17,551 research outputs found
The gravitational wave spectrum of non-axisymmetric, freely precessing neutron stars
Evidence for free precession has been observed in the radio signature of
several pulsars. Freely precessing pulsars radiate gravitationally at
frequencies near the rotation rate and twice the rotation rate, which for
rotation frequencies greater than Hz is in the LIGO band. In older
work, the gravitational wave spectrum of a precessing neutron star has been
evaluated to first order in a small precession angle. Here we calculate the
contributions to second order in the wobble angle, and we find that a new
spectral line emerges. We show that for reasonable wobble angles, the
second-order line may well be observable with the proposed advanced LIGO
detector for precessing neutron stars as far away as the galactic center.
Observation of the full second-order spectrum permits a direct measurement of
the star's wobble angle, oblateness, and deviation from axisymmetry, with the
potential to significantly increase our understanding of neutron star
structure.Comment: 22 pages, 1 figure. Minor changes in the text, typos correcte
Evaluation of non-intrusive flow measurement techniques for a re-entry flight experiment
This study evaluates various non-intrusive techniques for the measurement of the flow field on the windward side of the Space Shuttle orbiter or a similar reentry vehicle. Included are linear (Rayleigh, Raman, Mie, Laser Doppler Velocimetry, Resonant Doppler Velocimetry) and nonlinear (Coherent Anti-Stokes Raman, Laser-Induced Fluorescence) light scattering, electron-beam fluorescence, thermal emission, and mass spectroscopy. Flow-field properties were taken from a nonequilibrium flow model by Shinn, Moss, and Simmonds at the NASA Langley Research Center. Conclusions are, when possible, based on quantitative scaling of known laboratory results to the conditions projected. Detailed discussion with researchers in the field contributed further to these conclusions and provided valuable insights regarding the experimental feasibility of each of the techniques
Saddle Points and Stark Ladders: Exact Calculations of Exciton Spectra in Superlattices
A new, exact method for calculating excitonic absorption in superlattices is
described. It is used to obtain high resolution spectra showing the saddle
point exciton feature near the top of the miniband. The evolution of this
feature is followed through a series of structures with increasing miniband
width. The Stark ladder of peaks produced by an axial electric field is
investigated, and it is shown that for weak fields the line shapes are strongly
modified by coupling to continuum states, taking the form of Fano resonances.
The calculated spectra, when suitably broadened, are found to be in good
agreement with experimental results.Comment: 9 pages Revtex v3.0, followed by 4 uuencoded postscript figures,
SISSA-CM-94-00
Non-intrusive flow measurements on a reentry vehicle
This study evaluates the utility of various non-intrusive techniques for the measurement of the flow field on the windward side of the Space Shuttle or a similar re-entry vehicle. Included are linear (Rayleigh, Raman, Mie, Laser Doppler Velocimetry, Resonant Doppler Velocimetry) and nonlinear (Coherent Anti-Stokes Raman, Laser Induced Fluorescence) light scattering, electron beam fluorescence, thermal emission and mass spectroscopy. Flow field properties are taken from a nonequilibrium flow model by Shinn, Moss and Simmonds at NASA Langley. Conclusions are, when possible, based on quantitative scaling of known laboratory results to the conditions projected. Detailed discussion with researchers in the field contributed further to these conclusions and provided valuable insights regarding the experimental feasibility of each of the techniques
Conversion efficiency and luminosity for gamma-proton colliders based on the LHC-CLIC or LHC-ILC QCD Explorer scheme
Gamma-proton collisions allow unprecedented investigations of the low x and
high regions in quantum chromodynamics. In this paper, we investigate
the luminosity for "ILC"LHC ( TeV) and
"CLIC"LHC ( TeV) based colliders. Also
we determine the laser properties required for high conversion efficiency.Comment: 16, 6 figure
Fourier transform pure nuclear quadrupole resonance by pulsed field cycling
We report the observation of Fourier transform pure NQR by pulsed field cycling. For deuterium, well resolved spectra are obtained with high sensitivity showing the low frequency nu0 lines and allowing assignments of quadrupole couplings and asymmetry parameters to inequivalent deuterons. The technique is ideally applicable to nuclei with low quadrupolar frequencies (e.g., 2D, 7Li, 11B, 27Al, 23Na, 14N) and makes possible high resolution structure determination in polycrystalline or disordered materials
Stripe-hexagon competition in forced pattern forming systems with broken up-down symmetry
We investigate the response of two-dimensional pattern forming systems with a
broken up-down symmetry, such as chemical reactions, to spatially resonant
forcing and propose related experiments. The nonlinear behavior immediately
above threshold is analyzed in terms of amplitude equations suggested for a
and ratio between the wavelength of the spatial periodic forcing
and the wavelength of the pattern of the respective system. Both sets of
coupled amplitude equations are derived by a perturbative method from the
Lengyel-Epstein model describing a chemical reaction showing Turing patterns,
which gives us the opportunity to relate the generic response scenarios to a
specific pattern forming system. The nonlinear competition between stripe
patterns and distorted hexagons is explored and their range of existence,
stability and coexistence is determined. Whereas without modulations hexagonal
patterns are always preferred near onset of pattern formation, single mode
solutions (stripes) are favored close to threshold for modulation amplitudes
beyond some critical value. Hence distorted hexagons only occur in a finite
range of the control parameter and their interval of existence shrinks to zero
with increasing values of the modulation amplitude. Furthermore depending on
the modulation amplitude the transition between stripes and distorted hexagons
is either sub- or supercritical.Comment: 10 pages, 12 figures, submitted to Physical Review
Diffraction of a Bose-Einstein condensate from a Magnetic Lattice on a Micro Chip
We experimentally study the diffraction of a Bose-Einstein condensate from a
magnetic lattice, realized by a set of 372 parallel gold conductors which are
micro fabricated on a silicon substrate. The conductors generate a periodic
potential for the atoms with a lattice constant of 4 microns. After exposing
the condensate to the lattice for several milliseconds we observe diffraction
up to 5th order by standard time of flight imaging techniques. The experimental
data can be quantitatively interpreted with a simple phase imprinting model.
The demonstrated diffraction grating offers promising perspectives for the
construction of an integrated atom interferometer.Comment: 4 pages, 4 figure
Non-circular semiconductor nanorings of type I and II: Emission kinetics in the exciton Aharonov-Bohm effect
Transition energies and oscillator strengths of excitons in dependence on
magnetic field are investigated in type I and II semiconductor nanorings. A
slight deviation from circular (concentric) shape of the type II nanoring gives
a better observability of the Aharonov-Bohm oscillations since the ground state
is always optically active. Kinetic equations for the exciton occupation are
solved with acoustic phonon scattering as the major relaxation process, and
absorption and luminescence spectra are calculated showing deviations from
equilibrium. The presence of a non-radiative exciton decay leads to a quenching
of the integrated photoluminescence with magnetic field.Comment: The first version submitted to Phys. Rev. B on April 16, 2007.
Revised (this) version on July 31, 200
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