6,246 research outputs found
Oscillatons revisited
In this paper, we study some interesting properties of a spherically
symmetric oscillating soliton star made of a real time-dependent scalar field
which is called an oscillaton. The known final configuration of an oscillaton
consists of a stationary stage in which the scalar field and the metric
coefficients oscillate in time if the scalar potential is quadratic. The
differential equations that arise in the simplest approximation, that of
coherent scalar oscillations, are presented for a quadratic scalar potential.
This allows us to take a closer look at the interesting properties of these
oscillating objects. The leading terms of the solutions considering a quartic
and a cosh scalar potentials are worked in the so called stationary limit
procedure. This procedure reveals the form in which oscillatons and boson stars
may be related and useful information about oscillatons is obtained from the
known results of boson stars. Oscillatons could compete with boson stars as
interesting astrophysical objects, since they would be predicted by scalar
field dark matter models.Comment: 10 pages REVTeX, 10 eps figures. Updated files to match version
published in Classical and Quantum Gravit
Entropic Entanglement Criteria for Continuous Variables
We derive several entanglement criteria for bipartite continuous variable
quantum systems based on the Shannon entropy. These criteria are more sensitive
than those involving only second-order moments, and are equivalent to
well-known variance product tests in the case of Gaussian states. Furthermore,
they involve only a pair of quadrature measurements, and will thus should prove
extremely useful the experimental identification of entanglement.Comment: 4 pages, 2 figure
Evaluation of the phase discretization effect in transmitarrays formed by sub-wavelength patches
Most of the transmitarray phase shifting cell designs assume a periodic repetition of the same cell to account for the coupling effects of the neighboring cells. However the behavior in terms of magnitude and phase of the transmission coefficient calculated in this way may change when the neighboring cells have different phase, as normally happens in transmitarrays due to phase discretization. This paper characterizes this effect using a family of sub-wavelength patch cells covering the entire 360º phase shift interval, all with transmission coefficient better than -0.2 dB in the periodic structure. A stepped-phase structure is analysed considering different phase step values from 5º up to 90º. It is shown that the magnitude of the cells transmission factor changes considerably at the transition between different elements, with the unit cells with higher phase shift showing increased transmission while the lower phase shift cells in the transition tend to block the transmission more.info:eu-repo/semantics/acceptedVersio
Galactic Collapse of Scalar Field Dark Matter
We present a scenario for galaxy formation based on the hypothesis of scalar
field dark matter. We interpret galaxy formation through the collapse of a
scalar field fluctuation. We find that a cosh potential for the
self-interaction of the scalar field provides a reasonable scenario for
galactic formation, which is in agreement with cosmological observations and
phenomenological studies in galaxies.Comment: 4 pages, 3 figue
Edge phonons in black phosphorus
Exfoliated black phosphorus has recently emerged as a new two-dimensional
crystal that, due to its peculiar and anisotropic crystalline and electronic
band structures, may have potentially important applications in electronics,
optoelectronics and photonics. Despite the fact that the edges of layered
crystals host a range of singular properties whose characterization and
exploitation are of utmost importance for device development, the edges of
black phosphorus remain poorly characterized. In this work, the atomic
structure and the behavior of phonons near different black phosphorus edges are
experimentally and theoretically studied using Raman spectroscopy and density
functional theory calculations. Polarized Raman results show the appearance of
new modes at the edges of the sample, and their spectra depend on the atomic
structure of the edges (zigzag or armchair). Theoretical simulations confirm
that the new modes are due to edge phonon states that are forbidden in the
bulk, and originated from the lattice termination rearrangements.Comment: 15 pages, 4 figure
Alterações físico-químicas de um solo cultivado com Atriplex em função da irrigação com rejeito de dessalinizador.
Mg(, )Na reaction study for spectroscopy of Na
The Mg(, )Na reaction was measured at the Holifield
Radioactive Ion Beam Facility at Oak Ridge National Laboratory in order to
better constrain spins and parities of energy levels in Na for the
astrophysically important F()Ne reaction rate
calculation. 31 MeV proton beams from the 25-MV tandem accelerator and enriched
Mg solid targets were used. Recoiling He particles from the
Mg(, )Na reaction were detected by a highly segmented
silicon detector array which measured the yields of He particles over a
range of angles simultaneously. A new level at 6661 5 keV was observed in
the present work. The extracted angular distributions for the first four levels
of Na and Distorted Wave Born Approximation (DWBA) calculations were
compared to verify and extract angular momentum transfer.Comment: 11 pages, 6 figures, proceedings of the 18th International Conference
on Accelerators and Beam Utilization (ICABU2014
Hydrodynamics of galactic dark matter
We consider simple hydrodynamical models of galactic dark matter in which the
galactic halo is a self-gravitating and self-interacting gas that dominates the
dynamics of the galaxy. Modeling this halo as a sphericaly symmetric and static
perfect fluid satisfying the field equations of General Relativity, visible
barionic matter can be treated as ``test particles'' in the geometry of this
field. We show that the assumption of an empirical ``universal rotation curve''
that fits a wide variety of galaxies is compatible, under suitable
approximations, with state variables characteristic of a non-relativistic
Maxwell-Boltzmann gas that becomes an isothermal sphere in the Newtonian limit.
Consistency criteria lead to a minimal bound for particle masses in the range
and to a constraint between the central
temperature and the particles mass. The allowed mass range includes popular
supersymmetric particle candidates, such as the neutralino, axino and
gravitino, as well as lighter particles ( keV) proposed by numerical
N-body simulations associated with self-interactive CDM and WDM structure
formation theories.Comment: LaTeX article style, 16 pages including three figures. Final version
to appear in Classical and Quantum Gravit
Dilatonic Black Holes in Higher Curvature String Gravity
We give analytical arguments and demonstrate numerically the existence of
black hole solutions of the Effective Superstring Action in the presence
of Gauss-Bonnet quadratic curvature terms. The solutions possess non-trivial
dilaton hair. The hair, however, is of ``secondary" type", in the sense that
the dilaton charge is expressed in terms of the black hole mass. Our solutions
are not covered by the assumptions of existing proofs of the ``no-hair"
theorem. We also find some alternative solutions with singular metric
behaviour, but finite energy. The absence of naked singularities in this system
is pointed out.Comment: 22 pages, Latex file, 7 Latex figures already include
Experimental evaluation of a high gain dual-band beam steerable transmit-array
This paper presents the design and measurement of a novel transmit-array (TA) that is able to combine wide beam steering and high gain for two distinct frequency bands. The TA is designed for a mobile ground platform to operate at satellite communications in the Ka-bands: Rx (19.7-20.2 GHz) and Tx (29.5-30.0 GHz). A 28 dBi gain is obtained for 30 GHz and 24 dBi for 20 GHz. The zenith beam steering results from a linear displacement of the aperture in front of a stationary feed. A scanning range of 50º was obtained with 2dB scan loss. Overall, the dual-band TA performs as well as if two independent single band apertures were designedinfo:eu-repo/semantics/acceptedVersio
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