2,841 research outputs found
X-Raying the Dark Side of Venus - Scatter from Venus Magnetotail?
This work analyzes the X-ray, EUV and UV emission apparently coming from the
Earth-facing (dark) side of Venus as observed with Hinode/XRT and SDO/AIA
during a transit across the solar disk occurred in 2012. We have measured
significant X-Ray, EUV and UV flux from Venus dark side. As a check we have
also analyzed a Mercury transit across the solar disk, observed with Hinode/XRT
in 2006. We have used the latest version of the Hinode/XRT Point Spread
Function (PSF) to deconvolve Venus and Mercury X-ray images, in order to remove
possible instrumental scattering. Even after deconvolution, the flux from Venus
shadow remains significant while in the case of Mercury it becomes negligible.
Since stray-light contamination affects the XRT Ti-poly filter data from the
Venus transit in 2012, we performed the same analysis with XRT Al-mesh filter
data, which is not affected by the light leak. Even the Al-mesh filter data
show residual flux. We have also found significant EUV (304 A, 193 A, 335 A)
and UV (1700 A) flux in Venus shadow, as measured with SDO/AIA. The EUV
emission from Venus dark side is reduced when appropriate deconvolution methods
are applied; the emission remains significant, however. The light curves of the
average flux of the shadow in the X-ray, EUV, and UV bands appear different as
Venus crosses the solar disk, but in any of them the flux is, at any time,
approximately proportional to the average flux in a ring surrounding Venus, and
therefore proportional to the average flux of the solar regions around Venus
obscuring disk line of sight. The proportionality factor depends on the band.
This phenomenon has no clear origin; we suggest it may be due to scatter
occurring in the very long magnetotail of Venus.Comment: This paper has been accepted in The Astrophysical Journa
Chaotic Evolution in Quantum Mechanics
A quantum system is described, whose wave function has a complexity which
increases exponentially with time. Namely, for any fixed orthonormal basis, the
number of components required for an accurate representation of the wave
function increases exponentially.Comment: 8 pages (LaTeX 16 kB, followed by PostScript 2 kB for figure
Quantum Field Theory with Null-Fronted Metrics
There is a large class of classical null-fronted metrics in which a free
scalar field has an infinite number of conservation laws. In particular, if the
scalar field is quantized, the number of particles is conserved. However, with
more general null-fronted metrics, field quantization cannot be interpreted in
terms of particle creation and annihilation operators, and the physical meaning
of the theory becomes obscure.Comment: 11 page
Optical doping and damage formation in AIN by Eu implantation
AlN films grown on sapphire were implanted with 300 keV Eu ions to fluences from 3×1014 to 1.4×1017 atoms/cm2 in two different geometries: “channeled” along the c-axis and “random” with a 10° angle between the ion beam and the surface normal. A detailed study of implantation damage accumulation is presented. Strong ion channeling effects are observed leading to significantly decreased damage levels for the channeled implantation within the entire fluence range. For random implantation, a buried amorphous layer is formed at the highest fluences. Red Eu-related photoluminescence at room temperature is observed in all samples with highest intensities for low damage samples (low fluence and channeled implantation) after annealing. Implantation damage, once formed, is shown to be stable up to very high temperatures.FCT - POCI/FIS/57550/2004FCT - PTDC/FIS/66262/2006FCT - PTDC/CTM/100756/200
Entanglement dynamics of electron-electron scattering in low-dimensional semiconductor systems
We perform the quantitative evaluation of the entanglement dynamics in
scattering events between two insistinguishable electrons interacting via
Coulomb potential in 1D and 2D semiconductor nanostructures. We apply a
criterion based on the von Neumann entropy and the Schmidt decomposition of the
global state vector suitable for systems of identical particles. From the
timedependent numerical solution of the two-particle wavefunction of the
scattering carriers we compute their entanglement evolution for different spin
configurations: two electrons with the same spin, with different spin, singlet,
and triplet spin state. The procedure allows to evaluate the mechanisms that
govern entanglement creation and their connection with the characteristic
physical parameters and initial conditions of the system. The cases in which
the evolution of entanglement is similar to the one obtained for
distinguishable particles are discussed.Comment: 22 pages, 7 figures, submitted to Physical Review
Evolution of Liouville density of a chaotic system
An area-preserving map of the unit sphere, consisting of alternating twists
and turns, is mostly chaotic. A Liouville density on that sphere is specified
by means of its expansion into spherical harmonics. That expansion initially
necessitates only a finite number of basis functions. As the dynamical mapping
proceeds, it is found that the number of non-negligible coefficients increases
exponentially with the number of steps. This is to be contrasted with the
behavior of a Schr\"odinger wave function which requires, for the analogous
quantum system, a basis of fixed size.Comment: LaTeX 4 pages (27 kB) followed by four short PostScript files (2 kB +
2 kB + 1 kB + 4 kB
Kochen-Specker theorem for a single qubit using positive operator-valued measures
A proof of the Kochen-Specker theorem for a single two-level system is
presented. It employs five eight-element positive operator-valued measures and
a simple algebraic reasoning based on the geometry of the dodecahedron.Comment: REVTeX4, 4 pages, 2 figure
Filtering of the absolute value of photon-number difference for two-mode macroscopic quantum superpositions
We discuss a device capable of filtering out two-mode states of light with
mode populations differing by more than a certain threshold, while not
revealing which mode is more populated. It would allow engineering of
macroscopic quantum states of light in a way which is preserving specific
superpositions. As a result, it would enhance optical phase estimation with
these states as well as distinguishability of "macroscopic" qubits. We propose
an optical scheme, which is a relatively simple, albeit non-ideal, operational
implementation of such a filter. It uses tapping of the original polarization
two-mode field, with a polarization neutral beam splitter of low reflectivity.
Next, the reflected beams are suitably interfered on a polarizing beam
splitter. It is oriented such that it selects unbiased polarization modes with
respect to the original ones. The more an incoming two-mode Fock state is
unequally populated, the more the polarizing beam splitter output modes are
equally populated. This effect is especially pronounced for highly populated
states. Additionally, for such states we expect strong population correlations
between the original fields and the tapped one. Thus, after a photon-number
measurement of the polarizing beam splitter outputs, a feed-forward loop can be
used to let through a shutter the field, which was transmitted by the tapping
beam splitter. This happens only if the counts at the outputs are roughly
equal. In such a case, the transmitted field differs strongly in occupation
number of the two modes, while information on which mode is more populated is
non-existent (a necessary condition for preserving superpositions).Comment: 11 pages, 12 figure
Wigner's little group and Berry's phase for massless particles
The ``little group'' for massless particles (namely, the Lorentz
transformations that leave a null vector invariant) is isomorphic to
the Euclidean group E2: translations and rotations in a plane. We show how to
obtain explicitly the rotation angle of E2 as a function of and we
relate that angle to Berry's topological phase. Some particles admit both signs
of helicity, and it is then possible to define a reduced density matrix for
their polarization. However, that density matrix is physically meaningless,
because it has no transformation law under the Lorentz group, even under
ordinary rotations.Comment: 4 pages revte
Mass media destabilizes the cultural homogeneous regime in Axelrod's model
An important feature of Axelrod's model for culture dissemination or social
influence is the emergence of many multicultural absorbing states, despite the
fact that the local rules that specify the agents interactions are explicitly
designed to decrease the cultural differences between agents. Here we
re-examine the problem of introducing an external, global interaction -- the
mass media -- in the rules of Axelrod's model: in addition to their
nearest-neighbors, each agent has a certain probability to interact with a
virtual neighbor whose cultural features are fixed from the outset. Most
surprisingly, this apparently homogenizing effect actually increases the
cultural diversity of the population. We show that, contrary to previous claims
in the literature, even a vanishingly small value of is sufficient to
destabilize the homogeneous regime for very large lattice sizes
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