644 research outputs found
Flux Expulsion - Field Evolution in Neutron Stars
Models for the evolution of magnetic fields of neutron stars are constructed,
assuming the field is embedded in the proton superconducting core of the star.
The rate of expulsion of the magnetic flux out of the core, or equivalently the
velocity of outward motion of flux-carrying proton-vortices is determined from
a solution of the Magnus equation of motion for these vortices. A force due to
the pinning interaction between the proton-vortices and the neutron-superfluid
vortices is also taken into account in addition to the other more conventional
forces acting on the proton-vortices. Alternative models for the field
evolution are considered based on the different possibilities discussed for the
effective values of the various forces. The coupled spin and magnetic evolution
of single pulsars as well as those processed in low-mass binary systems are
computed, for each of the models. The predicted lifetimes of active pulsars,
field strengths of the very old neutron stars, and distribution of the magnetic
fields versus orbital periods in low-mass binary pulsars are used to test the
adopted field decay models. Contrary to the earlier claims, the buoyancy is
argued to be the dominant driving cause of the flux expulsion, for the single
as well as the binary neutron stars. However, the pinning is also found to play
a crucial role which is necessary to account for the observed low field binary
and millisecond pulsars.Comment: 23 pages, + 7 figures, accepted for publication in Ap
Wieldy Finger and Hand Motion Detection for Human Computer Interaction
We have developed a gesture based interface for human computer interaction under the research field of computer vision.Earlier system have used the costlier system devices to make an effective interaction with systems, instead we have worked on the web cam based gesture input system.Our goal was to propound lesser cost, wieldy, object detection technique using blobs for detection of fingers.And to give number of count of the same.In addition, we have also implemented the hand gesture recognition
Spin-down Rate of Pinned Superfluid
The spinning down (up) of a superfluid is associated with a radial motion of
its quantized vortices. In the presence of pinning barriers against the motion
of the vortices, a spin-down may be still realized through ``random unpinning''
and ``vortex motion,'' as two physically separate processes, as suggested
recently. The spin-down rate of a pinned superfluid is calculated, in this
framework, by directly solving the equation of motion applicable to only the
unpinned moving vortices, at any given time. The results indicate that the
pinned superfluid in the crust of a neutron star may as well spin down at the
same steady-state rate as the rest of the star, through random unpinning
events, while pinning conditions prevail and the superfluid rotational lag is
smaller than the critical lag value.Comment: to appear in ApJ (vol. 649 ?
Non-contact rack and pinion powered by the lateral Casimir force
The lateral Casimir force is employed to propose a design for a potentially
wear-proof rack and pinion with no contact, which can be miniaturized to
nano-scale. The robustness of the design is studied by exploring the relation
between the pinion velocity and the rack velocity in the different domains of
the parameter space. The effects of friction and added external load are also
examined. It is shown that the device can hold up extremely high velocities,
unlike what the general perception of the Casimir force as a weak interaction
might suggest.Comment: 4 pages, submitted for publication on 17 Jan 0
Diffusive transport of light in three-dimensional disordered Voronoi structures
The origin of diffusive transport of light in dry foams is still under
debate. In this paper, we consider the random walks of photons as they are
reflected or transmitted by liquid films according to the rules of ray optics.
The foams are approximately modeled by three-dimensional Voronoi tessellations
with varying degree of disorder. We study two cases: a constant intensity
reflectance and the reflectance of thin films. Especially in the second case,
we find that in the experimentally important regime for the film thicknesses,
the transport-mean-free path does not significantly depend on the topological
and geometrical disorder of the Voronoi foams including the periodic Kelvin
foam. This may indicate that the detailed structure of foams is not crucial for
understanding the diffusive transport of light. Furthermore, our theoretical
values for transport-mean-free path fall in the same range as the experimental
values observed in dry foams. One can therefore argue that liquid films
contribute substantially to the diffusive transport of light in {dry} foams.Comment: 8 pages, 8 figure
Bragg solitons in nonlinear PT-symmetric periodic potentials
It is shown that slow Bragg soliton solutions are possible in nonlinear
complex parity-time (PT) symmetric periodic structures. Analysis indicates that
the PT-symmetric component of the periodic optical refractive index can modify
the grating band structure and hence the effective coupling between the forward
and backward waves. Starting from a classical modified massive Thirring model,
solitary wave solutions are obtained in closed form. The basic properties of
these slow solitary waves and their dependence on their respective PT-symmetric
gain/loss profile are then explored via numerical simulations.Comment: 6 pages, 4 figures, published in Physical Review
Diffusive transport of light in two-dimensional granular materials
We study photon diffusion in a two-dimensional random packing of monodisperse
disks as a simple model of granular material. We apply ray optics approximation
to set up a persistent random walk for the photons. We employ Fresnel's
intensity reflectance with its rich dependence on the incidence angle and
polarization state of the light. We present an analytic expression for the
transport-mean-free path in terms of the refractive indices of grains and host
medium, grain radius, and packing fraction. We perform numerical simulations to
examine our analytical result.Comment: 9 pages, 3 figure
A two-parameter random walk with approximate exponential probability distribution
We study a non-Markovian random walk in dimension 1. It depends on two
parameters eps_r and eps_l, the probabilities to go straight on when walking to
the right, respectively to the left. The position x of the walk after n steps
and the number of reversals of direction k are used to estimate eps_r and
eps_l. We calculate the joint probability distribution p_n(x,k) in closed form
and show that, approximately, it belongs to the exponential family.Comment: 12 pages, updated reference to companion paper cond-mat/060126
Persistent random walk on a one-dimensional lattice with random asymmetric transmittances
We study the persistent random walk of photons on a one-dimensional lattice
of random asymmetric transmittances. Each site is characterized by its
intensity transmittance t (t') for photons moving to the right (left)
direction. Transmittances at different sites are assumed independent,
distributed according to a given probability density Distribution. We use the
effective medium approximation and identify two classes of probability density
distribution of transmittances which lead to the normal diffusion of photons.
Monte Carlo simulations confirm our predictions.Comment: 7 pages, submitted to Phys. Rev.
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