2,311 research outputs found
Meson and Quark Degrees of Freedom and the Radius of the Deuteron
The existing experimental data for the deuteron charge radius are discussed.
The data of elastic electron scattering are inconsistent with the value
obtained in a recent atomic physics experiment. Theoretical predictions based
on a nonrelativistic description of the deuteron with realistic nucleon-nucleon
potentials and with a rather complete set of meson-exchange contributions to
the charge operator are presented. Corrections arising from the quark-gluon
substructure of the nucleon are explored in a nonrelativistic quark model; the
quark-gluon corrections, not accounted for by meson exchange, are small. Our
prediction for the deuteron charge radius favors the value of a recent atomic
physics experiment.Comment: 20 pages, LaTeX, 4 Postscript figures, to appear in Few-Body-System
Zerebrale Manifestationen bei angioneurotischem Ă–dem
Es wird über zwei Patienten mit familiärem angioneurotischen Ödem berichtet. Eine 1929 geborene Patientin wies einen computertomographisch gesicherten Infarkt im Bereich der A. cerebri posterior rechts auf, der 20jährige Sohn kam wegen einer transitorischen ischämischen Attacke sowie eines prolongierten reversiblen Insultes, jeweils im Gebiet der A. cerebri media links, in neurologische Behandlung. Ätiologie, klinische Symptomatik, Therapie und Differentialdiagnose cerebraler Störungen beim angioneurotischen Ödem werden im Zusammenhang mit den bisher in der Literatur beschriebenen Fällen diskutiert
Infrared Spectra of Meteoritic SiC Grains
We present here the first infrared spectra of meteoritic SiC grains. The
mid-infrared transmission spectra of meteoritic SiC grains isolated from the
Murchison meteorite were measured in the wavelength range 2.5--16.5 micron, in
order to make available the optical properties of presolar SiC grains. These
grains are most likely stellar condensates with an origin predominately in
carbon stars. Measurements were performed on two different extractions of
presolar SiC from the Murchison meteorite. The two samples show very different
spectral appearance due to different grain size distributions. The spectral
feature of the smaller meteoritic SiC grains is a relatively broad absorption
band found between the longitudinal and transverse lattice vibration modes
around 11.3 micron, supporting the current interpretation about the presence of
SiC grains in carbon stars. In contrast to this, the spectral feature of the
large (> 5 micron) grains has an extinction minimum around 10 micron. The
obtained spectra are compared with commercially available SiC grains and the
differences are discussed. This comparison shows that the crystal structure
(e.g., beta-SiC versus alpha-SiC) of SiC grains plays a minor role on the
optical signature of SiC grains compared to e.g. grain size.Comment: 7 pages, 6 figures. To appear in A&
Imaging nonequilibrium atomic vibrations with x-ray diffuse scattering
For over a century, x-ray scattering has been the most powerful tool for
determining the equilibrium structure of crystalline materials. Deviations from
perfect periodicity, for example due to thermal motion of the atoms, reduces
the intensity of the Bragg peaks as well as produces structure in the diffuse
scattering background. Analysis of the thermal diffuse scattering (TDS) had
been used to determine interatomic force constants and phonon dispersion in
relatively simple cases before inelastic neutron scattering became the
preferred technique to study lattice dynamics. With the advent of intense
synchrotron x-ray sources, there was a renewed interest in TDS for measuring
phonon dispersion. The relatively short x-ray pulses emanating from these
sources also enables the measurement of phonon dynamics in the time domain.
Prior experiments on nonequilibrium phonons were either limited by
time-resolution and/or to relatively long wavelength excitations. Here we
present the first images of nonequilibrium phonons throughout the Brillouin
zone in photoexcited III-V semiconductors, indium-phosphide and
indium-antimonide, using picosecond time-resolved diffuse scattering. In each
case, we find that the lattice remain out of equilibrium for several hundred
picoseconds up to nanoseconds after laser excitation. The non-equilibrium
population is dominated by transverse acoustic phonons which in InP are
directed along high-symmetry directions. The results have wide implications for
the detailed study of electron-phonon and phonon-phonon coupling in solids.Comment: 10 pages, 3 figure
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