9,815 research outputs found
Parametrization of the Driven Betatron Oscillation
An AC dipole is a magnet which produces a sinusoidally oscillating dipole
field and excites coherent transverse beam motion in a synchrotron. By
observing this coherent motion, the optical parameters can be directly measured
at the beam position monitor locations. The driven oscillation induced by an AC
dipole will generate a phase space ellipse which differs from that of the free
oscillation. If not properly accounted for, this difference can lead to a
misinterpretation of the actual optical parameters, for instance, of 6% or more
in the cases of the Tevatron, RHIC, or LHC. The effect of an AC dipole on the
linear optics parameters is identical to that of a thin lens quadrupole. By
introducing a new amplitude function to describe this new phase space ellipse,
the motion produced by an AC dipole becomes easier to interpret. Beam position
data taken under the influence of an AC dipole, with this new interpretation in
mind, can lead to more precise measurements of the normal Courant-Snyder
parameters. This new parameterization of the driven motion is presented and is
used to interpret data taken in the FNAL Tevatron using an AC dipole.Comment: 8 pages, 8 figures, and 1 tabl
First Principles Study of Work Functions of Double Wall Carbon Nanotubes
Using first-principles density functional calculations, we investigated work
functions (WFs) of thin double-walled nanotubes (DWNTs) with outer tube
diameters ranging from 1nm to 1.5nm. The results indicate that work function
change within this diameter range can be up to 0.5 eV, even for DWNTs with same
outer diameter. This is in contrast with single-walled nanotubes (SWNTs) which
show negligible WF change for diameters larger than 1nm. We explain the WF
change and related charge redistribution in DWNTs using charge equilibration
model (CEM). The predicted work function variation of DWNTs indicates a
potential difficulty in their nanoelectronic device applications.Comment: 11 pages, 3 figures, to appear as rapid communication on Physical
Review
Signatures of LCDM substructure in tidal debris
In the past decade, surveys of the stellar component of the Galaxy have
revealed a number of streams from tidally disrupted dwarf galaxies and globular
clusters. Simulations of hierarchical structure formation in LCDM cosmologies
predict that the dark matter halo of a galaxy like the Milky Way contains
hundreds of subhalos with masses of ~10^8 solar masses and greater, and it has
been suggested that the existence of coherent tidal streams is incompatible
with the expected abundance of substructure. We investigate the effects of dark
matter substructure on tidal streams by simulating the disruption of a
self-gravitating satellite on a wide range of orbits in different host models
both with and without substructure. We find that the halo shape and the
specific orbital path more strongly determine the overall degree of disruption
of the satellite than does the presence or absence of substructure, i.e., the
changes in the large-scale properties of the tidal debris due to substructure
are small compared to variations in the debris from different orbits in a
smooth potential. Substructure typically leads to an increase in the degree of
clumpiness of the tidal debris in sky projection, and in some cases a more
compact distribution in line-of-sight velocity. Substructure also leads to
differences in the location of sections of debris compared to the results of
the smooth halo model, which may have important implications for the
interpretation of observed tidal streams. A unique signature of the presence of
substructure in the halo which may be detectable by upcoming surveys is
identified. We conclude, however, that predicted levels of substructure are
consistent with a detection of a coherent tidal stream from a dwarf galaxy.Comment: 15 pages, 13 figures, accepted for publication in ApJ. Matches
accepted versio
Ultrafast dephasing of coherent optical phonons in atomically controlled GeTe/SbTe superlattices
Femtosecond dynamics of coherent optical phonons in GeTe/SbTe
superlattices (SLs), a new class of semiconductor SLs with three different
states, have been investigated by using a reflection-type pump-probe technique
at various lattice temperatures. The time-resolved transient reflectivity (TR)
obtained in as-grown SLs exhibits the coherent A optical modes at 5.10
THz and 3.78 THz, while only the single A mode at 3.68 THz is observed in
annealed SLs. The decay rate of the A mode in annealed SLs is strongly
temperature dependent, while that in as-grown SLs is not temperature dependent.
This result indicates that the damping of the coherent A phonons in
amorphous SLs is governed by the phonon-defect (vacancy) scattering rather than
the anharmonic phonon-phonon coupling.Comment: 5 pages, 5 figure
Entanglement of orbital angular momentum states between an ensemble of cold atoms and a photon
Recently, atomic ensemble and single photons were successfully entangled by
using collective enhancement [D. N. Matsukevich, \textit{et al.}, Phys. Rev.
Lett. \textbf{95}, 040405(2005).], where atomic internal states and photonic
polarization states were correlated in nonlocal manner. Here we experimentally
clarified that in an ensemble of atoms and a photon system, there also exists
an entanglement concerned with spatial degrees of freedom. Generation of
higher-dimensional entanglement between remote atomic ensemble and an
application to condensed matter physics are also discussed.Comment: 5 pages, 3 figure
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