Photon Channelling in Foams

Experiments by Gittings, Bandyopadhyay, and Durian [Europhys. Lett.\ \textbf{65}, 414 (2004)] demonstrate that light possesses a higher probability to propagate in the liquid phase of a foam due to total reflection. The authors term this observation photon channelling which we investigate in this article theoretically. We first derive a central relation in the work of Gitting {\em et al.} without any free parameters. It links the photon's path-length fraction $f$ in the liquid phase to the liquid fraction $\epsilon$. We then construct two-dimensional Voronoi foams, replace the cell edges by channels to represent the liquid films and simulate photon paths according to the laws of ray optics using transmission and reflection coefficients from Fresnel's formulas. In an exact honeycomb foam, the photons show superdiffusive behavior. It becomes diffusive as soon as disorder is introduced into the foams. The dependence of the diffusion constant on channel width and refractive index is explained by a one-dimensional random-walk model. It contains a photon channelling state that is crucial for the understanding of the numerical results. At the end, we shortly comment on the observation that photon channelling only occurs in a finite range of $\epsilon$.Comment: 9 pages, minor change

Superconductivity and Magnetism at Nuclear-matter Densities: An Astronomical Challenge

We report on a study of the evolution of magnetic fields of neutron stars, driven by the expulsion of magnetic flux out of the proton superconducting core of the star. The rate of expulsion, or equivalently the velocity of outward motion of flux-carrying proton-vortices is determined from a solution of their equation of motion. A determination of the effective forces on the fluxoids moving through the quantum liquid interior of neutron stars is however confronted with many ambiguities about the properties of this special case of superconductivity in the nature. Also, the behaviour of the fluxoids at the core boundary, and the subsequent evolution of the expelled flux within the highly conductive surrounding crust, are other related issues that have not been so far explored in any great details.Comment: 8 papegs, 2 figures, accepted at the 1st Regional Conference on Magnetic and Superconducting Materials (MSM-99), Tehran, Sept. 199

The possible role of r-modes in post-glitch relaxation of Crab

The loss of angular momentum through gravitational radiation, driven by the excitation of r-modes, is considered in neutron stars having rotation frequencies smaller than the associated critical frequency. We find that for reasonable values of the initial amplitudes of such pulsation modes of the star, being excited at the event of a glitch in a pulsar, the total post-glitch losses correspond to a negligible fraction of the initial rise of the spin frequency in the case of Vela and the older pulsars. However, for the Crab pulsar the same effect would result, within a few months, in a decrease in its spin frequency by an amount larger than its glitch-induced frequency increase. This could provide an explanation for the peculiar behavior observed in the post-glitch relaxations of the Crab.Comment: 9 pages, 4 figures, RevTe

Inclusion, Participation and the Emergence of British Chinese Websites

Previous work has drawn attention to the relative absence of British Chinese voices in public culture. No one is more aware of this invisibility than British-born Chinese people themselves. Since 2000 the emergence of Internet discussion sites produced by British Chinese young people has provided an important forum for many of them to grapple with questions concerning their identities, experiences and status in Britain. In this paper we explore the ways in which Internet usage by British-born Chinese people has facilitated forms of self-expression, collective identity production and social and political action. This examination of British Chinese websites raises important questions about inclusion and exclusion, citizenship, participation and the development of a sense of belonging in Britain, issues which are usually overlooked in relation to a group which appears to be well integrated and successful in higher education

Smart expansion and fast calibration for jump diffusion

Using Malliavin calculus techniques, we derive an analytical formula for the price of European options, for any model including local volatility and Poisson jump process. We show that the accuracy of the formula depends on the smoothness of the payoff function. Our approach relies on an asymptotic expansion related to small diffusion and small jump frequency/size. Our formula has excellent accuracy (the error on implied Black-Scholes volatilities for call option is smaller than 2 bp for various strikes and maturities). Additionally, model calibration becomes very rapid.Comment: in Finance and Stochastics (2009) a paraitr