182 research outputs found
Edge spin accumulation in a ballistic regime
We consider a mesoscopic {\it ballistic} structure with Rashba spin-orbit
splitting of the electron spectrum. The ballistic region is attached to the
leads with a voltage applied between them. We calculate the edge spin density
which appears in the presence of a charge current through the structure due to
the difference in populations of electrons coming from different leads.
Combined effect of the boundary scattering and spin precession leads to
oscillations of the edge polarization with the envelope function decaying as a
power law of the distance from the boundary. The problem is solved with the use
of scattering states. The simplicity of the method allows to gain an insight
into the underlaying physics. We clarify the role of the unitarity of
scattering for the problem of edge spin accumulation. In case of a straight
boundary it leads to exact cancellation of all long-wave oscillations of the
spin density. As a result, only the Friedel-like spin density oscillations with
the momentum 2k_F survive. However, this appears to be rather exceptional case.
In general, the smooth spin oscillations with the spin precession length
recover, as it happens, e.g., for the wiggly boundary. We demonstrate also,
that there is no relation between the spin current in the bulk, which is zero
in the considered case, and the edge spin accumulation.Comment: Latex, 6 pages, 2 fig
A new rhynchocephalian from the late jurassic of Germany with a dentition that is unique amongst tetrapods.
Rhynchocephalians, the sister group of squamates (lizards and snakes), are only represented by the single genus Sphenodon today. This taxon is often considered to represent a very conservative lineage. However, rhynchocephalians were common during the late Triassic to latest Jurassic periods, but rapidly declined afterwards, which is generally attributed to their supposedly adaptive inferiority to squamates and/or Mesozoic mammals, which radiated at that time. New finds of Mesozoic rhynchocephalians can thus provide important new information on the evolutionary history of the group.
A new fossil relative of Sphenodon from the latest Jurassic of southern Germany, Oenosaurus muehlheimensis gen. et sp. nov., presents a dentition that is unique amongst tetrapods. The dentition of this taxon consists of massive, continuously growing tooth plates, probably indicating a crushing dentition, thus representing a previously unknown trophic adaptation in rhynchocephalians.
The evolution of the extraordinary dentition of Oenosaurus from the already highly specialized Zahnanlage generally present in derived rhynchocephalians demonstrates an unexpected evolutionary plasticity of these animals. Together with other lines of evidence, this seriously casts doubts on the assumption that rhynchocephalians are a conservative and adaptively inferior lineage. Furthermore, the new taxon underlines the high morphological and ecological diversity of rhynchocephalians in the latest Jurassic of Europe, just before the decline of this lineage on this continent. Thus, selection pressure by radiating squamates or Mesozoic mammals alone might not be sufficient to explain the demise of the clade in the Late Mesozoic, and climate change in the course of the fragmentation of the supercontinent of Pangaea might have played a major role
Spin Hall effect in clean two dimensional electron gases with Rashba spin-orbit coupling
We study the spin polarization induced by a current flow in clean two
dimensional electron gases with Rashba spin-orbit coupling. This geometric
effect originates from special properties of the electron's scattering at the
edges of the sample. In wide samples, the spin polarization has it largest
value at low energies (close to the bottom of the band) and goes to zero at
higher energies. In this case, the spin polarization is dominated by the
presence of evanescent modes which have an explicit spin component outside the
plane. In quantum wires, on the other hand, the spin polarization is dominated
by interference effects induced by multiple scattering at the edges. Here, the
spin polarization is quite sensitive to the value of the Fermi energy,
especially close to the point where a new channel opens up. We analyzed
different geometries and found that the spin polarization can be strongly
enhanced.Comment: 10 pages, 13 figures, high quality figures available upon reques
A study of Tycho's SNR at TeV energies with the HEGRA CT-System
Tycho's supernova remnant (SNR) was observed during 1997 and 1998 with the
HEGRA Cherenkov Telescope System in a search for gamma-ray emission at
energies above
~1 TeV. An analysis of these data, ~65 hours in total, resulted in no
evidence for TeV gamma-ray emission. The 3sigma upper limit to the gamma-ray
flux (>1 TeV) from Tycho is estimated at 5.78x10^{-13} photons cm^{-2} s^{-1},
or 33 milli-Crab. We interpret our upper limit within the framework of the
following scenarios:
(1) that the observed hard X-ray tail is due to synchrotron emission. A lower
limit on the magnetic field within Tycho may be estimated B>=22 microG,
assuming that the RXTE-detected
X-rays were due to synchrotron emission. However, using results from a
detailed model of the ASCA emission, a more conservative lower limit B>=6
microG is derived.
(2) the hadronic model of Drury, Aharonian & Voelk, and (3) the more recent
time-dependent kinetic theory of Berezhko & Voelk.
Our upper limit lies within the range of predicted values of both hadronic
models, according to uncertainties in physical parameters of Tycho, and shock
acceleration details. In the latter case, the model was scaled to suit the
parameters of Tycho and re-normalised to account for a simplification of the
original model.
We find that we cannot rule out Tycho as a potential contributor at an
average level to the Galactic cosmic-ray flux.Comment: 9 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
An agonistic anti-CD137 antibody disrupts lymphoid follicle structure and T-cell-dependent antibody responses
CD137 is a costimulatory receptor expressed on natural killer cells, T cells, and subsets of dendritic cells. An agonistic monoclonal antibody (mAb) against CD137 has been used to reduce tumor burden or reverse autoimmunity in animal models and clinical trials. Here, we show that mice treated with an agonistic anti-CD137 mAb have reduced numbers of germinal center (GC) B cells and follicular dendritic cells (FDCs) in lymphoid tissues, which impair antibody responses to multiple T-cell-dependent antigens, including infectious virus, viral proteins, and conjugated haptens. These effects are not due to enhanced apoptosis or impaired proliferation of B cells but instead correlate with changes in lymphoid follicle structure and GC B cell dispersal and are mediated by CD137 signaling in CD
Acoustically Levitated Whispering-Gallery Mode Microlasers
Acoustic levitation has become a crucial technique for contactless manipulation in several fields, particularly in biological applications. However, its application in the photonics field remains largely unexplored. In this study, we implement an affordable and innovative phased-array levitator that enables stable trapping in the air of micrometer dye-doped droplets, thereby enabling the creation of microlasers. For the first time, this paper presents a detailed performance of the levitated microlaser cavity, supported by theoretical analysis concerning the hybrid technology based on the combination of whispering-gallery modes and acoustic fields. The pressure field distribution inside the acoustic cavity is numerically solved and qualitatively matched with the schlieren deflectometry technique. The optical lasing features of the levitated microlasers are highly comparable with those devices based On-a-Chip registering maximum Q-factors of ~ 105, and minimum lasing thresholds ~ 150 nJ cm−2. The emission comb is explained as a sum of multiple individual-supported whispering-gallery modes. The use of novel touchless micrometric lasers, produced with an acoustic levitator brings new technological opportunities based on photonic-acoustic technological platforms
An agonistic anti-CD137 antibody disrupts lymphoid follicle structure and T-cell-dependent antibody responses
CD137 is a costimulatory receptor expressed on natural killer cells, T cells, and subsets of dendritic cells. An agonistic monoclonal antibody (mAb) against CD137 has been used to reduce tumor burden or reverse autoimmunity in animal models and clinical trials. Here, we show that mice treated with an agonistic anti-CD137 mAb have reduced numbers of germinal center (GC) B cells and follicular dendritic cells (FDCs) in lymphoid tissues, which impair antibody responses to multiple T-cell-dependent antigens, including infectious virus, viral proteins, and conjugated haptens. These effects are not due to enhanced apoptosis or impaired proliferation of B cells but instead correlate with changes in lymphoid follicle structure and GC B cell dispersal and are mediated by CD137 signaling in CD
AKARI and BLAST Observations of the Cassiopeia A Supernova Remnant and Surrounding Interstellar Medium
We use new large area far infrared maps ranging from 65 - 500 microns
obtained with the AKARI and the Balloon-borne Large Aperture Submillimeter
Telescope (BLAST) missions to characterize the dust emission toward the
Cassiopeia A supernova remnant (SNR). Using the AKARI high resolution data we
find a new "tepid" dust grain population at a temperature of ~35K and with an
estimated mass of 0.06 solar masses. This component is confined to the central
area of the SNR and may represent newly-formed dust in the unshocked supernova
ejecta. While the mass of tepid dust that we measure is insufficient by itself
to account for the dust observed at high redshift, it does constitute an
additional dust population to contribute to those previously reported. We fit
our maps at 65, 90, 140, 250, 350, and 500 microns to obtain maps of the column
density and temperature of "cold" dust (near 16 K) distributed throughout the
region. The large column density of cold dust associated with clouds seen in
molecular emission extends continuously from the surrounding interstellar
medium to project on the SNR, where the foreground component of the clouds is
also detectable through optical, X-ray, and molecular extinction. At the
resolution available here, there is no morphological signature to isolate any
cold dust associated only with the SNR from this confusing interstellar
emission. Our fit also recovers the previously detected "hot" dust in the
remnant, with characteristic temperature 100 K.Comment: Accepted for publication in the Astrophysical Journal. Maps and
related data are available at http://blastexperiment.info
The Relation Between the Surface Brightness and the Diameter for Galactic Supernova Remnants
In this work, we have constructed a relation between the surface brightness
() and diameter (D) of Galactic C- and S-type supernova remnants
(SNRs). In order to calibrate the -D dependence, we have carefully
examined some intrinsic (e.g. explosion energy) and extrinsic (e.g. density of
the ambient medium) properties of the remnants and, taking into account also
the distance values given in the literature, we have adopted distances for some
of the SNRs which have relatively more reliable distance values. These
calibrator SNRs are all C- and S-type SNRs, i.e. F-type SNRs (and S-type SNR
Cas A which has an exceptionally high surface brightness) are excluded. The
Sigma-D relation has 2 slopes with a turning point at D=36.5 pc: (at 1
GHz)=8.4 D
WmHzster (for
WmHzster and D36.5 pc) and (at 1
GHz)=2.7 10 D
WmHzster (for
WmHzster and D36.5 pc). We discussed the theoretical
basis for the -D dependence and particularly the reasons for the change
in slope of the relation were stated. Added to this, we have shown the
dependence between the radio luminosity and the diameter which seems to have a
slope close to zero up to about D=36.5 pc. We have also adopted distance and
diameter values for all of the observed Galactic SNRs by examining all the
available distance values presented in the literature together with the
distances found from our -D relation.Comment: 45 pages, 2 figures, accepted for publication in Astronomical and
Astrophysical Transaction
Six Years of Chandra Observations of Supernova Remnants
We present a review of the first six years of Chandra X-ray Observatory
observations of supernova remnants. From the official "first-light" observation
of Cassiopeia A that revealed for the first time the compact remnant of the
explosion, to the recent million-second spectrally-resolved observation that
revealed new details of the stellar composition and dynamics of the original
explosion, Chandra observations have provided new insights into the supernova
phenomenon. We present an admittedly biased overview of six years of these
observations, highlighting new discoveries made possible by Chandra's unique
capabilities.Comment: 82 pages, 28 figures, for the book Astrophysics Update
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