2,435 research outputs found
Valley filter and valley valve in graphene
It is known that the lowest propagating mode in a narrow ballistic ribbon of
graphene may lack the twofold valley degeneracy of higher modes. Depending on
the crystallographic orientation of the ribbon axis, the lowest mode mixes both
valleys or lies predominantly in a single valley (chosen by the direction of
propagation). We show, using a tight-binding model calculation, that a
nonequilibrium valley polarization can be realized in a sheet of graphene, upon
injection of current through a ballistic point contact with zigzag edges. The
polarity can be inverted by local application of a gate voltage to the point
contact region. Two valley filters in series may function as an
electrostatically controlled ``valley valve'', representing a
zero-magnetic-field counterpart to the familiar spin valve.Comment: RevTeX, 4 pages, 5 figure
Probing the close environment of young stellar objects with interferometry
The study of Young Stellar Objects (YSOs) is one of the most exciting topics
that can be undertaken by long baseline optical interferometry. The magnitudes
of these objects are at the edge of capabilities of current optical
interferometers, limiting the studies to a few dozen, but are well within the
capability of coming large aperture interferometers like the VLT
Interferometer, the Keck Interferometer, the Large Binocular Telescope or
'OHANA. The milli-arcsecond spatial resolution reached by interferometry probes
the very close environment of young stars, down to a tenth of an astronomical
unit. In this paper, I review the different aspects of star formation that can
be tackled by interferometry: circumstellar disks, multiplicity, jets. I
present recent observations performed with operational infrared
interferometers, IOTA, PTI and ISI, and I show why in the next future one will
extend these studies with large aperture interferometers.Comment: Review to be published in JENAM'2002 proceedings "The Very Large
Telescope Interferometer Challenges for the future
Bipolar supercurrent in graphene
Graphene -a recently discovered one-atom-thick layer of graphite- constitutes
a new model system in condensed matter physics, because it is the first
material in which charge carriers behave as massless chiral relativistic
particles. The anomalous quantization of the Hall conductance, which is now
understood theoretically, is one of the experimental signatures of the peculiar
transport properties of relativistic electrons in graphene. Other unusual
phenomena, like the finite conductivity of order 4e^2/h at the charge
neutrality (or Dirac) point, have come as a surprise and remain to be
explained. Here, we study the Josephson effect in graphene. Our experiments
rely on mesoscopic superconducting junctions consisting of a graphene layer
contacted by two closely spaced superconducting electrodes, where the charge
density can be controlled by means of a gate electrode. We observe a
supercurrent that, depending on the gate voltage, is carried by either
electrons in the conduction band or by holes in the valence band. More
importantly, we find that not only the normal state conductance of graphene is
finite, but also a finite supercurrent can flow at zero charge density. Our
observations shed light on the special role of time reversal symmetry in
graphene and constitute the first demonstration of phase coherent electronic
transport at the Dirac point.Comment: Under review, 12 pages, 4 Figs., suppl. info (v2 identical, resolved
file problems
The Dold-Kan Correspondence and Coalgebra Structures
By using the Dold-Kan correspondence we construct a Quillen adjunction
between the model categories of non-cocommutative coassociative simplicial and
differential graded coalgebras over a field. We restrict to categories of
connected coalgebras and prove a Quillen equivalence between them.Comment: 24 pages. Accepted by the Journal of Homotopy and Related Structures.
Online 28 November 201
Multi-product firms and exporting: a developing country perspective
In this paper we make the distinction between single-product and multi-product firms to contribute to our understanding of the complex relationship between multinational enterprises (MNEs), exporting and economic development. Using firm-level data for Thailand we show that the number of goods produced causes a larger variation in exports volumes than production volumes. Whilst the number of products and the total volume of exports are positively correlated we find, in contrast to US studies, a negative correlation between the number of products produced and the volume of production per product. We then investigate the characteristics associated with multi-product firms and find a distinction between foreign-owned and domestic firms. The presence of foreign firms producing single products solely for the domestic market as well as those producing many products for export demonstrates the diversity of behaviour of foreign-owned firms in developing countries
Accreting Neutron Stars in Low-Mass X-Ray Binary Systems
Using the Rossi X-ray Timing Explorer (RossiXTE), astronomers have discovered
that disk-accreting neutron stars with weak magnetic fields produce three
distinct types of high-frequency X-ray oscillations. These oscillations are
powered by release of the binding energy of matter falling into the strong
gravitational field of the star or by the sudden nuclear burning of matter that
has accumulated in the outermost layers of the star. The frequencies of the
oscillations reflect the orbital frequencies of gas deep in the gravitational
field of the star and/or the spin frequency of the star. These oscillations can
therefore be used to explore fundamental physics, such as strong-field gravity
and the properties of matter under extreme conditions, and important
astrophysical questions, such as the formation and evolution of millisecond
pulsars. Observations using RossiXTE have shown that some two dozen neutron
stars in low-mass X-ray binary systems have the spin rates and magnetic fields
required to become millisecond radio-emitting pulsars when accretion ceases,
but that few have spin rates above about 600 Hz. The properties of these stars
show that the paucity of spin rates greater than 600 Hz is due in part to the
magnetic braking component of the accretion torque and to the limited amount of
angular momentum that can be accreted in such systems. Further study will show
whether braking by gravitational radiation is also a factor. Analysis of the
kilohertz oscillations has provided the first evidence for the existence of the
innermost stable circular orbit around dense relativistic stars that is
predicted by strong-field general relativity. It has also greatly narrowed the
possible descriptions of ultradense matter.Comment: 22 pages, 7 figures, updated list of sources and references, to
appear in "Short-period Binary Stars: Observation, Analyses, and Results",
eds. E.F. Milone, D.A. Leahy, and D. Hobill (Dordrecht: Springer,
http://www.springerlink.com
The usefulness of contrast during exercise echocardiography for the assessment of systolic pulmonary pressure
<p>Abstract</p> <p>Background</p> <p>The systolic pulmonary artery pressure (PAPs) can be accurately estimated, non-invasively, using continuous-wave Doppler (CWD) ultrasound measurement of the peak velocity of a tricuspid regurgitant (TR) jet.</p> <p>However, it is often difficult to obtain adequate tricuspid regurgitation signals for measurement of PAPs, what could lead to its underestimation. Therefore, utilization of air-blood-saline contrast has been implemented for the improvement of Doppler signal in several clinical contexts.</p> <p>It is now recommended in the evaluation of patients with pulmonary hypertension. Physical activity is severely restricted in patients with PAH, being exertional dypnea the most typical symptom. Exercise stress echo-Doppler imaging allows assessment of the response to exercise. It is an excellent screening test for patients with suspected PAH. Our purpose was to evaluate the value and accuracy of agitated saline with blood contrast echocardiography, in the improvement of the Doppler signal, to quantify PAPs during treadmill exercise-echocardiography.</p> <p>Purpose</p> <p>To evaluate the value of contrast echocardiography, using agitated saline with blood, in the improvement of the Doppler signal used to quantify the pulmonary artery systolic pressure during exercise.</p> <p>Methods</p> <p>From a total of 41 patients (pts), we studied 38 pts (93%), 35 women, aged 54 ± 12 years-old. 27 with the diagnosis of systemic sclerosis, 10 with history of pulmonary embolism and one patient with a suspected idiopathic PAH, who were referred to the Unity of Heart Failure and Pulmonary Hypertension for screening of PAH. According to the Unity protocol, a transthoracic echocardiogram was made, in left decubitus (LD), with evaluation of right ventricle-right atria gradient (RV/RAg). A peripheral venous access was obtained, with a 3-way stopcock and the patients were placed in orthostatism (O), with a new evaluation of RV/RAg. Exercise echocardiography (EE) was begun, with evaluation of RV/RAg at peak exercise (P) and afterwards agitated saline (8 cc with 1 cc of air and 1 cc of blood) was injected, followed by a new evaluation of RV/RAg (PC) and then the interruption of the EE. Pulmonary Hypertension was diagnosed when RV/RAg at the end of the exercise was superior to 40 mmHg.</p> <p>Results</p> <p>The quality of Doppler signal was deteriorated in 5 pts, maintained in 6 pts and improved in 26 pts, with the use of contrast. In one patient, an interventricular septal defect was diagnosed. In 6 pts, a Doppler signal was only obtained with the use of contrast. In 15 pts, a RV/RAg superior to 40 mmHg was only obtained with the use of contrast. Of these, 9 have already been submitted to right heart cathetherism, that confirmed the diagnosis of pulmonary hypertension in 5 of them (56%). RV/RAg (P) was 44 ± 11 mmHg and RV/RAg (PC) was 54 ± 11 mmHg, p < 0,001.</p> <p>Conclusion</p> <p>1. The method is applicable in a large number of patients. 2. RV/RA gradients obtained at peak exercise are higher with the use of contrast, and the clinical meaning of this difference should be evaluated in a larger number of pts submitted to right heart cathetherism. The high number of false positives should lead to a higher diagnostic threshold. 3. This method seems to have relevant clinical value in the diagnosis of pulmonary arterial hypertension.</p
NCF1 gene and pseudogene pattern: association with parasitic infection and autoimmunity
<p>Abstract</p> <p>Background</p> <p>Neutrophil cytosolic factor 1, p47<sup>phox </sup>(NCF1) is a component of the leukocyte NADPH oxidase complex mediating formation of reactive oxygen intermediates (ROI) which play an important role in host defense and autoimmunity. An individual genomic pattern of <it>ncf1 </it>and its two types of pseudogenes (reflected by the ΔGT/GTGT ratio) may influence the individual capacity to produce ROI.</p> <p>Methods</p> <p>NCF1ΔGT/GTGT ratios were correlated with clinical parameters and ROI production during <it>Plasmodium falciparum </it>malaria and with susceptibility to the autoimmune disease multiple sclerosis (MS).</p> <p>Results</p> <p>Among Gabonese children with severe malaria, ROI production from peripheral blood tended to be higher in individuals with a ΔGT/GTGT ratio ≤ 1:1. ΔGT/GTGT ratios were not associated with susceptibility to MS, but to age-of-onset among MS patients.</p> <p>Conclusion</p> <p>The genomic pattern of <it>NCF1 </it>and its pseudogenes might influence ROI production but only marginally influence susceptibility to and outcome of malaria and MS.</p
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