5,039 research outputs found
Single-cycle THz pulses with amplitudes exceeding 1 MV/cm generated by optical rectification in LiNbO3
Using the tilted-pulse-intensity-front scheme, we generate single-cycle
terahertz (THz) pulses by optical rectification of femtosecond laser pulses in
LiNbO3. In the THz generation setup, the condition that the image of the
grating coincides with the tilted-optical-pulse front is fulfilled to obtain
optimal THz beam characteristics and pump-to-THz conversion efficiency. The
designed focusing geometry enables tight focus of the collimated THz beam with
a spot size close to the diffraction limit, and the maximum THz electric field
of 1.2 MV/cm is obtained
Directional motion of forced polymer chains with hydrodynamic interaction
We study the propulsion of a one-dimensional (1D) polymer chain under
sinusoidal external forces in the overdamped (low Reynolds number) regime. We
show that, when hydrodynamical interactions are included, the polymer presents
directional motion which depends on the phase differences of the external force
applied along the chain. Moreover, the velocity shows a maximum as a function
of the frequency. We discuss the relevance of all these results in light of
recent nanotechnology experiments.Comment: 5 pages, 6 figure
Nonlinear waves in a model for silicate layers
Some layered silicates are composed of positive ions, surrounded by layers of ions with opposite sign. Mica muscovite is a particularly interesting material, because there exist fossil and experimental evidence for nonlinear excitations transporting localized energy and charge along the cation rows within the potassium layers. This evidence suggest that there are different kinds of excitations with different energies and properties. Some of the authors proposed recently a one-dimensional model based in physical principles and the silicate structure. The main characteristic of the model is that it has a hard substrate potential and two different repulsion terms, between ions and nuclei. In a previous work with this model, it was found the propagation of crowdions, i.e., lattice kinks in a lattice with substrate potential that transport mass and charge. They have a single specific velocity and energy coherent with the experimental data. In the present work we perform a much more thorough search for nonlinear excitations in the same model using the pseudospectral method to obtain exact nanopteron solutions, which are single kinks with tails, crowdions and bi-crowdions. We analyze their velocities, energies and stability or instability and the possible reasons for the latter. We relate the different excitations with their possible origin from recoils from different beta decays and with the fossil tracks. We explore the consequences of some variation of the physical parameters because their values are not perfectly known. Through a different method, we also have found stationary and moving breathers, that is, localized nonlinear excitations with an internal vibration. Moving breathers have small amplitude and energy, which is also coherent with the fossil evidence.MINECO (Spain) FIS2015-65998-C2-2-PJunta de Andalucía 2017/FQM-280Universidad de Sevilla (España) grants VI PPIT-US-201
Cation Transport in Polymer Electrolytes: A Microscopic Approach
A microscopic theory for cation diffusion in polymer electrolytes is
presented. Based on a thorough analysis of molecular dynamics simulations on
PEO with LiBF the mechanisms of cation dynamics are characterised. Cation
jumps between polymer chains can be identified as renewal processes. This
allows us to obtain an explicit expression for the lithium ion diffusion
constant D_{Li} by invoking polymer specific properties such as the Rouse
dynamics. This extends previous phenomenological and numerical approaches. In
particular, the chain length dependence of D_{Li} can be predicted and compared
with experimental data. This dependence can be fully understood without
referring to entanglement effects.Comment: 4 pages, 4 figures, Physical Review Letters in pres
Viscoelastic Effect on Hydrodynamic Relaxation in Polymer Solutions
The viscoelastic effect on the hydrodynamic relaxation in semidilute polymer
solutions is investigated. From the linearized two-fluid model equations, we
predict that the dynamical asymmetry coupling between the velocity fluctuations
and the viscoelastic stress influences on the hydrodynamic relaxation process,
resulting in a wave-number-dependent shear viscosity.Comment: 7pages; To be published in Journal of the Physical Society of
Japan,Vol 72,No2,(2003
Self-consistent variational theory for globules
A self-consistent variational theory for globules based on the uniform
expansion method is presented. This method, first introduced by Edwards and
Singh to estimate the size of a self-avoiding chain, is restricted to a good
solvent regime, where two-body repulsion leads to chain swelling. We extend the
variational method to a poor solvent regime where the balance between the
two-body attractive and the three-body repulsive interactions leads to
contraction of the chain to form a globule. By employing the Ginzburg
criterion, we recover the correct scaling for the -temperature. The
introduction of the three-body interaction term in the variational scheme
recovers the correct scaling for the two important length scales in the globule
- its overall size , and the thermal blob size . Since these two
length scales follow very different statistics - Gaussian on length scales
, and space filling on length scale - our approach extends the
validity of the uniform expansion method to non-uniform contraction rendering
it applicable to polymeric systems with attractive interactions. We present one
such application by studying the Rayleigh instability of polyelectrolyte
globules in poor solvents. At a critical fraction of charged monomers, ,
along the chain backbone, we observe a clear indication of a first-order
transition from a globular state at small , to a stretched state at large
; in the intermediate regime the bistable equilibrium between these two
states shows the existence of a pearl-necklace structure.Comment: 7 pages, 1 figur
Weak Magnetism in Two Neutrino Double Beta Decay
We have extended the formalism for the two-neutrino double beta decay by
including the weak-magnetism term, as well as other second-forbidden
corrections. The weak magnetism diminishes the calculated half-lives in , independently of the nuclear structure. Numerical computations were
performed within the pn-QRPA, for , , ,
and nuclei. No one of the second-forbidden corrections modifies
significantly the spectrum shapes. The total reduction in the calculated half
lives varies from 6% up to 32%, and strongly depend on the nuclear interaction
in the particle-particle channel. We conclude that the higher order
effects in the weak Hamiltonian would hardly be observed in the two-neutrino
double beta experiments.Comment: 8 pages, latex, 1ps figure, to appear in Phys. Lett.
Scoping a public health impact assessment of aquaculture with particular reference to tilapia in the UK
Background. The paper explores shaping public health impact assessment tools for tilapia, a novel emergent aquaculture sector in the UK. This Research Council’s UK Rural Economy and Land Use project embraces technical, public health, and marketing perspectives scoping tools to assess possible impacts of the activity. Globally, aquaculture produced over 65 million tonnes of food in 2008 and will grow significantly requiring apposite global public health impact assessment tools.<p></p>
Methods. Quantitative and qualitative methods incorporated data from a tridisciplinary literature. Holistic tools scoped tilapia farming impact assessments. Laboratory-based tilapia production generated data on impacts in UK and Thailand along with 11 UK focus groups involving 90 consumers, 30 interviews and site visits, 9 visits to UK tilapia growers and 2 in The Netherlands.<p></p>
Results. The feasibility, challenges, strengths, and weaknesses of creating a tilapia Public Health Impact Assessment are analysed. Occupational and environmental health benefits and risks attached to tilapia production were identified.<p></p>
Conclusions. Scoping public health impacts of tilapia production is possible at different levels and forms for producers, retailers, consumers, civil society and governmental bodies that may contribute to complex and interrelated public health assessments of aquaculture projects. Our assessment framework constitutes an innovatory perspective in the field
Sudden collapse of a colloidal gel
Metastable gels formed by weakly attractive colloidal particles display a
distinctive two-stage time-dependent settling behavior under their own weight.
Initially a space-spanning network is formed that for a characteristic time,
which we define as the lag time \taud, resists compaction. This solid-like
behavior persists only for a limited time. Gels whose age \tw is greater than
\taud yield and suddenly collapse. We use a combination of confocal
microscopy, rheology and time-lapse video imaging to investigate both the
process of sudden collapse and its microscopic origin in an refractive-index
matched emulsion-polymer system. We show that the height of the gel in the
early stages of collapse is well described by the surprisingly simple
expression, h(\ts) = \h0 - A \ts^{3/2}, with \h0 the initial height and
\ts = \tw-\taud the time counted from the instant where the gel first yields.
We propose that this unexpected result arises because the colloidal network
progressively builds up internal stress as a consequence of localized
rearrangement events which leads ultimately to collapse as thermal equilibrium
is re-established.Comment: 14 pages, 11 figures, final versio
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