface polariton which is supported by the plane interface.
There exists also an infinite family of surface polaritons of
whispering-gallery type which have no analogs in the plane interface case and
which are specific to left-handed materials.Comment: published version. v3: reference list correcte
R-modes in Neutron Stars with Crusts: Turbulent Saturation, Spin-down, and Crust Melting
Rossby waves (r-modes) have been suggested as a means to regulate the spin
periods of young or accreting neutron stars, and also to produce observable
gravitational wave radiation. R-modes involve primarily transverse,
incompressive motions of the star's fluid core. However, neutron stars gain
crusts early in their lives: therefore, r-modes also imply shear in the fluid
beneath the crust. We examine the criterion for this shear layer to become
turbulent, and derive the rate of dissipation in the turbulent regime. Unlike
dissipation from a viscous boundary layer, turbulent energy loss is nonlinear
in mode energy and can therefore cause the mode to saturate at amplitudes
typically much less than unity. This energy loss also reappears as heat below
the crust. We study the possibility of crust melting as well as its
implications for the spin evolution of low-mass X-ray binaries. Lastly, we
identify some universal features of the spin evolution that may have
observational consequences.Comment: 12 pages, 4 figures, submitted to Ap
Qudit Quantum State Tomography
Recently quantum tomography has been proposed as a fundamental tool for
prototyping a few qubit quantum device. It allows the complete reconstruction
of the state produced from a given input into the device. From this
reconstructed density matrix, relevant quantum information quantities such as
the degree of entanglement and entropy can be calculated. Generally orthogonal
measurements have been discussed for this tomographic reconstruction. In this
paper, we extend the tomographic reconstruction technique to two new regimes.
First we show how non-orthogonal measurement allow the reconstruction of the
state of the system provided the measurements span the Hilbert space. We then
detail how quantum state tomography can be performed for multi qudits with a
specific example illustrating how to achieve this in one and two qutrit
systems.Comment: 6 pages, 4 figures, submitted to PR
Multi-layer mucilage of Plantago ovata seeds: Rheological differences arise from variations in arabinoxylan side chains
Abstract not availableLong Yu, Gleb E. Yakubov, Wei Zeng, Xiaohui Xing, John Stenson, Vincent Bulone, Jason R. Stoke
A phenomenological approach to normal form modeling: a case study in laser induced nematodynamics
An experimental setting for the polarimetric study of optically induced
dynamical behavior in nematic liquid crystal films has allowed to identify most
notably some behavior which was recognized as gluing bifurcations leading to
chaos. This analysis of the data used a comparison with a model for the
transition to chaos via gluing bifurcations in optically excited nematic liquid
crystals previously proposed by G. Demeter and L. Kramer. The model of these
last authors, proposed about twenty years before, does not have the central
symmetry which one would expect for minimal dimensional models for chaos in
nematics in view of the time series. What we show here is that the simplest
truncated normal forms for gluing, with the appropriate symmetry and minimal
dimension, do exhibit time signals that are embarrassingly similar to the ones
found using the above mentioned experimental settings. The gluing bifurcation
scenario itself is only visible in limited parameter ranges and substantial
aspect of the chaos that can be observed is due to other factors. First, out of
the immediate neighborhood of the homoclinic curve, nonlinearity can produce
expansion leading to chaos when combined with the recurrence induced by the
homoclinic behavior. Also, pairs of symmetric homoclinic orbits create extreme
sensitivity to noise, so that when the noiseless approach contains a rich
behavior, minute noise can transform the complex damping into sustained chaos.
Leonid Shil'nikov taught us that combining global considerations and local
spectral analysis near critical points is crucial to understand the
phenomenology associated to homoclinic bifurcations. Here this helps us
construct a phenomenological approach to modeling experiments in nonlinear
dissipative contexts.Comment: 25 pages, 9 figure
Strain-induced kinetics of intergrain defects as the mechanism of slow dynamics in the nonlinear resonant response of humid sandstone bars
A closed-form description is proposed to explain nonlinear and slow dynamics
effects exhibited by sandstone bars in longitudinal resonance experiments.
Along with the fast subsystem of longitudinal nonlinear displacements we
examine the strain-dependent slow subsystem of broken intergrain and
interlamina cohesive bonds. We show that even the simplest but
phenomenologically correct modelling of their mutual feedback elucidates the
main experimental findings typical for forced longitudinal oscillations of
sandstone bars, namely, (i) hysteretic behavior of a resonance curve on both
its up- and down-slopes, (ii) linear softening of resonant frequency with
increase of driving level, and (iii) gradual recovery (increase) of resonant
frequency at low dynamical strains after the sample was conditioned by high
strains. In order to reproduce the highly nonlinear elastic features of
sandstone grained structure a realistic non-perturbative form of strain
potential energy was adopted. In our theory slow dynamics associated with the
experimentally observed memory of peak strain history is attributed to
strain-induced kinetic changes in concentration of ruptured inter-grain and
inter-lamina cohesive bonds causing a net hysteretic effect on the elastic
Young's modulus. Finally, we explain how enhancement of hysteretic phenomena
originates from an increase in equilibrium concentration of ruptured cohesive
bonds that are due to water saturation.Comment: 5 pages, 3 figure
High-pressure phase and transition phenomena in ammonia borane NH3BH3 from X-ray diffraction, Landau theory, and ab initio calculations
Structural evolution of a prospective hydrogen storage material, ammonia
borane NH3BH3, has been studied at high pressures up to 12 GPa and at low
temperatures by synchrotron powder diffraction. At 293 K and above 1.1 GPa a
disordered I4mm structure reversibly transforms into a new ordered phase. Its
Cmc21 structure was solved from the diffraction data, the positions of N and B
atoms and the orientation of NH3 and BH3 groups were finally assigned with the
help of density functional theory calculations. Group-theoretical analysis
identifies a single two-component order parameter, combining ordering and
atomic displacement mechanisms, which link an orientationally disordered parent
phase I4mm with ordered distorted Cmc21, Pmn21 and P21 structures. We propose a
generic phase diagram for NH3BH3, mapping three experimentally found and one
predicted (P21) phases as a function of temperature and pressure, along with
the evolution of the corresponding structural distortions. Ammonia borane
belongs to the class of improper ferroelastics and we show that both
temperature- and pressure-induced phase transitions can be driven to be of the
second order. The role of N-H...H-B dihydrogen bonds and other intermolecular
interactions in the stability of NH3BH3 polymorphs is examined.Comment: 23 pages, 7 figure
Elastic turbulence in curvilinear flows of polymer solutions
Following our first report (A. Groisman and V. Steinberg, \sl Nature , 53 (2000)) we present an extended account of experimental observations of
elasticity induced turbulence in three different systems: a swirling flow
between two plates, a Couette-Taylor (CT) flow between two cylinders, and a
flow in a curvilinear channel (Dean flow). All three set-ups had high ratio of
width of the region available for flow to radius of curvature of the
streamlines. The experiments were carried out with dilute solutions of high
molecular weight polyacrylamide in concentrated sugar syrups. High polymer
relaxation time and solution viscosity ensured prevalence of non-linear elastic
effects over inertial non-linearity, and development of purely elastic
instabilities at low Reynolds number (Re) in all three flows. Above the elastic
instability threshold, flows in all three systems exhibit features of developed
turbulence. Those include: (i)randomly fluctuating fluid motion excited in a
broad range of spatial and temporal scales; (ii) significant increase in the
rates of momentum and mass transfer (compared to those expected for a steady
flow with a smooth velocity profile). Phenomenology, driving mechanisms, and
parameter dependence of the elastic turbulence are compared with those of the
conventional high Re hydrodynamic turbulence in Newtonian fluids.Comment: 23 pages, 26 figure
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