35,244 research outputs found
Observation of fine one-dimensionally disordered layers in silicon carbide
The improved resolution of synchrotron edge-topography is enabling thinner (less than 100 microns), silicon carbide crystals to be studied, and is providing a more detailed and wider database on polytype depth profiles. Fine long-period and one-dimensionally-disordered layers, 5-25 microns thick, can now be confidently resolved and are found to be very common features, often in association with high-defect density bands. These features are illustrated in this paper using three examples. A new long period polytype LPP (152H/456R) has been discovered and reported here for the first time
Plume mapping and isotopic characterisation of anthropogenic methane sources
Methane stable isotope analysis, coupled with mole fraction measurement, has been used to link isotopic signature to methane emissions from landfill sites, coal mines and gas leaks in the United Kingdom. A mobile Picarro G2301 CRDS (Cavity Ring-Down Spectroscopy) analyser was installed on a vehicle, together with an anemometer and GPS receiver, to measure atmospheric methane mole fractions and their relative location while driving at speeds up to 80 kph. In targeted areas, when the methane plume was intercepted, air samples were collected in Tedlar bags, for delta C-13-CH4 isotopic analysis by CF-GC-IRMS (Continuous Flow Gas Chromatography-Isotope Ratio Mass Spectrometry). This method provides high precision isotopic values, determining delta C-13-CH4 to +/- 0.05 per mil. The bulk signature of the methane plume into the atmosphere from the whole source area was obtained by Keeling plot analysis, and a delta C-13 -CH4 signature, with the relative uncertainty, allocated to each methane source investigated. Both landfill and natural gas emissions in SE England have tightly constrained isotopic signatures. The averaged delta C-13-CH4 for landfill sites is -58 +/- 3%o. The delta C-13-CH4 signature for gas leaks is also fairly constant around -36 +/- 2 parts per thousand, a value characteristic of homogenised North Sea supply. In contrast, signatures for coal mines in N. England and Wales fall in a range of -51.2 +/- 0.3 parts per thousand to 30.9 +/- 1.4 parts per thousand, but can be tightly constrained by region. The study demonstrates that CRDS-based mobile methane measurement coupled with off-line high precision isotopic analysis of plume samples is an efficient way of characterising methane sources. It shows that iiotopic measurements allow type identification, and possible location of previously unknown methane sources. In modelling studies this measurement provides an independent constraint to determine the contributions of different sources to the regional methane budget and in the verification of inventory source distribution. (C) 2015 Elsevier Ltd. All rights reserved
Charge dynamics of the spin-density-wave state in BaFeAs
We report on a thorough optical investigation of BaFeAs over a broad
spectral range and as a function of temperature, focusing our attention on its
spin-density-wave (SDW) phase transition at K. While
BaFeAs remains metallic at all temperatures, we observe a depletion in
the far infrared energy interval of the optical conductivity below ,
ascribed to the formation of a pseudogap-like feature in the excitation
spectrum. This is accompanied by the narrowing of the Drude term consistent
with the transport results and suggestive of suppression of scattering
channels in the SDW state. About 20% of the spectral weight in the far infrared
energy interval is affected by the SDW phase transition
Random Vibrational Networks and Renormalization Group
We consider the properties of vibrational dynamics on random networks, with
random masses and spring constants. The localization properties of the
eigenstates contrast greatly with the Laplacian case on these networks. We
introduce several real-space renormalization techniques which can be used to
describe this dynamics on general networks, drawing on strong disorder
techniques developed for regular lattices. The renormalization group is capable
of elucidating the localization properties, and provides, even for specific
network instances, a fast approximation technique for determining the spectra
which compares well with exact results.Comment: 4 pages, 3 figure
Models of planetary rings
The Voyager occultations provide several uniform and high quality data sets for Saturn, Uranus, and Neptune. These data are intercompared, and theoretical models for the particle sizes and the particle transport are developed. The major topics covered include: ring size distribution, torques and resonances, and satellite wakes
Eddy Current measurement of Fiber Volume Fraction in Metal Matrix Composite Extrusions
The objective of this work was to develop an eddy current method for measuring fiber volume fraction in continuous-fiber metal matrix composites. Because an eddy current measurement can be affected by the spatial distribution of fibers as well as the overall fiber density, the measurement method had to be tolerant of possible variations in spatial distribution that might be encountered in practice. For this reason, the work began with the development of models of the effective resistivity tensor for a composite with an arbitrary fiber distribution and the resulting eddy current probe response [1,2]. The intent was to use these models to help design a measurement method and to test the method for ordered and disordered arrangements of fibers
Phase Diagram for Splay Glass Superconductivity
Localization of flux lines to splayed columnar pins is studied. A sine-Gordon
type renormalization group study reveals the existence of a Splay glass phase
and yields an analytic form for the transition temperature into the glass
phase. As an independent test, the characteristics are determined via a
Molecular Dynamics code. The glass transition temperature supports the RG
results convincingly. The full phase diagram of the model is constructed.Comment: 14 pages, uuencoded compressed tar file with 3 postscript figure
Exact Solution of the Discrete (1+1)-dimensional RSOS Model in a Slit with Field and Wall Interactions
We present the solution of a linear Restricted Solid--on--Solid (RSOS) model
confined to a slit. We include a field-like energy, which equivalently weights
the area under the interface, and also include independent interaction terms
with both walls. This model can also be mapped to a lattice polymer model of
Motzkin paths in a slit interacting with both walls and including an osmotic
pressure. This work generalises previous work on the RSOS model in the
half-plane which has a solution that was shown recently to exhibit a novel
mathematical structure involving basic hypergeometric functions .
Because of the mathematical relationship between half-plane and slit this work
hence effectively explores the underlying -orthogonal polynomial structure
to that solution. It also generalises two other recent works: one on Dyck paths
weighted with an osmotic pressure in a slit and another concerning Motzkin
paths without an osmotic pressure term in a slit
Positive noise cross-correlations in superconducting hybrids: Roles of interfaces and interactions
Shot noise cross-correlations in normal metal-superconductor-normal metal
structures are discussed at arbitrary interface transparencies using both the
scattering approach of Blonder, Tinkham and Klapwik and a microscopic Green's
function approach. Surprisingly, negative crossed conductance in such set-ups
[R. Melin and D. Feinberg, Phys. Rev. B 70, 174509 (2004)] does not preclude
the possibility of positive noise cross-correlations for almost transparent
contacts. We conclude with a phenomenological discussion of interactions in the
one dimensional leads connected to the superconductor, which induce sign
changes in the noise cross-correlations.Comment: 15 pages, 9 figure
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