8,542 research outputs found
Full-vector analysis of a realistic photonic crystal fiber
We analyze the guiding problem in a realistic photonic crystal fiber using a
novel full-vector modal technique, a biorthogonal modal method based on the
nonselfadjoint character of the electromagnetic propagation in a fiber.
Dispersion curves of guided modes for different fiber structural parameters are
calculated along with the 2D transverse intensity distribution of the
fundamental mode. Our results match those achieved in recent experiments, where
the feasibility of this type of fiber was shown.Comment: 3 figures, submitted to Optics Letter
Exact Hairy Black Holes and their Modification to the Universal Law of Gravitation
In this paper two things are done. First, it is pointed out the existence of
exact asymptotically flat, spherically symmetric black holes when a self
interacting, minimally coupled scalar field is the source of the energy
momentum of the Einstein equations in four dimensions. The scalar field
potential is the recently found to be compatible with the hairy generalization
of the Plebanski-Demianski solution of general relativity. This paper describes
the spherically symmetric solutions that smoothly connect the Schwarzschild
black hole with its hairy counterpart. The geometry and scalar field are
everywhere regular except at the usual Schwarzschild like singularity inside
the black hole. The scalar field energy momentum tensor satisfies the null
energy condition in the static region of the spacetime. The first law holds
when the parameters of the scalar field potential are fixed under
thermodynamical variation. Secondly, it is shown that an extra, dimensionless
parameter, present in the hairy solution, allows to modify the gravitational
field of a spherically symmetric black hole in a remarkable way. When the
dimensionless parameter is increased, the scalar field generates a flat
gravitational potential, that however asymptotically matches the Schwarzschild
gravitational field. Finally, it is shown that a positive cosmological constant
can render the scalar field potential convex if the parameters are within a
specific rank.Comment: Two new references, 10 pages, 2 figure
Ga+, In+ and Tl+ Impurities in Alkali Halide Crystals: Distortion Trends
A computational study of the doping of alkali halide crystals (AX: A = Na, K;
X = Cl, Br) by ns2 cations (Ga+, In+ and Tl+) is presented. Active clusters of
increasing size (from 33 to 177 ions) are considered in order to deal with the
large scale distortions induced by the substitutional impurities. Those
clusters are embedded in accurate quantum environments representing the
surrounding crystalline lattice. The convergence of the distortion results with
the size of the active cluster is analyced for some selected impurity systems.
The most important conclusion from this study is that distortions along the
(100) and (110) crystallographic directions are not independent. Once a
reliable cluster model is found, distortion trends as a function of impurity,
alkali cation and halide anion are identified and discussed. These trends may
be useful when analycing other cation impurities in similar host lattices.Comment: LaTeX file. 7 pages and 2 pictures. Accepted for publication in J.
Chem. Phy
Microscopic calculations of double and triple Giant Resonance excitation in heavy ion collisions
We perform microscopic calculations of the inelastic cross sections for the
double and triple excitation of giant resonances induced by heavy ion probes
within a semicalssical coupled channels formalism. The channels are defined as
eigenstates of a bosonic quartic Hamiltonian constructed in terms of collective
RPA phonons. Therefore, they are superpositions of several multiphonon states,
also with different numbers of phonons and the spectrum is anharmonic. The
inclusion of (n+1) phonon configurations affects the states whose main
component is a n-phonon one and leads to an appreacible lowering of their
energies. We check the effects of such further anharmonicities on the previous
published results for the cross section for the double excitation of Giant
Resonances. We find that the only effect is a shift of the peaks towards lower
energies, the double GR cross section being not modified by the explicity
inclusion of the three-phonon channels in the dynamical calculations. The
latters give an important contribution to the cross section in the triple GR
energy region which however is still smaller than the experimental available
data. The inclusion of four phonon configurations in the structure calculations
does not modify the results.Comment: Revtex4, to be published in PR
Asymptotically (anti) de Sitter Black Holes and Wormholes with a Self Interacting Scalar Field in Four Dimensions
The aim of this paper is to report on the existence of a wide variety of
exact solutions, ranging from black holes to wormholes, when a conformally
coupled scalar field with a self interacting potential containing a linear, a
cubic and a quartic self interaction is taken as a source of the
energy-momentum tensor, in the Einstein theory with a cosmological constant.
Among all the solutions there are two particularly interesting. On the one
hand, the spherically symmetric black holes when the cosmological constant is
positive; they are shown to be everywhere regular, namely there is no
singularity neither inside nor outside the event horizon. On the other hand,
there are spherically symmetric and topological wormholes that connect two
asymptotically (anti) de Sitter regions with a different value for the
cosmological constant. The regular black holes and the wormholes are supported
by everywhere regular scalar field configurations.Comment: Final versio
Multicomponent fluids of hard hyperspheres in odd dimensions
Mixtures of hard hyperspheres in odd space dimensionalities are studied with
an analytical approximation method. This technique is based on the so-called
Rational Function Approximation and provides a procedure for evaluating
equations of state, structure factors, radial distribution functions, and
direct correlations functions of additive mixtures of hard hyperspheres with
any number of components and in arbitrary odd-dimension space. The method gives
the exact solution of the Ornstein--Zernike equation coupled with the
Percus--Yevick closure, thus extending to arbitrary odd dimension the solution
for hard-sphere mixtures [J. L. Lebowitz, Phys.\ Rev.\ \textbf{133}, 895
(1964)]. Explicit evaluations for binary mixtures in five dimensions are
performed. The results are compared with computer simulations and a good
agreement is found.Comment: 16 pages, 8 figures; v2: slight change of notatio
Probing the Structure of Halo Nuclei
Our understanding of halo nuclei has so far relied on high-energy scattering
and reactions, but a number of uncertainties remain. I discuss in general terms
the new range of observables which will be measured by experiments around the
Coulomb barrier, and how some details of the reaction mechanisms still need to
be clarified.Comment: Proceedings of FUSION97 conference (March 1997), South Durras,
Australia. Submitted to J. Physics G: special issue `Heavy ion collisions at
near barrier energies'. No figures; uses IOPConf.sty (included
Long range absorption in the scattering of 6He on 208Pb and 197Au at 27 MeV
Quasi-elastic scattering of 6He at E_lab=27 MeV from 197Au has been measured
in the angular range of 6-72 degrees in the laboratory system employing LEDA
and LAMP detection systems. These data, along with previously analysed data of
6He + 208Pb at the same energy, are analyzed using Optical Model calculations.
The role of Coulomb dipole polarizability has been investigated. Large
imaginary diffuseness parameters are required to fit the data. This result is
an evidence for long range absorption mechanisms in 6He induced reactions.Comment: 10 pages, 10 figures, minor corrections. To appear in Nucl. Phys.
Critical Behavior of a Heavy Particle in a Granular Fluid
Behavior analogous to a second order phase transition is observed for the
homogeneous cooling state of a heavy impurity particle in a granular fluid. The
order parameter is the ratio of impurity mean square velocity to that
of the fluid, with a conjugate field proportional to the mass ratio. A
parameter , measuring the fluid cooling rate relative to the
impurity--fluid collision rate, is the analogue of the inverse temperature. For
the fluid is ``normal'' with at , as in the case of a
system with elastic collisions. For an ``ordered'' state with occurs at , representing an extreme breakdown of equipartition.
Critical slowing and qualitative changes in the velocity distribution function
for the impurity particle near the transition are notedComment: 4 pages (4 figures included
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