575 research outputs found
Propagation of a laser beam in a plasma
This paper shows that for a nonabsorbing medium with a prescribed index of refraction, the effects of beam stability, line focusing, and beam distortion can be predicted from simple ray optics. When the paraxial approximation is used, diffraction effects are examined for Gaussian, Lorentzian, and square beams. Most importantly, it is shown that for a Gaussian beam, diffraction effects can be included simply by adding imaginary solutions to the paraxial ray equations. Also presented are several procedures to extend the paraxial approximation so that the solution will have a domain of validity of greater extent
Discoveries and challenges en route to swinhoeisterolâ A
In this work, a full account of the authorsâ synthetic studies is reported that culminated in the first synthesis of 13(14â8),14(8â7)diabeoâsteroid swinhoeisterolâ
A as well as the related dankasterones A and B, 13(14â8)abeoâsteroids, and periconiastoneâ
A, a 13(14â8)abeoâ4,14âcycloâsteroid. Experiments are described in detail that provided further insight into the mechanism of the switchable radical framework reconstruction approach. By discussing failed strategies and tactics towards swinhoeisterolâ
A, the successful route that also allowed an access to structurally closely related analogues, such as Î22â24âepiâswinhoeisterolâ
A, is eventually presented
A Method of Hospital Infection Surveillance Incorporating the Use of the Computer
The records of all patients in the hospital on a particular date were studied for hospital-acquired infections. Results were compared with a continuing surveillance based on discharge reporting. Collection of data was programmed for analysis by using the hospital computer. Thirteen per cent of the patients manifested an infection after admission, but before or on the survey day. Results elsewhere are similar. Areas of the hospital with a relatively higher incidence of infection did not have clusters of particular pathogens
Bogoliubov spectrum and Bragg spectroscopy of elongated Bose-Einstein condensates
The behavior of the momentum transferred to a trapped Bose-Einstein
condensate by a two-photon Bragg pulse reflects the structure of the underlying
Bogoliubov spectrum. In elongated condensates, axial phonons with different
number of radial nodes give rise to a multibranch spectrum which can be
resolved in Bragg spectroscopy, as shown by Steinhauer {\it et al.} [Phys. Rev.
Lett. {\bf 90}, 060404 (2003)]. Here we present a detailed theoretical analysis
of this process. We calculate the momentum transferred by numerically solving
the time dependent Gross-Pitaevskii equation. In the case of a cylindrical
condensate, we compare the results with those obtained by linearizing the
Gross-Pitaevskii equation and using a quasiparticle projection method. This
analysis shows how the axial-phonon branches affect the momentum transfer, in
agreement with our previous interpretation of the observed data. We also
discuss the applicability of this type of spectroscopy to typical available
condensates, as well as the role of nonlinear effects.Comment: 8 pages, 7 figures, minor changes, typos correcte
Dynamics of fluctuations in an optical analog of the Laval nozzle
Using the analogy between the description of coherent light propagation in a
medium with Kerr nonlinearity by means of nonlinear Schr\"odinger equation and
that of a dissipationless liquid we propose an optical analogue of the Laval
nozzle. The optical Laval nozzle will allow one to form a transonic flow in
which one can observe and study a very unusual dynamics of classical and
quantum fluctuations including analogue of the Hawking radiation of real black
holes. Theoretical analysis of this dynamics is supported by numerical
calculations and estimates for a possible experimental setup are presented.Comment: 7 pages, 4 figure
Outbursts on normal stars. FH Leo misclassified as a novalike variable
We present high resolution spectroscopy of the common proper motion system FH
Leo (components HD 96273 and BD+07 2411B), which has been classified as a
novalike variable due to an outburst observed by Hipparcos, and we present and
review the available photometry. We show from our spectra that neither star can
possibly be a cataclysmic variable, instead they are perfectly normal late-F
and early-G stars. We measured their radial velocities and derived the
atmospheric fundamental parameters, abundances of several elements including
Fe, Ni, Cr, Co, V, Sc, Ti, Ca and Mg, and we derive the age of the system. From
our analysis we conclude that the stars do indeed constitute a physical binary.
However, the observed outburst cannot be readily explained. We examine several
explanations, including pollution with scattered light from Jupiter, binarity,
microlensing, background supernovae, interaction with unseen companions and
planetary engulfment. While no explanation is fully satisfactory, the scattered
light and star-planet interaction scenarios emerge as the least unlikely ones,
and we give suggestions for further study.Comment: 8 pages, 7 figures. Accepted for publication in A&
Transmittivity of a Bose-Einstein condensate on a lattice: interference from period doubling and the effect of disorder
We evaluate the particle current flowing in steady state through a
Bose-Einstein condensate subject to a constant force in a quasi-onedimensional
lattice and to attractive interactions from fermionic atoms that are localized
in various configurations inside the lattice wells. The system is treated
within a Bose-Hubbard tight binding model by an out-of-equilibrium Green's
function approach. A new band gap opens up when the lattice period is doubled
by locating the fermions in alternate wells and yields an interference pattern
in the transmittivity on varying the intensity of the driving force. The
positions of the transmittivity minima are determined by matching the period of
Bloch oscillations and the time for tunnelling across the band gap. Massive
disorder in the distribution of the fermions will wash out the interference
pattern, but the same period doubling of the lattice can be experimentally
realized in a four-beam set-up. We report illustrative numerical results for a
mixture of 87Rb and 40K atoms in an optical lattice created by laser beams with
a wavelength of 763 nm.Comment: 13 pages, 5 figure
Inverse anisotropic diffusion from power density measurements in two dimensions
This paper concerns the reconstruction of an anisotropic diffusion tensor
from knowledge of internal functionals
of the form with for
solutions of the elliptic equation on a two
dimensional bounded domain with appropriate boundary conditions. We show that
for I=4 and appropriately chosen boundary conditions, may uniquely and
stably be reconstructed from such internal functionals, which appear in
coupled-physics inverse problems involving the ultrasound modulation of
electrical or optical coefficients. Explicit reconstruction procedures for the
diffusion tensor are presented and implemented numerically.Comment: 27 pages, 6 figure
Quantum-fluid dynamics of microcavity polaritons
Semiconductor microcavities offer a unique system to investigate the physics
of weakly interacting bosons. Their elementary excitations, polaritons--a
mixture of excitons and photons--behave, in the low density limit, as bosons
that can undergo a phase transition to a regime characterised by long range
coherence. Condensates of polaritons have been advocated as candidates for
superfluidity; and the formation of vortices as well as elementary excitations
with a linear dispersion are actively sought after. In this work, we have
created and set in motion a macroscopically degenerate state of polaritons and
let it collide with a variety of defects present in the sample. Our experiments
show striking manifestations of a coherent light-matter packet that displays
features of a superfluid, although one of a highly unusual character as it
involves an out-of-equilibrium dissipative system where it travels at
ultra-fast velocity of the order of 1% the speed of light. Our main results are
the observation of i) a linear polariton dispersion accompanied with
diffusion-less motion, ii) flow without resistance when crossing an obstacle,
iii) suppression of Rayleigh scattering and iv) splitting into two fluids when
the size of the obstacle is comparable with the size of the wavepacket. This
work opens the way to the investigation of new phenomenology of
out-of-equilibrium condensates.Comment: 22 pages, 5 figure
- âŠ