8,032 research outputs found
Damping rate of plasmons and photons in a degenerate nonrelativistic plasma
A calculation is presented of the plasmon and photon damping rates in a dense
nonrelativistic plasma at zero temperature, following the resummation program
of Braaten-Pisarski. At small soft momentum , the damping is dominated by scattering processes corresponding to double longitudinal Landau
damping. The dampings are proportional to , where
is the Fermi velocity.Comment: 9 pages, 2 figure
Locating the ‘radical’ in 'Shoot the Messenger'
This is the author's accepted manuscript. The final published article is available from the link below, copyright 2013 @ Edinburgh University Press.The 2006 BBC drama Shoot the Messenger is based on the psychological journey of a Black schoolteacher, Joe Pascale, accused of assaulting a Black male pupil. The allegation triggers Joe's mental breakdown which is articulated, through Joe's first-person narration, as a vindictive loathing of Black people. In turn, a range of common stereotypical characterisations and discourses based on a Black culture of hypocrisy, blame and entitlement is presented. The text is therefore laid wide open to a critique of its neo-conservatism and hegemonic narratives of Black Britishness. However, the drama's presentation of Black mental illness suggests that Shoot the Messenger may also be interpreted as a critique of social inequality and the destabilising effects of living with ethnicised social categories. Through an analysis of issues of representation, the article reclaims this controversial text as a radical drama and examines its implications for and within a critical cultural politics of ‘race’ and representation
Why is the bandwidth of sodium observed to be narrower in photoemission experiments?
The experimentally predicted narrowing in the bandwidth of sodium is
interpreted in terms of the non-local self-energy effect on quasi-particle
energies of the electron liquid. The calculated self-energy correction is a
monotonically increasing function of the wavenumber variable. The usual
analysis of photo-emission experiments assumes the final state energies on the
nearly-free-electron-like model and hence it incorrectly ascribes the non-local
self-energy correction to the final state energies to the occupied state
energies, thus leading to a seeming narrowing in the bandwidth.Comment: 9 page
Dynamic exchange-correlation potentials for the electron gas in dimensionality D=3 and D=2
Recent progress in the formulation of a fully dynamical local approximation
to time-dependent Density Functional Theory appeals to the longitudinal and
transverse components of the exchange and correlation kernel in the linear
current-density response of the homogeneous fluid at long wavelength. Both
components are evaluated for the electron gas in dimensionality D=3 and D=2 by
an approximate decoupling in the equation of motion for the current density,
which accounts for processes of excitation of two electron-hole pairs. Each
pair is treated in the random phase approximation, but the role of exchange and
correlation is also examined; in addition, final-state exchange processes are
included phenomenologically so as to satisfy the exactly known high-frequency
behaviours of the kernel. The transverse and longitudinal spectra involve the
same decay channels and are similar in shape. A two-plasmon threshold in the
spectrum for two-pair excitations in D=3 leads to a sharp minimum in the real
part of the exchange and correlation kernel at twice the plasma frequency. In
D=2 the same mechanism leads to a broad spectral peak and to a broad minimum in
the real part of the kernel, as a consequence of the dispersion law of the
plasmon vanishing at long wavelength. The numerical results have been fitted to
simple analytic functions.Comment: 13 pages, 11 figures included. Accepted for publication in Phys. Rev.
Fuzzy cellular model for on-line traffic simulation
This paper introduces a fuzzy cellular model of road traffic that was
intended for on-line applications in traffic control. The presented model uses
fuzzy sets theory to deal with uncertainty of both input data and simulation
results. Vehicles are modelled individually, thus various classes of them can
be taken into consideration. In the proposed approach, all parameters of
vehicles are described by means of fuzzy numbers. The model was implemented in
a simulation of vehicles queue discharge process. Changes of the queue length
were analysed in this experiment and compared to the results of NaSch cellular
automata model.Comment: The original publication is available at http://www.springerlink.co
New variables, the gravitational action, and boosted quasilocal stress-energy-momentum
This paper presents a complete set of quasilocal densities which describe the
stress-energy-momentum content of the gravitational field and which are built
with Ashtekar variables. The densities are defined on a two-surface which
bounds a generic spacelike hypersurface of spacetime. The method used
to derive the set of quasilocal densities is a Hamilton-Jacobi analysis of a
suitable covariant action principle for the Ashtekar variables. As such, the
theory presented here is an Ashtekar-variable reformulation of the metric
theory of quasilocal stress-energy-momentum originally due to Brown and York.
This work also investigates how the quasilocal densities behave under
generalized boosts, i. e. switches of the slice spanning . It is
shown that under such boosts the densities behave in a manner which is similar
to the simple boost law for energy-momentum four-vectors in special relativity.
The developed formalism is used to obtain a collection of two-surface or boost
invariants. With these invariants, one may ``build" several different mass
definitions in general relativity, such as the Hawking expression. Also
discussed in detail in this paper is the canonical action principle as applied
to bounded spacetime regions with ``sharp corners."Comment: Revtex, 41 Pages, 4 figures added. Final version has been revised and
improved quite a bit. To appear in Classical and Quantum Gravit
An Effective Superstring Spectral Action
A supersymmetric theory in two-dimensions has enough data to define a
noncommutative space thus making it possible to use all the tools of
noncommutative geometry. In particular, we apply this to the N=1 supersymmetric
non-linear sigma model and derive an expression for the generalized loop space
Dirac operator, in presence of a general background, using canonical
quantization. The spectral action principle is used to show that the
superstring partition function is also a spectral action valid for the
fluctuations of the string modes.Comment: 31 pages, Latex fil
On certain quasi-local spin-angular momentum expressions for small spheres
The Ludvigsen-Vickers and two recently suggested quasi-local spin-angular
momentum expressions, based on holomorphic and anti-holomorphic spinor fields,
are calculated for small spheres of radius about a point . It is shown
that, apart from the sign in the case of anti-holomorphic spinors in
non-vacuum, the leading terms of all these expressions coincide. In non-vacuum
spacetimes this common leading term is of order , and it is the product of
the contraction of the energy-momentum tensor and an average of the approximate
boost-rotation Killing vector that vanishes at and of the 3-volume of the
ball of radius . In vacuum spacetimes the leading term is of order ,
and the factor of proportionality is the contraction of the Bel-Robinson tensor
and an other average of the same approximate boost-rotation Killing vector.Comment: 16 pages, Plain Te
Interpreting an action from what we perceive and what we expect
International audienceIn update logic as studied by Baltag, Moss, Solecki and van Benthem, little attention is paid to the interpretation of an action by an agent, which is just assumed to depend on the situation. This is actually a complex issue that nevertheless complies to some logical dynamics. In this paper, we tackle this topic. We also deal with actions that change propositional facts of the situation. In parallel, we propose a formalism to accurately represent an agent's epistemic state based on hyperreal numbers. In that respect, we use infinitesimals to express what would surprise the agents (and by how much) by contradicting their beliefs. We also use a subjective probability to model the notion of belief. It turns out that our probabilistic update mechanism satisfies the AGM postulates of belief revision
Crystal structures and freezing of dipolar fluids
We investigate the crystal structure of classical systems of spherical
particles with an embedded point dipole at T=0. The ferroelectric ground state
energy is calculated using generalizations of the Ewald summation technique.
Due to the reduced symmetry compared to the nonpolar case the crystals are
never strictly cubic. For the Stockmayer (i.e., Lennard-Jones plus dipolar)
interaction three phases are found upon increasing the dipole moment:
hexagonal, body-centered orthorhombic, and body-centered tetragonal. An even
richer phase diagram arises for dipolar soft spheres with a purely repulsive
inverse power law potential . A crossover between qualitatively
different sequences of phases occurs near the exponent . The results are
applicable to electro- and magnetorheological fluids. In addition to the exact
ground state analysis we study freezing of the Stockmayer fluid by
density-functional theory.Comment: submitted to Phys. Rev.
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