7,298 research outputs found
Intermittent energy dissipation by turbulent reconnection
Magnetic reconnection—the process responsible for many explosive phenomena in both nature and laboratory—is efficient at dissipating magnetic energy into particle energy. To date, exactly how this dissipation happens remains unclear, owing to the scarcity of multipoint measurements of the “diffusion region” at the sub-ion scale. Here we report such a measurement by Cluster—four spacecraft with separation of 1/5 ion scale. We discover numerous current filaments and magnetic nulls inside the diffusion region of magnetic reconnection, with the strongest currents appearing at spiral nulls (O-lines) and the separatrices. Inside each current filament, kinetic-scale turbulence is significantly increased and the energy dissipation, E′ ⋅ j, is 100 times larger than the typical value. At the jet reversal point, where radial nulls (X-lines) are detected, the current, turbulence, and energy dissipations are surprisingly small. All these features clearly demonstrate that energy dissipation in magnetic reconnection occurs at O-lines but not X-lines
Topological Crystalline Insulators in the SnTe Material Class
Topological crystalline insulators are new states of matter in which the
topological nature of electronic structures arises from crystal symmetries.
Here we predict the first material realization of topological crystalline
insulator in the semiconductor SnTe, by identifying its nonzero topological
index. We predict that as a manifestation of this nontrivial topology, SnTe has
metallic surface states with an even number of Dirac cones on high-symmetry
crystal surfaces such as {001}, {110} and {111}. These surface states form a
new type of high-mobility chiral electron gas, which is robust against disorder
and topologically protected by reflection symmetry of the crystal with respect
to {110} mirror plane. Breaking this mirror symmetry via elastic strain
engineering or applying an in-plane magnetic field can open up a continuously
tunable band gap on the surface, which may lead to wide-ranging applications in
thermoelectrics, infrared detection, and tunable electronics. Closely related
semiconductors PbTe and PbSe also become topological crystalline insulators
after band inversion by pressure, strain and alloying.Comment: submitted on Feb. 10, 2012; to appear in Nature Communications; 5
pages, 4 figure
Discovery (theoretical prediction and experimental observation) of a large-gap topological-insulator class with spin-polarized single-Dirac-cone on the surface
Recent theories and experiments have suggested that strong spin-orbit
coupling effects in certain band insulators can give rise to a new phase of
quantum matter, the so-called topological insulator, which can show macroscopic
entanglement effects. Such systems feature two-dimensional surface states whose
electrodynamic properties are described not by the conventional Maxwell
equations but rather by an attached axion field, originally proposed to
describe strongly interacting particles. It has been proposed that a
topological insulator with a single spin-textured Dirac cone interfaced with a
superconductor can form the most elementary unit for performing fault-tolerant
quantum computation. Here we present an angle-resolved photoemission
spectroscopy study and first-principle theoretical calculation-predictions that
reveal the first observation of such a topological state of matter featuring a
single-surface-Dirac-cone realized in the naturally occurring BiSe
class of materials. Our results, supported by our theoretical predictions and
calculations, demonstrate that undoped compound of this class of materials can
serve as the parent matrix compound for the long-sought topological device
where in-plane surface carrier transport would have a purely quantum
topological origin. Our study further suggests that the undoped compound
reached via n-to-p doping should show topological transport phenomena even at
room temperature.Comment: 3 Figures, 18 pages, Submitted to NATURE PHYSICS in December 200
Passive phloem loading and long-distance transport in a synthetic tree-on-a-chip
Vascular plants rely on differences of osmotic pressure to export sugars from
regions of synthesis (mature leaves) to sugar sinks (roots, fruits). In this
process, known as M\"unch pressure flow, the loading of sugars from
photosynthetic cells to the export conduit (the phloem) is crucial, as it sets
the pressure head necessary to power long-distance transport. Whereas most
herbaceous plants use active mechanisms to increase phloem concentration above
that of the photosynthetic cells, in most tree species, for which transport
distances are largest, loading seems to occur via passive symplastic diffusion
from the mesophyll to the phloem. Here, we use a synthetic microfluidic model
of a passive loader to explore the nonlinear dynamics that arise during export
and determine the ability of passive loading to drive long-distance transport.
We first demonstrate that in our device, phloem concentration is set by the
balance between the resistances to diffusive loading from the source and
convective export through the phloem. Convection-limited export corresponds to
classical models of M\"unch transport, where phloem concentration is close to
that of the source; in contrast, diffusion-limited export leads to small phloem
concentrations and weak scaling of flow rates with the hydraulic resistance. We
then show that the effective regime of convection-limited export is predominant
in plants with large transport resistances and low xylem pressures. Moreover,
hydrostatic pressures developed in our synthetic passive loader can reach
botanically relevant values as high as 10 bars. We conclude that passive
loading is sufficient to drive long-distance transport in large plants, and
that trees are well suited to take full advantage of passive phloem loading
strategies
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Finite element analysis and calculation method of residual flexural capacity of post-fire RC beams
Fire tests and subsequent bending tests of fourreinforced concrete (RC) beamswere performed. Based on these tests, the post-fire performance of RCbeams was further studied using finite element simulation through reasonable selection of suitable thermal and thermodynamic parameters of steel and concrete materials. A thermodynamic model of RC beams with three sides under fire was built using finite element analysis(FEA)software ABAQUS. The FEA model was validated with the results of fire tests. Different factors were taken into account for further parametric studies in fire using the proposed FE model.The results show that the main factors affecting the fire resistance of the beamsare the thickness of the concretecover, reinforcement ratio of longitudinal steel,the fire exposure timeandthe fire exposure sides. Based on the strength reduction formula at high temperature of steel and concrete, animproved section method was proposed to develop a calculation formula to calculate the flexural capacity of RC beams after fire. The theoretical calculation method proposed in this paper shows good agreement with FEA results, which can be used to calculate the flexuralcapacity of RC beams after fire
Linear Sigma Models with Torsion
Gauged linear sigma models with (0,2) supersymmetry allow a larger choice of
couplings than models with (2,2) supersymmetry. We use this freedom to find a
fully linear construction of torsional heterotic compactifications, including
models with branes. As a non-compact example, we describe a family of metrics
which correspond to deformations of the heterotic conifold by turning on
H-flux. We then describe compact models which are gauge-invariant only at the
quantum level. Our construction gives a generalization of symplectic reduction.
The resulting spaces are non-Kahler analogues of familiar toric spaces like
complex projective space. Perturbatively conformal models can be constructed by
considering intersections.Comment: 40 pages, LaTeX, 1 figure; references added; a new section on
supersymmetry added; quantization condition revisite
Compactification on negatively curved manifolds
We show that string/M theory compactifications to maximally symmetric
space-times using manifolds whose scalar curvature is everywhere negative, must
have significant warping, large stringy corrections, or both.Comment: 18 pages, JHEP3.cl
Heterotic Sigma Models with N=2 Space-Time Supersymmetry
We study the non-linear sigma model realization of a heterotic vacuum with
N=2 space-time supersymmetry. We examine the requirements of (0,2) + (0,4)
world-sheet supersymmetry and show that a geometric vacuum must be described by
a principal two-torus bundle over a K3 manifold.Comment: 20 pages, uses xy-pic; v3: typos corrected, reference added,
discussion of constraints on Hermitian form modifie
Comparison of two independent systematic reviews of trials of recombinant human bone morphogenetic protein-2 (rhBMP-2) : The Yale Open Data Access Medtronic Project
Background: It is uncertain whether the replication of systematic reviews, particularly those with the same objectives and resources, would employ similar methods and/or arrive at identical findings. We compared the results and conclusions of two concurrent systematic reviews undertaken by two independent research teams provided with the same objectives, resources, and individual participant-level data. Methods: Two centers in the USA and UK were each provided with participant-level data on 17 multi-site clinical trials of recombinant human bone morphogenetic protein-2 (rhBMP-2). The teams were blinded to each other's methods and findings until after publication. We conducted a retrospective structured comparison of the results of the two systematic reviews. The main outcome measures included (1) trial inclusion criteria; (2) statistical methods; (3) summary efficacy and risk estimates; and (4) conclusions. Results: The two research teams' meta-analyses inclusion criteria were broadly similar but differed slightly in trial inclusion and research methodology. They obtained similar results in summary estimates of most clinical outcomes and adverse events. Center A incorporated all trials into summary estimates of efficacy and harms, while Center B concentrated on analyses stratified by surgical approach. Center A found a statistically significant, but small, benefit whereas Center B reported no advantage. In the analysis of harms, neither showed an increased cancer risk at 48 months, although Center B reported a significant increase at 24 months. Conclusions reflected these differences in summary estimates of benefit balanced with small but potentially important risk of harm. Conclusions: Two independent groups given the same research objectives, data, resources, funding, and time produced broad general agreement but differed in several areas. These differences, the importance of which is debatable, indicate the value of the availability of data to allow for more than a single approach and a single interpretation of the data. Systematic review registration: PROSPERO CRD42012002040and CRD42012001907
Altered hemodynamics and vascular reactivity in a mouse model with severe pericyte deficiency
Pericytes are the mural cells of the microvascular network that are in close contact with underlying endothelial cells. Endothelial-secreted PDGFB leads to recruitment of pericytes to the vessel wall, but this is disrupted in Pdgfb mice when the PDGFB retention motif is deleted. This results in severely reduced pericyte coverage on blood vessels. In this study, we investigated vascular abnormalities and hemodynamics in Pdgfb mice throughout the cerebrovascular network and in different cortical layers by in vivo two-photon microscopy. We confirmed that Pdgfb mice are severely deficient in pericytes throughout the vascular network, with enlarged brain blood vessels and a reduced number of vessel branches. Red blood cell velocity, linear density, and tube hematocrit were reduced in Pdgfb mice, which may impair oxygen delivery to the tissue. We also measured intravascular PO and found that concentrations were higher in cortical Layer 2/3 in Pdgfb mice, indicative of reduced blood oxygen extraction. Finally, we found that Pdgfb mice had a reduced capacity for vasodilation in response to an acetazolamide challenge during functional MRI imaging. Taken together, these results suggest that severe pericyte deficiency can lead to vascular abnormalities and altered cerebral blood flow, reminiscent of pathologies such as arteriovenous malformations
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