15,846 research outputs found
Charge redistribution in the formation of one-dimensional lithium wires on Cu(001)
We describe the formation of one-dimensional lithium wires on a Cu(001) substrate, providing an atomic-scale description of the onset of metallization in this prototypical adsorption system. A combination of helium atom scattering and density-functional theory reveals pronounced changes in the electronic charge distribution on the formation of the c(5√2×√2)R45° Li/Cu(001) structure, as in-plane bonds are created. Charge donation from Li-substrate bonds is found to facilitate the formation of stable, bonded, and depolarized chains of Li adatoms that coexist with an interleaved phase of independent adatoms. The resultant overlayer has a commensurate charge distribution and lattice modulations but differs fundamentally from structurally similar charge-density wave systems
Minimal Brownian Ratchet: An Exactly Solvable Model
We develop an exactly-solvable three-state discrete-time minimal Brownian
ratchet (MBR), where the transition probabilities between states are
asymmetric. By solving the master equations we obtain the steady-state
probabilities. Generally the steady-state solution does not display detailed
balance, giving rise to an induced directional motion in the MBR. For a reduced
two-dimensional parameter space we find the null-curve on which the net current
vanishes and detailed balance holds. A system on this curve is said to be
balanced. On the null-curve, an additional source of external random noise is
introduced to show that a directional motion can be induced under the zero
overall driving force. We also indicate the off-balance behavior with biased
random noise.Comment: 4 pages, 4 figures, RevTex source, General solution added. To be
appeared in Phys. Rev. Let
Cytotoxic T Lymphocyte Antigen-4 Accumulation in the Immunological Synapse Is Regulated by TCR Signal Strength
AbstractCD28 and CTLA-4 engagement with B7 expressed by APCs generates critical regulatory signals for T cell activation. CD28 is expressed on the T cell surface and enhances T cell expansion, while CTLA-4 localizes primarily to an intracellular compartment and inhibits T cell proliferation. We demonstrate that CTLA-4 has several unique trafficking properties that may regulate its ability to attenuate a T cell response. Importantly, accumulation of CTLA-4 at the immunological synapse is proportional to the strength of the TCR signal, suggesting that cells receiving stronger stimuli are more susceptible to CTLA-4-mediated inhibition. This may represent a novel feedback control mechanism in which a stimulatory signal regulates the recruitment of an inhibitory receptor to a functionally relevant site on the cell surface
Simulations of Galactic Cosmic Rays Impacts on the Herschel/PACS Photoconductor Arrays with Geant4 Code
We present results of simulations performed with the Geant4 software code of
the effects of Galactic Cosmic Ray impacts on the photoconductor arrays of the
PACS instrument. This instrument is part of the ESA-Herschel payload, which
will be launched in late 2007 and will operate at the Lagrangian L2 point of
the Sun-Earth system. Both the Satellite plus the cryostat (the shield) and the
detector act as source of secondary events, affecting the detector performance.
Secondary event rates originated within the detector and from the shield are of
comparable intensity. The impacts deposit energy on each photoconductor pixel
but do not affect the behaviour of nearby pixels. These latter are hit with a
probability always lower than 7%. The energy deposited produces a spike which
can be hundreds times larger than the noise. We then compare our simulations
with proton irradiation tests carried out for one of the detector modules and
follow the detector behaviour under 'real' conditions.Comment: paper submitted to Experimental Astronomy in March 200
Primary isolated extramedullary plasmacytoma of the colon: a case report
The original publication is available at http://www.samj.org.zaA 10th case of extramedullary plasmacytoma (EMP) of the colon is reported. The term 'primary isolated extramedullary plasmacytoma' is preferred to those terms previously used. A broad classification of plasmacytoma of the gastrointestinal tract is suggested and a management policy, based on this classification, is outlined.Publishers' versio
Polyphenolic Phenomena: Transgenic Analysis of Some of the Factors that Regulate the Cell-Specific Accumulation of Condensed Tannins (Proanthocyanidins) in Forage Crops
Condensed tannins biosynthesised within crops are a well-established mechanism for protecting plant protein in the rumen of grazing livestock. Protein protection mediated by these polymeric flavonoid molecules has been characterised in Lotus spp. and offers an interesting contrast to the polyphenol oxidase (PPO) system that confers protein protection in red clover
Fiber Optic Thermal Health Monitoring of Aerospace Structures and Materials
A new technique is presented for thermographic detection of flaws in materials and structures by performing temperature measurements with fiber Bragg gratings. Individual optical fibers with multiple Bragg gratings employed as surface temperature sensors were bonded to the surfaces of structures with subsurface defects or thickness variations. Both during and following the application of a thermal heat flux to the surface, the individual Bragg grating sensors measured the temporal and spatial temperature variations. The investigated structures included a 10-ply composite specimen with subsurface delaminations of various sizes and depths. The data obtained from grating sensors were further analyzed with thermal modeling to reveal particular characteristics of the interested areas. These results were found to be consistent with those from conventional thermography techniques. Limitations of the technique were investigated using both experimental and numerical simulation techniques. Methods for performing in-situ structural health monitoring are discussed
The arctic curve of the domain-wall six-vertex model in its anti-ferroelectric regime
An explicit expression for the spatial curve separating the region of
ferroelectric order (`frozen' zone) from the disordered one (`temperate' zone)
in the six-vertex model with domain wall boundary conditions in its
anti-ferroelectric regime is obtained.Comment: 12 pages, 1 figur
Algebraic arctic curves in the domain-wall six-vertex model
The arctic curve, i.e. the spatial curve separating ordered (or `frozen') and
disordered (or `temperate) regions, of the six-vertex model with domain wall
boundary conditions is discussed for the root-of-unity vertex weights. In these
cases the curve is described by algebraic equations which can be worked out
explicitly from the parametric solution for this curve. Some interesting
examples are discussed in detail. The upper bound on the maximal degree of the
equation in a generic root-of-unity case is obtained.Comment: 15 pages, no figures; v2: metadata correcte
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