1,155 research outputs found
PPM-Extended (PPMX) - a catalogue of positions and proper motions
Aims: We build a catalogue PPM-Extended (PPMX) on the ICRS system which is
complete down to a well-defined limiting magnitude and contains the best
presently available proper motions to be suited for kinematical studies in the
Galaxy.
Methods: We perform a rigorous weighted least-squares adjustment of
individual observations, spread over more than a century, to determine mean
positions and proper motions. The stellar content of PPMX is taken from GSC 1.2
supplemented by catalogues like ARIHIP, PPM and Tycho-2 at the bright end. All
observations have been weighted according to their individual accuracy. The
catalogue has been screened towards rejecting false entries in the various
source catalogues.
Results: PPM-Extended (PPMX) is a catalogue of 18,088,920 stars containing
astrometric and photometric information. Its limiting magnitude is about 15.2
in the GSC photometric system. PPMX consists of three parts: a) a survey
complete down to R_U = 12.8 in the magnitude system of UCAC2; b) additional
stars of high-precision proper motions, and c) all other stars from GSC 1.2
identified in 2MASS. The typical accuracy of the proper motions is 2mas/y for
66 percent of the survey stars (a) and the high-precision stars (b), and about
10 mas/y for all other stars. PPMX contains photometric information from
ASCC-2.5 and 2MASS.Comment: 9 pages, 8 figures, accepted for publication in Astronomy and
Astrophysic
Optical control of coherent interactions between quantum dot electron spins
Coherent interactions between spins in quantum dots are a key requirement for
quantum gates. We have performed pump-probe experiments in which pulsed lasers
emitting at different photon energies manipulate two distinct subsets of
electron spins within an inhomogeneous InGaAs quantum dot ensemble. The spin
dynamics are monitored through their precession about an external magnetic
field. These measurements demonstrate spin precession phase shifts and
modulations of the magnitude of one subset of oriented spins after optical
orientation of the second subset. The observations are consistent with results
from a model using a Heisenberg-like interaction with microeV-strength.Comment: 5 pages, 4 figure
Effect of pump-probe detuning on the Faraday rotation and ellipticity signals of mode-locked spins in InGaAs quantum dots
We have studied the Faraday rotation and ellipticity signals in ensembles of
singly-charged (In,Ga)As/GaAs quantum dots by pump-probe spectroscopy. For
degenerate pump and probe we observe that the Faraday rotation signal amplitude
first grows with increasing the time separation between pump and probe before a
decay is observed for large temporal separations. The temporal behavior of the
ellipticity signal, on the other hand, is regular: its amplitude decays with
the separation. By contrast, for detuned pump and probe the Faraday rotation
and ellipticty signals both exhibit similar and conventional behavior. The
experimental results are well described in the frame of a recently developed
microscopic theory [Phys. Rev. B 80, 104436 (2009)]. The comparison between
calculations and experimental data allows us to provide insight into the
spectral dependence of the electron spin precession frequencies and extract the
electron g-factor dependence on energy.Comment: 9 pages, 7 figure
Identification of a Protein in Several Borrelia Species which is Related to OspC of the Lyme Disease Spirochetes.
Using oligonucleotide probes which have previously been shown to be specific for the ospC gene found in the Lyme disease spirochete species Borrelia burgdorferi, B. garinii, and group VS461, we detected an ospC homolog in other Borrelia species including B. coriaceae, B. hermsii, B. anserina, B. turicatae, and B. parkeri. In contrast to the Lyme disease spirochetes, which carry the ospC gene on a 26-kb circular plasmid, we mapped the gene in other Borrelia species to linear plasmids which varied in size among the isolates tested. Some isolates carry multiple copies of the gene residing on linear plasmids of different sizes. The analyses conducted here also demonstrate that these Borrelia species contain a linear chromosome. Northern (RNA) blot analyses demonstrated that the gene is transcriptionally expressed in all species examined. High levels of transcriptional expression were observed in some B. hermsii isolates. Transcriptional start site analyses revealed that the length of the untranslated leader sequence was identical to that observed in the Lyme disease spirochete species. By Western blotting (immunoblotting) with antiserum (polyclonal) raised against the OspC protein of B. burgdorferi, we detected an immunoreactive protein of the same molecular weight as the OspC found in Lyme disease spirochete species. The results presented here demonstrate the presence of a protein that is genetically and antigenically related to OspC which is expressed in all species of the genus Borrelia tested
Parton Distributions and New Physics Searches: the Drell-Yan Forward-Backward Asymmetry as a Case Study
We discuss the sensitivity of theoretical predictions of observables used in
searches for new physics to parton distributions (PDFs) at large momentum
fraction . Specifically, we consider the neutral-current Drell-Yan
production of gauge bosons with invariant masses in the TeV range, for which
the forward-backward asymmetry of charged leptons from the decay of the gauge
boson in its rest frame is a traditional probe of new physics. We show that the
qualitative behaviour of the asymmetry depends strongly on the assumptions made
in determining the underlying PDFs. We discuss and compare the large-
behaviour of various different PDF sets, and find that they differ
significantly. Consequently, the shape of the asymmetry observed at lower
dilepton invariant masses, where all PDF sets are in reasonable agreement
because of the presence of experimental constraints, is not necessarily
reproduced at large masses where the PDFs are mostly unconstrained by data. It
follows that the shape of the asymmetry at high masses may depend on
assumptions made in the PDF parametrization, and thus deviations from the
traditionally expected behaviour cannot be taken as a reliable indication of
new physics. We demonstrate that forward-backward asymmetry measurements could
help in constraining PDFs at large and discuss the accuracy that would be
required to disentangle the effects of new physics from uncertainties in the
PDFs in this region.Comment: 27 pages, 20 figure
The prolate-to-oblate shape transition of phospholipid vesicles in response to frequency variation of an AC electric field can be explained by the dielectric anisotropy of a phospholipid bilayer
The external electric field deforms flaccid phospholipid vesicles into
spheroidal bodies, with the rotational axis aligned with its direction.
Deformation is frequency dependent: in the low frequency range (~ 1 kHz), the
deformation is typically prolate, while increasing the frequency to the 10 kHz
range changes the deformation to oblate. We attempt to explain this behaviour
with a theoretical model, based on the minimization of the total free energy of
the vesicle. The energy terms taken into account include the membrane bending
energy and the energy of the electric field. The latter is calculated from the
electric field via the Maxwell stress tensor, where the membrane is modelled as
anisotropic lossy dielectric. Vesicle deformation in response to varying
frequency is calculated numerically. Using a series expansion, we also derive a
simplified expression for the deformation, which retains the frequency
dependence of the exact expression and may provide a better substitute for the
series expansion used by Winterhalter and Helfrich, which was found to be valid
only in the limit of low frequencies. The model with the anisotropic membrane
permittivity imposes two constraints on the values of material constants:
tangential component of dielectric permittivity tensor of the phospholipid
membrane must exceed its radial component by approximately a factor of 3; and
the membrane conductivity has to be relatively high, approximately one tenth of
the conductivity of the external aqueous medium.Comment: 17 pages, 6 figures; accepted for publication in J. Phys.: Condens.
Matte
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