4,850 research outputs found
Determination of electromagnetic medium from the Fresnel surface
We study Maxwell's equations on a 4-manifold where the electromagnetic medium
is described by an antisymmetric -tensor . In this setting,
the Tamm-Rubilar tensor density determines a polynomial surface of fourth order
in each cotangent space. This surface is called the Fresnel surface and acts as
a generalisation of the light-cone determined by a Lorentz metric; the Fresnel
surface parameterises electromagnetic wave-speed as a function of direction.
Favaro and Bergamin have recently proven that if has only a principal
part and if the Fresnel surface of coincides with the light cone for a
Lorentz metric , then is proportional to the Hodge star operator of
. That is, under additional assumptions, the Fresnel surface of
determines the conformal class of . The purpose of this paper is
twofold. First, we provide a new proof of this result using Gr\"obner bases.
Second, we describe a number of cases where the Fresnel surface does not
determine the conformal class of the original -tensor . For
example, if is invertible we show that and have
the same Fresnel surfaces.Comment: 23 pages, 1 figur
Differential gaze behavior towards sexually preferred and non-preferred human figures
The gaze pattern associated with image exploration is a sensitive index of our attention, motivation and preference. To examine whether an individualâs gaze behavior
can reflect his/her sexual interest, we compared gaze patterns of young heterosexual men and women (M = 19.94 years, SD = 1.05) while viewing photos of plain-clothed male and female figures aged from birth to sixty years old. Our analysis revealed a clear gender difference in viewing sexually preferred figure images. Men displayed a distinctive gaze pattern only when viewing twenty-year-old female images, with more fixations and longer viewing time dedicated to the upper body and waist-hip region. Women also
directed more attention at the upper body on female images in comparison to male images, but this difference was not age-specific. Analysis of local image salience revealed that observersâ eye-scanning strategies could not be accounted for by low-level processes, such as analyzing local image contrast and structure, but were associated with
attractiveness judgments. The results suggest that the difference in cognitive processing of sexually preferred and non-preferred figures can be manifested in gaze patterns
associated with figure viewing. Thus, eye-tracking holds promise as a potential sensitive measure for sexual preference, particularly in men
Transport coefficients for inelastic Maxwell mixtures
The Boltzmann equation for inelastic Maxwell models is used to determine the
Navier-Stokes transport coefficients of a granular binary mixture in
dimensions. The Chapman-Enskog method is applied to solve the Boltzmann
equation for states near the (local) homogeneous cooling state. The mass, heat,
and momentum fluxes are obtained to first order in the spatial gradients of the
hydrodynamic fields, and the corresponding transport coefficients are
identified. There are seven relevant transport coefficients: the mutual
diffusion, the pressure diffusion, the thermal diffusion, the shear viscosity,
the Dufour coefficient, the pressure energy coefficient, and the thermal
conductivity. All these coefficients are {\em exactly} obtained in terms of the
coefficients of restitution and the ratios of mass, concentration, and particle
sizes. The results are compared with known transport coefficients of inelastic
hard spheres obtained analytically in the leading Sonine approximation and by
means of Monte Carlo simulations. The comparison shows a reasonably good
agreement between both interaction models for not too strong dissipation,
especially in the case of the transport coefficients associated with the mass
flux.Comment: 9 figures, to be published in J. Stat. Phy
Thomas-Fermi-Dirac-von Weizsacker hydrodynamics in laterally modulated electronic systems
We have studied the collective plasma excitations of a two-dimensional
electron gas with an arbitrary lateral charge-density modulation. The dynamics
is formulated using a previously developed hydrodynamic theory based on the
Thomas-Fermi-Dirac-von Weizsacker approximation. In this approach, both the
equilibrium and dynamical properties of the periodically modulated electron gas
are treated in a consistent fashion. We pay particular attention to the
evolution of the collective excitations as the system undergoes the transition
from the ideal two-dimensional limit to the highly-localized one-dimensional
limit. We also calculate the power absorption in the long-wavelength limit to
illustrate the effect of the modulation on the modes probed by far-infrared
(FIR) transmission spectroscopy.Comment: 27 page Revtex file, 15 Postscript figure
Manifestation of the Hofstadter butterfly in far-infrared absorption
The far-infrared absorption of a two-dimensional electron gas with a
square-lattice modulation in a perpendicular constant magnetic field is
calculated self-consistently within the Hartree approximation. For strong
modulation and short period we obtain intra- and intersubband magnetoplasmon
modes reflecting the subbands of the Hofstadter butterfly in two or more Landau
bands. The character of the absorption and the correlation of the peaks to the
number of flux quanta through each unit cell of the periodic potential depends
strongly on the location of the chemical potential with respect to the
subbands, or what is the same, on the density of electrons in the system.Comment: RevTeX file + 4 postscript figures, to be published Phys. Rev. B
Rapid Com
The FIR-absorption of short period quantum wires and the transition from one to two dimensions
We investigate the FIR-absorption of short period parallel quantum wires in a
perpendicular quantizing magnetic field. The external time-dependent electric
field is linearly polarized along the wire modulation. The mutual Coulomb
interaction of the electrons is treated self-consistently in the ground state
and in the absorption calculation within the Hartree approximation. We consider
the effects of a metal gate grating coupler, with the same or with a different
period as the wire modulation, on the absorption. The evolution of the
magnetoplasmon in the nonlocal region where it is split into several Bernstein
modes is discussed in the transition from: narrow to broad wires, and isolated
to overlapping wires. We show that in the case of narrow and not strongly
modulated wires the absorption can be directly correlated with the underlying
electronic bandstructure.Comment: 15 pages, 9 figures, Revtex, to appear in Phys. Rev.
Near-threshold production of -mesons in and NN collisions and -mixing
We consider near-threshold -meson production in and
collisions. An effective Lagrangian approach with one-pion exchange is applied
to analyze different contributions to the cross section for different isospin
channels. The Reggeon exchange mechanism is also evaluated for comparison. The
results from reactions are used to calculate the contribution of the
meson to the cross sections and invariant mass distributions
of the reactions and . It is found that
the experimental observation of mesons in the reaction is much more promising than the observation of mesons in
the reaction . Effects of isospin violation in the reactions
, , and , which are induced by -- mixing, are also analyzed.Comment: 43 pages, including 16 eps figures, to be bublished in Phys. Atom.
Nucl. (Yad. Fiz.) vol. 65, No. 11 (2002
Butterfly-like spectra and collective modes of antidot superlattices in magnetic fields
We calculate the energy band structure for electrons in an external periodic
potential combined with a perpendicular magnetic field. Electron-electron
interactions are included within a Hartree approximation. The calculated energy
spectra display a considerable degree of self-similarity, just as the
``Hofstadter butterfly.'' However, screening affects the butterfly, most
importantly the bandwidths oscillate with magnetic field in a characteristic
way. We also investigate the dynamic response of the electron system in the
far-infrared (FIR) regime. Some of the peaks in the FIR absorption spectra can
be interpreted mainly in semiclassical terms, while others originate from
inter(sub)band transitions.Comment: 4 pages with 2 embeded eps figures. Uses revtex, multicol and
graphicx styles. Accepted for publication in PRB Brief Report
Absence of Scaling in the Integer Quantum Hall Effect
We have studied the conductivity peak in the transition region between the
two lowest integer Quantum Hall states using transmission measurements of edge
magnetoplasmons. The width of the transition region is found to increase
linearly with frequency but remains finite when extrapolated to zero frequency
and temperature. Contrary to prevalent theoretical pictures, our data does not
show the scaling characteristics of critical phenomena.These results suggest
that a different mechanism governs the transition in our experiment.Comment: Minor changes and new references include
Theory of Exciton Recombination from the Magnetically Induced Wigner Crystal
We study the theory of itinerant-hole photoluminescence of two-dimensional
electron systems in the regime of the magnetically induced Wigner crystal. We
show that the exciton recombination transition develops structure related to
the presence of the Wigner crystal. The form of this structure depends strongly
on the separation between the photo-excited hole and the plane of the
two-dimensional electron gas. When is small compared to the magnetic
length, additional peaks appear in the spectrum due to the recombination of
exciton states with wavevectors equal to the reciprocal lattice vectors of the
crystal. For larger than the magnetic length, the exciton becomes strongly
confined to an interstitial site of the lattice, and the structure in the
spectrum reflects the short-range correlations of the Wigner crystal. We derive
expressions for the energies and the radiative lifetimes of the states
contributing to photoluminescence, and discuss how the results of our analysis
compare with experimental observations.Comment: 10 pages, no figures, uses Revtex and multicol.st
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