54,204 research outputs found
Dynamic homogenisation of Maxwellâs equations with applications to photonic crystals and localised waveforms on gratings
A two-scale asymptotic theory is developed to generate continuum equations that model the macroscopic be- haviour of electromagnetic waves in periodic photonic structures when the wavelength is not necessarily long relative to the periodic cell dimensions; potentially highly-oscillatory short-scale detail is encapsulated through integrated quantities. The resulting equations include tensors that represent effective refractive indices near band edge frequencies along all principal axes directions, and these govern scalar functions providing long-scale mod- ulation of short-scale Bloch eigenstates, which can be used to predict the propagation of waves at frequencies outside of the long wavelength regime; these results are outside of the remit of typical homogenisation schemes. The theory we develop is applied to two topical examples, the first being the case of aligned dielectric cylin- ders, which has great importance in modelling photonic crystal fibres. Results of the asymptotic theory are veri- fied against numerical simulations by comparing photonic band diagrams and evanescent decay rates for guided modes. The second example is the propagation of electromagnetic waves localised within a planar array of di- electric spheres; at certain frequencies strongly directional propagation is observed, commonly described as dy- namic anisotropy. Computationally this is a challenging three-dimensional calculation, which we perform, and then demonstrate that the asymptotic theory captures the effect, giving highly accurate qualitative and quantitative comparisons as well as providing interpretation for the underlying change from elliptic to hyperbolic behaviour
Surprising simplicity in the modeling of dynamic granular intrusion
Granular intrusions, such as dynamic impact or wheel locomotion, are complex
multiphase phenomena where the grains exhibit solid-like and fluid-like
characteristics together with an ejected gas-like phase. Despite decades of
modeling efforts, a unified description of the physics in such intrusions is as
yet unknown. Here we show that a continuum model based on the simple notions of
frictional flow and tension-free separation describes complex granular
intrusions near free surfaces. This model captures dynamics in a variety of
experiments including wheel locomotion, plate intrusions, and running legged
robots. The model reveals that three effects (a static contribution and two
dynamic ones) primarily give rise to intrusion forces in such scenarios.
Identification of these effects enables the development of a further
reduced-order technique (Dynamic Resistive Force Theory) for rapid modeling of
granular locomotion of arbitrarily shaped intruders. The continuum-motivated
strategy we propose for identifying physical mechanisms and corresponding
reduced-order relations has potential use for a variety of other materials.Comment: 41 pages including supplementary document, 10 figures, and 8 vide
Local Optical Probe of Motion and Stress in a multilayer graphene NEMS
Nanoelectromechanical systems (NEMSs) are emerging nanoscale elements at the
crossroads between mechanics, optics and electronics, with significant
potential for actuation and sensing applications. The reduction of dimensions
compared to their micronic counterparts brings new effects including
sensitivity to very low mass, resonant frequencies in the radiofrequency range,
mechanical non-linearities and observation of quantum mechanical effects. An
important issue of NEMS is the understanding of fundamental physical properties
conditioning dissipation mechanisms, known to limit mechanical quality factors
and to induce aging due to material degradation. There is a need for detection
methods tailored for these systems which allow probing motion and stress at the
nanometer scale. Here, we show a non-invasive local optical probe for the
quantitative measurement of motion and stress within a multilayer graphene NEMS
provided by a combination of Fizeau interferences, Raman spectroscopy and
electrostatically actuated mirror. Interferometry provides a calibrated
measurement of the motion, resulting from an actuation ranging from a
quasi-static load up to the mechanical resonance while Raman spectroscopy
allows a purely spectral detection of mechanical resonance at the nanoscale.
Such spectroscopic detection reveals the coupling between a strained
nano-resonator and the energy of an inelastically scattered photon, and thus
offers a new approach for optomechanics
Directed current in the Holstein system
We propose a mechanism to rectify charge transport in the semiclassical
Holstein model. It is shown that localised initial conditions, associated with
a polaron solution, in conjunction with a nonreversion symmetric static
electron on-site potential constitute minimal prerequisites for the emergence
of a directed current in the underlying periodic lattice system. In particular,
we demonstrate that for unbiased spatially localised initial conditions,
violation of parity prevents the existence of pairs of counter-propagating
trajectories, thus allowing for a directed current despite the
time-reversibility of the equations of motion. Occurrence of long-range
coherent charge transport is demonstrated
Description of Multi Quasi Particle Bands by the Tilted Axis Cranking Model
The selfconsistent cranking approach is extended to the case of rotation
about an axis which is tilted with respect to the principal axes of the
deformed potential (Tilted Axis Cranking). Expressions for the energies and the
intra bands electromagnetic transition probabilities are given.
The mean field solutions are interpreted in terms of quantal rotational
states. The construction of the quasiparticle configurations and the
elimination of spurious states is discussed. The application of the theory to
high spin data is demonstrated by analyzing the multi quasiparticle bands in
the nuclide-s with and .Comment: 23 pages 27 figure
A review of the effectiveness of lower limb orthoses used in cerebral palsy
To produce this review, a systematic literature search was conducted for relevant articles published in the period between the date of the previous ISPO consensus conference report on cerebral palsy (1994) and April 2008. The search terms were 'cerebral and pals* (palsy, palsies), 'hemiplegia', 'diplegia', 'orthos*' (orthoses, orthosis) orthot* (orthotic, orthotics), brace or AFO
Shape of an elastica under growth restricted by friction
We investigate the quasi-static growth of elastic fibers in the presence of
dry or viscous friction. An unusual form of destabilization beyond a critical
length is described. In order to characterize this phenomenon, a new definition
of stability against infinitesimal perturbations over finite time intervals is
proposed and a semi-analytical method for the determination of the critical
length is developed. The post-critical behavior of the system is studied by
using an appropriate numerical scheme based on variational methods. We find
post-critical shapes for uniformly distributed as well as for concentrated
growth and demonstrate convergence to a figure-8 shape for large lengths when
self-crossing is allowed. Comparison with simple physical experiments yields
reasonable accuracy of the theoretical predictions
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