4,440 research outputs found
Isotropic properties of the photonic band gap in quasicrystals with low-index contrast
We report on the formation and development of the photonic band gap in
two-dimensional 8-, 10- and 12-fold symmetry quasicrystalline lattices of low
index contrast. Finite size structures made of dielectric cylindrical rods were
studied and measured in the microwave region, and their properties compared
with a conventional hexagonal crystal. Band gap characteristics were
investigated by changing the direction of propagation of the incident beam
inside the crystal. Various angles of incidence from 0 \degree to 30\degree
were used in order to investigate the isotropic nature of the band gap. The
arbitrarily high rotational symmetry of aperiodically ordered structures could
be practically exploited to manufacture isotropic band gap materials, which are
perfectly suitable for hosting waveguides or cavities.Comment: 16 pages, 7 figures, submitted to PR
HOM-free accelerating dielectric cavities with metallic inclusions
Previous investigations demonstrated that point-defected photonic bandgap cavities based on periodic and aperiodic dielectric arrangements can be successfully employed as single cell in particle accelerators. In this paper, we present a study aimed at highlighting the possible advantages of using hybrid structures based on the above dielectric configurations, but featuring metallic rods in the outermost regions, for the design of accelerating resonators of extremely high-quality factor. In this framework, we consider diverse configurations, with different (periodic and aperiodic) lattice geometries, sizes, and dielectric/metal fractions. The use of superconducting plates to further increase the cavity performances is taken into account. Results from our comparative studies, based on numerical full-wave simulations backed by experimental validations (at room and cryogenic temperatures) in the microwave region, identify the candidate parametric configurations capable of yielding the highest quality factor
Critical-state effects on microwave losses in type-II superconductors
We discuss the microwave energy losses in superconductors in the critical
state. The field-induced variations of the surface resistance are determined,
in the framework of the Coffey and Clem model, by taking into account the
distribution of the vortex magnetic field inside the sample. It is shown that
the effects of the critical state cannot generally be disregarded to account
for the experimental data. Results obtained in bulk niobium at low temperatures
are quantitatively justified.Comment: 4 pages, 4 embedded figures, to be published on Eur. Phys. J.
Twenty-first semiannual report to Congress, 1 January - 30 June 1969
Manned space flights, satellite observations, space sciences, and air traffic control - NASA report to Congress for 1 Jan. to 30 June 196
Microwave Harmonic Emission in MgB2 Superconductor: Comparison with YBaCuO
We report results of microwave second-harmonic generation in ceramic samples
of MgB2, prepared by different methods. The SH signal has been investigated as
a function of the temperature and the static magnetic field. The results are
discussed in the framework of models reported in the literature. We show that
the peculiarities of the SH signal are related to the specific properties of
the sample. A comparison with the results obtained in ceramic and crystalline
YBa(2)Cu(3)O(7) shows that the second-harmonic emission in MgB2 is weaker than
that observed in ceramic YBa(2)Cu(3)O(7).Comment: 13 pages, 6 figures; Proceedings of Third Workshop on Metamaterials
and Special Materials for Electromagnetic Applications and TLC (Rome, 30-31
March, 2006
Nonlocal field theory driven by a deformed product: Generalization of Kalb-Ramond duality
A modification of the standard product used in local field theory by means of an associative
deformed product is proposed. We present a class of deformed products, one for
every spin S = 0, 1/2, 1, that induces a nonlocal theory, displaying different form for
different fields. This type of deformed product is naturally supersymmetric and it has
an intriguing duality
The Scalar wave equation in a non-commutative spherically symmetric space-time
Recent work in the literature has studied a version of non-commutative Schwarzschild
black holes where the effects of non-commutativity are described by a mass function
depending on both the radial variable r and a non-commutativity parameter θ. The
present paper studies the asymptotic behavior of solutions of the zero-rest-mass scalar
wave equation in such a modified Schwarzschild space-time in a neighborhood of spatial
infinity. The analysis is eventually reduced to finding solutions of an inhomogeneous
Euler–Poisson–Darboux equation, where the parameter θ affects explicitly the functional
form of the source term. Interestingly, for finite values of θ, there is full qualitative
agreement with general relativity: the conformal singularity at spacelike infinity reduces
in a considerable way the differentiability class of scalar fields at future null infinity.
In the physical space-time, this means that the scalar field has an asymptotic behavior
with a fall-off going on rather more slowly than in flat space-time
The spectroscopic evolution of the recurrent nova T Pyxidis during its 2011 outburst. II.The optically thin phase and the structure of the ejecta in recurrent novae
We continue our study of the physical properties of the recurrent nova T Pyx,
focussing on the structure of the ejecta in the nebular stage of expansion
during the 2011 outburst. The nova was observed contemporaneously with the
Nordic Optical Telescope (NOT), at high resolution spectroscopic resolution (R
~ 65000) on 2011 Oct. 11 and 2012 Apr. 8 (without absolute flux calibration),
and with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble
Space Telescope, at high resolution (R ~ 30000) on 2011 Oct. 10 and 2012 Mar.
28 (absolute fluxes). We use standard plasma diagnostics (e.g. [O III] and [N
II] line ratios and the H line fluxes) to constrain electron densities
and temperatures. Using Monte Carlo modeling of the ejecta, we derive the
structure and filling factor from comparisons to the optical and ultraviolet
line profiles. The ejecta can be modeled using an axisymmetric conical --
bipolar -- geometry with a low inclination of the axis to the line of sight,
i=15+/-5 degrees, compatible with published results from high angular
resolution optical spectro-interferometry. The structure is similar to that
observed in the other short orbital period recurrent novae during their nebular
stages. We show that the electron density scales as as expected from a
ballistically ejected constant mass shell; there is no need to invoke a
continuing mass outflow following the eruption. The derived mass for the ejecta
with filling factor f ~ 3%, M_ej ~ 2E-6$M_sun is similar to that obtained for
other recurrent nova ejecta but inconsistent with the previously reported
extended optically thick epoch of the explosion. We suggest that the system
underwent a common envelope phase following the explosion that produced the
recombination event. Implications for the dynamics of the recurrent novae are
discussed. (truncated)Comment: accepted for publication in A&A (10 Nov. 2012), 10 pgs, 16 fig
- …