5,875 research outputs found
Recent Results on the Decay of Metastable Phases
We review some aspects of current knowledge regarding the decay of metastable
phases in many-particle systems. In particular we emphasize recent theoretical
and computational developments and numerical results regarding homogeneous
nucleation and growth in kinetic Ising and lattice-gas models. An introductory
discussion of the droplet theory of homogeneous nucleation is followed by a
discussion of Monte Carlo and transfer-matrix methods commonly used for
numerical study of metastable decay, including some new algorithms. Next we
discuss specific classes of systems. These include a brief discussion of recent
progress for fluids, and more exhaustive considerations of ferromagnetic Ising
models ({\it i.e.}, attractive lattice-gas models) with weak long-range
interactions and with short-range interactions. Whereas weak-long-range-force
(WLRF) models have infinitely long-lived metastable phases in the
infinite-range limit, metastable phases in short-range-force (SRF) models
eventually decay, albeit extremely slowly. Recent results on the finite-size
scaling of metastable lifetimes in SRF models are reviewed, and it is pointed
out that such effects may be experimentally observable.Comment: 34 pages, LaTex, 8 ps figs. on request, preprint FSU-SCRI-94-6
Strain-Modified RKKY Interaction in Carbon Nanotubes
For low-dimensional metallic structures, such as nanotubes, the exchange
coupling between localized magnetic dopants is predicted to decay slowly with
separation. The long-range character of this interaction plays a significant
role in determining the magnetic order of the system. It has previously been
shown that the interaction range depends on the conformation of the magnetic
dopants in both graphene and nanotubes. Here we examine the RKKY interaction in
carbon nanotubes in the presence of uniaxial strain for a range of different
impurity configurations. We show that strain is capable of amplifying or
attenuating the RKKY interaction, significantly increasing certain interaction
ranges, and acting as a switch: effectively turning on or off the interaction.
We argue that uniaxial strain can be employed to significantly manipulate
magnetic interactions in carbon nanotubes, allowing an interplay between
mechanical and magnetic properties in future spintronic devices. We also
examine the dimensional relationship between graphene and nanotubes with
regards to the decay rate of the RKKY interaction.Comment: 7 pages, 6 figures, submitte
Education pathways to the labour market for 16-year-olds who struggle to achieve maths and English in General Certificate of Secondary Education
We examine the post age-16 educational pathways taken by the 44% of young people who do not gain âgoodâ grades in English and Maths at age 16 years. We then assess the causal effects of attending General Further Education (GFE) colleges on education and labour market outcomes for this group. We use the Longitudinal Education Outcomes dataset, which comprises linked administrative education, employment and income records for the population of English school pupils aged 16 in 2011. To summarise complex post-16 education trajectories, we present Sankey charts stratified by indicators of disadvantage. We study the effects of attending GFE at age 17 on whether a pupil gains a Level 3 qualification by age 19, and their earnings and employment status at age 24. To estimate a causal impact, we use distance from home to the closest GFE college as an instrumental variable, controlling for a rich set of background characteristics. Our graphical results highlight the complexity of post-16 educational pathways and transitions, which are differentiated by disadvantage. Over 50% have GFE as their first post-16 destination. Results from instrumental variable analyses show a positive association between attending GFE and gaining a Level 3 qualification by age 19, among pupils who do not gain a âgoodâ pass in the General Certificate of Secondary Education (GCSE) in either English and/or Maths. Restricting analyses to the bottom of the distribution â those who gain an E, F or G grade in both English and Maths - we do not detect an impact of GFE on qualifications at age 19. Among both subgroups, we do not detect any impact of attending GFE on earnings and employment at age 24 years. While the post-16 pathways taken by disadvantaged, lower-attaining pupils do increase qualification attainment for some, the value these have in the labour market appears limited. These results may indicate the importance of âsoft-skillsâ and early employment experiences for this subgroup of lower-attainers
RKKY interaction between extended magnetic defect lines in graphene
Of fundamental interest in the field of spintronics is the mechanism of
indirect exchange coupling between magnetic impurities embedded in metallic
hosts. A range of physical features, such as magnetotransport and overall
magnetic moment formation, are predicated upon this magnetic coupling, often
referred to as the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. Recent
theoretical studies on the RKKY in graphene have been motivated by possible
spintronic applications of magnetically doped graphene systems. In this work a
combination of analytic and numerical techniques are used to examine the
effects of defect dimensionality on such an interaction. We show, in a
mathematically transparent manner, that moving from single magnetic impurities
to extended lines of impurities effectively reduces the dimensionality of the
system and increases the range of the interaction. This has important
consequences for the spintronic application of magnetically-doped and we
illustrate this with a simple magnetoresistance device.Comment: 6 pages, 5 figure
Orbital assembly and maintenance study. Executive summary
A sound, practical approach for the assembly and maintenance of very large structures in space is presented. The methods and approaches for assembling two large structures are examined. The maintenance objectives include the investigation of methods to maintain five geosynchronous satellites. The two assembly examples are a 200-meter-diameter radio astronomy telescope and a 1,000-meter-diameter microwave power transmission system. The radio astronomy telescope operates at an 8,000-mile altitude and receives RF signals from space. The microwave power transmission system is part of a solar power satellite that will be used to transmit converted solar energy to microwave ground receivers. Illustrations are included
Inhibition by Anandamide of 6-Hydroxydopamine-Induced Cell Death in PC12 Cells
6-hydroxydopamine (6-OHDA) is a selective neurotoxin that is widely used to investigate cell death and protective strategies in models of Parkinson's disease. Here, we investigated the effects of the endogenous cannabinoid, anandamide, on 6-OHDA-induced toxicity in rat adrenal phaeochromocytoma PC12 cells. Morphological analysis and caspase-3 activity assay revealed that anandamide inhibited 6-OHDA-induced apoptosis. The protection was not affected by antagonists of either cannabinoid receptors (CB1 or CB2) or the vanilloid receptor TRPV1. Anandamide-dependent protection was reduced by pretreatment with LY294002 (inhibitor of phosphatidylinositol 3-kinase, PI3K) and unaffected by U0126 (inhibitor of extracellularly-regulated kinase). Interestingly, phosphorylation of c-Jun-NH2-terminal kinase (JNK) in cells exposed to 6-OHDA was strongly reduced by anandamide pre-treatment. Furthermore, 6-OHDA induced c-Jun activation and increased Bim expression, both of which were inhibited by anandamide. Together, these data demonstrate antiapoptotic effects of anandamide and also suggest a role for activation of PI3K and inhibition of JNK signalling in anandamide-mediated protection against 6-OHDA
Strain-induced modulation of magnetic interactions in graphene
The ease with which the physical properties of graphene can be tuned suggests
a wide range of possible applications. Recently, strain engineering of these
properties has been of particular interest. Possible spintronic applications of
magnetically doped graphene systems have motivated recent theoretical
investigations of the so-called Ruderman-Kittel-Kasuya-Yosida (RKKY)
interaction between localized moments in graphene. In this work a combination
of analytic and numerical techniques are used to examine the effects of
uniaxial strain on such an interaction. A range of interesting features are
uncovered depending on the separation and strain directions. Amplification,
suppression, and oscillatory behavior are reported as a function of the strain
and mathematically transparent expressions predicting these features are
derived. Since a wide range of effects, including overall moment formation and
magnetotransport response, are underpinned by such interactions we predict that
the ability to manipulate the coupling by applying strain may lead to
interesting spintronic applications.Comment: 6 pages, 3 figure
Near-field optical spectroscopy and microscopy of self-assembled GaNâAlN nanostructures
The spatial distribution and emission properties of small clusters of GaNquantum dots in an AlN matrix are studied using high-resolution electron and optical microscopy. High-resolution transmission electron microscopy reveals near vertical correlation among the GaNdots due to a sufficiently thin AlN spacer layer thickness, which allows strain induced stacking. Scanning electron and atomic force microscopy show lateral coupling due to a surface roughness of âź50â60nm. Near-field photoluminescence in the illumination mode (both spatially and spectrally resolved) at 10K revealed emission from individual dots, which exhibits size distribution of GaNdots from localized sites in the stacked nanostructure. Strong spatial localization of the excitons is observed in GaNquantum dots formed at the tip of self-assembled hexagonal pyramid shapes with six [101ÂŻ1ÂŻ] facets
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