10,522 research outputs found

    Thermopower of gapped bilayer graphene

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    We calculate thermopower of clean and impure bilayer graphene systems. Opening a band gap through the application of an external electric field is shown to greatly enhance the thermopower of bilayer graphene, which is more than four times that of the monolayer graphene and gapless bilayer graphene at room temperature. The effect of scattering by dilute charged impurities is discussed in terms of the self-consistent Born approximation. Temperature dependence of the thermopower is also analyzed.Comment: 8 pages, 5 figures; An inconsistency in the definitions of Eq.(17) and (18) in version 1 is found and correcte

    Gallium Arsenide preparation and QE Lifetime Studies using the ALICE Photocathode Preparation Facility

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    In recent years, Gallium Arsenide (GaAs) type photocathodes have become widely used as electron sources in modern Energy Recovery Linac based light sources such as the Accelerators and Lasers in Combined Experiments (ALICE) at Daresbury Laboratory and as polarised electron source for the proposed International Linear Collider (ILC). Once activated to a Low Electron Affinity (LEA) state and illuminated by a laser, these materials can be used as a high-brightness source of both polarised and un-polarised electrons. This paper presents an effective multi-stage preparation procedure including heat cleaning, atomic hydrogen cleaning and the activation process for a GaAs photocathode. The stability of quantum efficiency (QE) and lifetime of activated to LEA state GaAs photocathode have been studied in the ALICE load-lock photocathode preparation facility which has a base pressure in the order of 10^-11 mbar. These studies are supported by further experimental evidence from surface science techniques such as X-ray Photoelectron Spectroscopy (XPS) to demonstrate the processes at the atomic level.Comment: Presented at First International Particle Accelerator Conference, IPAC'10, Kyoto, Japan, from 23 to 28 May 201

    A newly discovered population of the critically endangered false limpet Siphonaria compressa Allanson, 1958 (Pulmonata: Siphonariidae), with observations on its reproductive biology

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    The endangered false limpet, Siphonaria compressa, originally described by Allanson in 1958 from Langebaan Lagoon, Saldanha, has been found living in intertidal eelgrass meadows in the embayment of the Knysna River estuary at Bollard Bay, Leisure Isle, Knysna, South Africa. This represents only the second known site of occurrence of the species. Although classified by the IUCN as critically endangered, the Knysna population is viable.Acrobat Distiller 6.0 (Windows

    Embryonic Pattern Scaling Achieved by Oppositely Directed Morphogen Gradients

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    Morphogens are proteins, often produced in a localised region, whose concentrations spatially demarcate regions of differing gene expression in developing embryos. The boundaries of expression must be set accurately and in proportion to the size of the one-dimensional developing field; this cannot be accomplished by a single gradient. Here, we show how a pair of morphogens produced at opposite ends of a developing field can solve the pattern-scaling problem. In the most promising scenario, the morphogens effectively interact according to the annihilation reaction A+BA+B\to\emptyset and the switch occurs according to the absolute concentration of AA or BB. In this case embryonic markers across the entire developing field scale approximately with system size; this cannot be achieved with a pair of non-interacting gradients that combinatorially regulate downstream genes. This scaling occurs in a window of developing-field sizes centred at a few times the morphogen decay length.Comment: 24 pages; 11 figures; uses iopar

    An alternative derivation of the gravitomagnetic clock effect

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    The possibility of detecting the gravitomagnetic clock effect using artificial Earth satellites provides the incentive to develop a more intuitive approach to its derivation. We first consider two test electric charges moving on the same circular orbit but in opposite directions in orthogonal electric and magnetic fields and show that the particles take different times in describing a full orbit. The expression for the time difference is completely analogous to that of the general relativistic gravitomagnetic clock effect in the weak-field and slow-motion approximation. The latter is obtained by considering the gravitomagnetic force as a small classical non-central perturbation of the main central Newtonian monopole force. A general expression for the clock effect is given for a spherical orbit with an arbitrary inclination angle. This formula differs from the result of the general relativistic calculations by terms of order c^{-4}.Comment: LaTex2e, 11 pages, 1 figure, IOP macros. Submitted to Classical and Quantum Gravit

    Mid-Infrared Ethane Emission on Neptune and Uranus

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    We report 8- to 13-micron spectral observations of Neptune and Uranus from the NASA Infrared Telescope Facility spanning more than a decade. The spectroscopic data indicate a steady increase in Neptune's mean atmospheric 12-micron ethane emission from 1985 to 2003, followed by a slight decrease in 2004. The simplest explanation for the intensity variation is an increase in stratospheric effective temperature from 155 +/- 3 K in 1985 to 176 +/- 3 K in 2003 (an average rate of 1.2 K/year), and subsequent decrease to 165 +/- 3 K in 2004. We also detected variation of the overall spectral structure of the ethane band, specifically an apparent absorption structure in the central portion of the band; this structure arises from coarse spectral sampling coupled with a non-uniform response function within the detector elements. We also report a probable direct detection of ethane emission on Uranus. The deduced peak mole fraction is approximately an order of magnitude higher than previous upper limits for Uranus. The model fit suggests an effective temperature of 114 +/- 3 K for the globally-averaged stratosphere of Uranus, which is consistent with recent measurements indicative of seasonal variation.Comment: Accepted for publication in ApJ. 16 pages, 10 figures, 2 table

    Measurement and simulation of anisotropic magnetoresistance in single GaAs/MnAs core/shell nanowires

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    We report four probe measurements of the low field magnetoresistance in single core/shell GaAs/MnAs nanowires synthesized by molecular beam epitaxy, demonstrating clear signatures of anisotropic magnetoresistance that track the field-dependent magnetization. A comparison with micromagnetic simulations reveals that the principal characteristics of the magnetoresistance data can be unambiguously attributed to the nanowire segments with a zinc blende GaAs core. The direct correlation between magnetoresistance, magnetization and crystal structure provides a powerful means of characterizing individual hybrid ferromagnet/semiconductor nanostructures.Comment: Submitted to Applied Physics Letters; some typos corrected and a defective figure replace

    A universal constraint between charge and rotation rate for degenerate black holes surrounded by matter

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    We consider stationary, axially and equatorially symmetric systems consisting of a central rotating and charged degenerate black hole and surrounding matter. We show that a2+Q2=M2a^2+Q^2=M^2 always holds provided that a continuous sequence of spacetimes can be identified, leading from the Kerr-Newman solution in electrovacuum to the solution in question. The quantity a=J/Ma=J/M is the black hole's intrinsic angular momentum per unit mass, QQ its electric charge and MM the well known black hole mass parameter introduced by Christodoulou and Ruffini.Comment: 19 pages, 2 figures, replaced with published versio
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