10,001 research outputs found
Prediction of rotating-blade vortex noise from noise of nonrotating blades
Measurements were conducted in an acoustic wind tunnel to determine vortex noise of nonrotating circular cylinders and NACA 0012 airfoils. Both constant-width and spanwise tapered models were tested at a low turbulence level. The constant-diameter cylinder and constant-chord airfoil also were tested in the turbulent wake generated by an upstream cylinder or airfoil. Vortex noise radiation from nonrotating circular cylinders at Reynolds numbers matching those of the rotating-blade tests were found to be strongly dependent on surface conditions and Reynolds number. Vortex noise of rotating circular cylinder blades, operating with and without the shed wake blown downstream, could be predicted using data for nonrotating circular cylinders as functions of Reynolds number. Vortex noise of nonrotating airfoils was found to be trailing-edge noise at a time frequence equal to that predicted for maximum-amplitude Tollmein-Schlichting instability waves at the trailing edge
The X-ray Iron Emission from Tycho's Supernova Remnant
We present the results of broadband fits to the X-ray spectrum of Tycho's
supernova remnant obtained by the Solid-State Imaging Spectrometers on the ASCA
Observatory. We use single-temperature, single-ionization-age, nonequilibrium
ionization models to characterize the ejecta and the blast-shocked interstellar
medium. Based on the Fe K emission at 6.5 keV, previous spectral studies have
suggested that the Fe ejecta in this Type Ia remnant are stratified interior to
the other ejecta. The ASCA data provide important constraints from the Fe L
emission near 1 keV as well as the Fe K emission. We find that the simplest
models, with emission from the ejecta and blast wave each at a single
temperature and ionization age, severely underestimate the Fe K flux. We show
that there is little Fe emission associated with the Si and S ejecta shell. The
blast-shocked interstellar medium has abundances roughly 0.3 times the solar
value, while the ejecta, with the exception of Fe, have relative abundances
that are typical of Type Ia supernovae. The addition of another component of Fe
emission, which we associate with ejecta, at a temperature at least two times
higher and an ionization age 100 times lower than the Si ejecta, does
provide a good fit to the spectrum. This model is consistent with X-ray imaging
results. Although fluorescent emission from dust in the remnant may contribute
to the Fe K flux, we conclude that it is unlikely to dominate.Comment: 23 pages, LaTex; 4 postscript figures, 2 postscript tables. To appear
in ApJ, vol 49
Solution to the twin image problem in holography
While the invention of holography by Dennis Gabor truly constitutes an
ingenious concept, it has ever since been troubled by the so called twin image
problem limiting the information that can be obtained from a holographic
record. Due to symmetry reasons there are always two images appearing in the
reconstruction process. Thus, the reconstructed object is obscured by its
unwanted out of focus twin image. Especially for emission electron as well as
for x- and gamma-ray holography, where the source-object distances are small,
the reconstructed images of atoms are very close to their twin images from
which they can hardly be distinguished. In some particular instances only,
experimental efforts could remove the twin images. More recently, numerical
methods to diminish the effect of the twin image have been proposed but are
limited to purely absorbing objects failing to account for phase shifts caused
by the object. Here we show a universal method to reconstruct a hologram
completely free of twin images disturbance while no assumptions about the
object need to be imposed. Both, amplitude and true phase distributions are
retrieved without distortion
Electron-phonon coupling in 122 Fe pnictides analyzed by femtosecond time-resolved photoemission
Based on results from femtosecond time-resolved photoemission, we compare
three different methods for determination of the electron-phonon coupling
constant {\lambda} in Eu and Ba-based 122 FeAs compounds. We find good
agreement between all three methods, which reveal a small {\lambda} < 0.2. This
makes simple electron-phonon mediated superconductivity unlikely in these
compounds.Comment: 11 pages, 3 figure
Exceptional type-I superconductivity of the layered silver oxide AgPbO
We report zero-resistivity transition and the details of magnetic transition
of a layered silver oxide AgPbO single crystal, which make
definitive evidence of superconductivity in this compound. In the AC
susceptibility of a mono-crystal, we observed large supercooling, as well as
positive peaks in the real part of the susceptibility indicating the
reversibility of magnetic process. These observations reveal that
AgPbO is probably the first oxide that shows type-I
superconductivity. Evaluation of the superconducting parameters not only gives
confirming evidence of type-I superconductivity, but also indicates that it is
a dirty-limit superconductor. We also analyze supercooling to determine the
upper limit of the Ginzburg-Landau parameter.Comment: v2: PACS numbers are adde
Momentum dependent ultrafast electron dynamics in antiferromagnetic EuFe2As2
Employing the momentum-sensitivity of time- and angle-resolved photoemission
spectroscopy we demonstrate the analysis of ultrafast single- and many-particle
dynamics in antiferromagnetic EuFe2As2. Their separation is based on a
temperature-dependent difference of photo-excited hole and electron relaxation
times probing the single particle band and the spin density wave gap,
respectively. Reformation of the magnetic order occurs at 800 fs, which is four
times slower compared to electron-phonon equilibration due to a smaller
spin-dependent relaxation phase space
Manifestation of spin-charge separation in the dynamic dielectric response of one--dimensional Sr2CuO3
We have determined the dynamical dielectric response of a one-dimensional,
correlated insulator by carrying out electron energy-loss spectroscopy on
Sr2CuO3 single crystals. The observed momentum and energy dependence of the
low-energy features, which correspond to collective transitions across the gap,
are well described by an extended one-band Hubbard model with moderate nearest
neighbor Coulomb interaction strength. An exciton-like peak appears with
increasing momentum transfer. These observations provide experimental evidence
for spin-charge separation in the relevant excitations of this compound, as
theoretically expected for the one-dimensional Hubbard model.Comment: RevTex, 4 pages+2 figures, to appear in PRL (July 13
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