759 research outputs found
Relativistic Attosecond Electron Bunches from Laser-Illuminated Droplets
The generation of relativistic attosecond electron bunches is observed in
three-dimensional, relativistic particle-in-cell simulations of the interaction
of intense laser light with droplets. The electron bunches are emitted under
certain angles which depend on the ratios of droplet radius to wavelength and
plasma frequency to laser frequency. The mechanism behind the multi-MeV
attosecond electron bunch generation is investigated using Mie theory. It is
shown that the angular distribution and the high electron energies are due to a
parameter-sensitive, time-dependent local field enhancement at the droplet
surface.Comment: 4 pages, 5 figures, REVTeX-styl
Two center multipole expansion method: application to macromolecular systems
We propose a new theoretical method for the calculation of the interaction
energy between macromolecular systems at large distances. The method provides a
linear scaling of the computing time with the system size and is considered as
an alternative to the well known fast multipole method. Its efficiency,
accuracy and applicability to macromolecular systems is analyzed and discussed
in detail.Comment: 23 pages, 7 figures, 1 tabl
Selective decay by Casimir dissipation in fluids
The problem of parameterizing the interactions of larger scales and smaller
scales in fluid flows is addressed by considering a property of two-dimensional
incompressible turbulence. The property we consider is selective decay, in
which a Casimir of the ideal formulation (enstrophy in 2D flows, helicity in 3D
flows) decays in time, while the energy stays essentially constant. This paper
introduces a mechanism that produces selective decay by enforcing Casimir
dissipation in fluid dynamics. This mechanism turns out to be related in
certain cases to the numerical method of anticipated vorticity discussed in
\cite{SaBa1981,SaBa1985}. Several examples are given and a general theory of
selective decay is developed that uses the Lie-Poisson structure of the ideal
theory. A scale-selection operator allows the resulting modifications of the
fluid motion equations to be interpreted in several examples as parameterizing
the nonlinear, dynamical interactions between disparate scales. The type of
modified fluid equation systems derived here may be useful in modelling
turbulent geophysical flows where it is computationally prohibitive to rely on
the slower, indirect effects of a realistic viscosity, such as in large-scale,
coherent, oceanic flows interacting with much smaller eddies
Formation and Evolution of Singularities in Anisotropic Geometric Continua
Evolutionary PDEs for geometric order parameters that admit propagating
singular solutions are introduced and discussed. These singular solutions arise
as a result of the competition between nonlinear and nonlocal processes in
various familiar vector spaces. Several examples are given. The motivating
example is the directed self assembly of a large number of particles for
technological purposes such as nano-science processes, in which the particle
interactions are anisotropic. This application leads to the derivation and
analysis of gradient flow equations on Lie algebras. The Riemann structure of
these gradient flow equations is also discussed.Comment: 38 pages, 4 figures. Physica D, submitte
Coherent Excitation of a Two-Level Atom driven by a far off-resonant Classical Field: Analytical Solutions
We present an analytical treatment of coherent excitation of a Two-Level Atom
driven by a far-off resonant classical field. A class of pulse envelope is
obtained for which this problem is exactly solvable. The solutions are given in
terms of Heun function which is a generalization of the Hypergeometric
function. The degeneracy of Heun to Hypergeometric equation can give all the
exactly solvable pulse shapes of Gauss Hypergeometric form, from the
generalized pulse shape obtained here. We discuss the application of the
results obtained to the generation of XUV.Comment: 9 Pages, 8 Figures. Accepted for Physical Review A as a regular
articl
Semiclassical Particle Spectrum of Double Sine-Gordon Model
We present new theoretical results on the spectrum of the quantum field
theory of the Double Sine Gordon model. This non-integrable model displays
different varieties of kink excitations and bound states thereof. Their mass
can be obtained by using a semiclassical expression of the matrix elements of
the local fields. In certain regions of the coupling-constants space the
semiclassical method provides a picture which is complementary to the one of
the Form Factor Perturbation Theory, since the two techniques give information
about the mass of different types of excitations. In other regions the two
methods are comparable, since they describe the same kind of particles.
Furthermore, the semiclassical picture is particularly suited to describe the
phenomenon of false vacuum decay, and it also accounts in a natural way the
presence of resonance states and the occurrence of a phase transition.Comment: 32 pages, latex, 8 figure
Speech rhythm: a metaphor?
Is speech rhythmic? In the absence of evidence for a traditional view that languages strive to coordinate either syllables or stress-feet with regular time intervals, we consider the alternative that languages exhibit contrastive rhythm subsisting merely in the alternation of stronger and weaker elements. This is initially plausible, particularly for languages with a steep âprominence gradientâ, i.e. a large disparity between stronger and weaker elements; but we point out that alternation is poorly achieved even by a âstress-timedâ language such as English, and, historically, languages have conspicuously failed to adopt simple phonological remedies that would ensure alternation. Languages seem more concerned to allow âsyntagmatic contrastâ between successive units and to use durational effects to support linguistic functions than to facilitate rhythm. Furthermore, some languages (e.g. Tamil, Korean) lack the lexical prominence which would most straightforwardly underpin prominence alternation. We conclude that speech is not incontestibly rhythmic, and may even be antirhythmic. However, its linguistic structure and patterning allow the metaphorical extension of rhythm in varying degrees and in different ways depending on the language, and that it is this analogical process which allows speech to be matched to external rhythms
Defining the molecular signatures of Achilles tendinopathy and anterior cruciate ligament ruptures: A whole-exome sequencing approach
Musculoskeletal soft tissue injuries are complex phenotypes with genetics being one of
many proposed risk factors. Case-control association studies using the candidate gene
approach have predominately been used to identify risk loci for these injuries. However, the
ability to identify all risk conferring variants using this approach alone is unlikely. Therefore,
this study aimed to further define the genetic profile of these injuries using an integrated
omics approach involving whole exome sequencing and a customised analyses pipeline.
The exomes of ten exemplar asymptomatic controls and ten exemplar cases with Achilles
tendinopathy were individually sequenced using a platform that included the coverage of
the untranslated regions and miRBase miRNA genes. Approximately 200 000 variants were
identified in the sequenced samples. Previous research was used to guide a targeted analysis
of the genes encoding the tenascin-C (TNC) glycoprotein and the α1 chain of type XXVII
collagen (COL27A1) located on chromosome 9. Selection of variants within these genes
were; however, not predetermined but based on a tiered filtering strategy. Four variants in
TNC (rs1061494, rs1138545, rs2104772 and rs1061495) and three variants in the upstream
COL27A1 gene (rs2567706, rs2241671 and rs2567705) were genotyped in larger Achilles
tendinopathy and anterior cruciate ligament (ACL) rupture sample groups. The CC genotype
of TNC rs1061494 (C/T) was associated with the risk of Achilles tendinopathy (p = 0.018,
OR: 2.5 95% CI: 1.2â5.1). Furthermore, the AA genotype of the TNC rs2104772 (A/T) variant
was significantly associated with ACL ruptures in the female subgroup (p = 0.035, OR:
2.3 95% CI: 1.1â5.5). An inferred haplotype in the TNC gene was also associated with the
risk of Achilles tendinopathy. These results provide a proof of concept for the use of a customised
pipeline for the exploration of a larger genomic dataset. This approach, using previous
research to guide a targeted analysis of the data has generated new genetic signatures
in the biology of musculoskeletal soft tissue injuries.IS
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Livelihood diversification and the expansion of artisanal mining in rural Tanzania: drivers and policy implications
This paper provides an extended analysis of livelihood diversification in rural Tanzania, with special emphasis on artisanal and small-scale mining (ASM). Over the past decade, this sector of industry, which is labour-intensive and comprises an array of rudimentary and semi-mechanized operations, has become an indispensable economic activity throughout Sub-Saharan Africa, providing employment to a host of redundant public sector workers, retrenched large-scale mine labourers and poor farmers. In many of the regionâs rural areas, it is overtaking subsistence agriculture as the primary industry. Such a pattern appears
to be unfolding within the Morogoro and Mbeya regions of southern Tanzania, where findings from recent research suggest that a growing number of smallholder farmers are turning to ASM for employment and financial support. It is imperative that national rural development programmes take this trend into account and provide support to these people
B\"acklund transformations for the second Painlev\'e hierarchy: a modified truncation approach
The second Painlev\'e hierarchy is defined as the hierarchy of ordinary
differential equations obtained by similarity reduction from the modified
Korteweg-de Vries hierarchy. Its first member is the well-known second
Painlev\'e equation, P2.
In this paper we use this hierarchy in order to illustrate our application of
the truncation procedure in Painlev\'e analysis to ordinary differential
equations. We extend these techniques in order to derive auto-B\"acklund
transformations for the second Painlev\'e hierarchy. We also derive a number of
other B\"acklund transformations, including a B\"acklund transformation onto a
hierarchy of P34 equations, and a little known B\"acklund transformation for P2
itself.
We then use our results on B\"acklund transformations to obtain, for each
member of the P2 hierarchy, a sequence of special integrals.Comment: 12 pages in LaTeX 2.09 (uses ioplppt.sty), to appear in Inverse
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