967 research outputs found
First Principles Calculations of Ionic Vibrational Frequencies in PbMg1/3Nb2/3O3
Lattice dynamics for several ordered supercells with composition
PbMg1/3Nb2/3O (PMN) were calculated with first-principles frozen phonon
methods. Nominal symmetries of the supercells studied are reduced by lattice
instabilities. Lattice modes corresponding to these instabilities, equilibrium
ionic positions, and infrared (IR) reflectivity spectra are reported.Comment: 6 pages; Fundamental physics of Ferroelectrics 200
Lattice dynamics of BaTiO3, PbTiO3 and PbZrO3: a comparative first-principles study
The full phonon dispersion relations of lead titanate and lead zirconate in
the cubic perovskite structure are computed using first-principles variational
density-functional perturbation theory, with ab initio pseudopotentials and a
plane-wave basis set. Comparison with the results previously obtained for
barium titanate shows that the change of a single constituent (Ba to Pb, Ti to
Zr) has profound effects on the character and dispersion of unstable modes,
with significant implications for the nature of the phase transitions and the
dielectric and piezoelectric responses of the compounds. Examination of the
interatomic force constants in real space, obtained by a transformation which
correctly treats the long-range dipolar contribution, shows that most are
strikingly similar, while it is the differences in a few key interactions which
produce the observed changes in the phonon dispersions. These trends suggest
the possibility of the transferability of force constants to predict the
lattice dynamics of perovskite solid solutions.Comment: 9 pages, 2 figures (one in colors), revised version (small changes
essentially in Sec. III
The EVERT (effective verruca treatments) trial protocol: a randomised controlled trial to evaluate cryotherapy versus salicylic acid for the treatment of verrucae
<p>Abstract</p> <p>Background</p> <p>Verrucae are a common, infectious and sometimes painful problem. The optimal treatment for verrucae is unclear due to a lack of high quality randomised controlled trials. The primary objective of this study is to compare the clinical effectiveness of two common treatments for verrucae: cryotherapy using liquid nitrogen versus salicylic acid. Secondary objectives include a comparison of the cost-effectiveness of the treatments, and an investigation of time to clearance of verrucae, recurrence/clearance of verrucae at six months, patient satisfaction with treatment, pain associated with treatment, and use of painkillers for the treatments.</p> <p>Methods/Design</p> <p>This is an open, pragmatic, multicentre, randomised controlled trial with two parallel groups: cryotherapy using liquid nitrogen delivered by a healthcare professional for a maximum of 4 treatments (treatments 2-3 weeks apart) or daily self-treatment with 50% salicylic acid for a maximum of 8 weeks. Two hundred and sixty-six patients aged 12 years and over with a verruca are being enrolled into the study. The primary outcome is complete clearance of all verrucae as observed on digital photographs taken at 12 weeks compared with baseline and assessed by an independent healthcare professional. Secondary outcomes include self-reported time to clearance of verrucae, self-reported clearance of verrucae at 6 months, cost-effectiveness of the treatments compared to one another, and patient acceptability of both treatments including possible side effects such as pain. The primary analysis will be intention to treat. It is planned that recruitment will be completed by December 2009 and results will be available by June 2010.</p> <p>Trial registration</p> <p>Current Controlled Trials ISRCTN18994246.</p
Diffusion of Point Defects in Two-Dimensional Colloidal Crystals
We report the first study of the dynamics of point defects, mono and
di-vacancies, in a confined 2-D colloidal crystal in real space and time using
digital video microscopy. The defects are introduced by manipulating individual
particles with optical tweezers. The diffusion rates are measured to be
Hz for mono-vacancies and
Hz for di-vacancies. The elementary diffusion
processes are identified and it is found that the diffusion of di-vacancies is
enhanced by a \textit{dislocation dissociation-recombination} mechanism.
Furthermore, the defects do not follow a simple random walk but their hopping
exhibits memory effects, due to the reduced symmetry (compared to the
triangular lattice) of their stable configurations, and the slow relaxation
rates of the lattice modes.Comment: 6 pages (REVTEX), 5 figures (PS
Clusters, phason elasticity, and entropic stabilisation: a theory perspective
Personal comments are made about the title subjects, including: the relation
of Friedel oscillations to Hume-Rothery stabilisation; how calculations may
resolve the random-tiling versus ideal pictures of quasicrystals; and the role
of entropies apart from tile-configurational.Comment: IOP macros; 8pp, 1 figure. In press, Phil. Mag. A (Proc. Intl. Conf.
on Quasicrystals 9, Ames Iowa, May 2005
Building effective models from sparse but precise data
A common approach in computational science is to use a set of of highly
precise but expensive calculations to parameterize a model that allows less
precise, but more rapid calculations on larger scale systems. Least-squares
fitting on a model that underfits the data is generally used for this purpose.
For arbitrarily precise data free from statistic noise, e.g. ab initio
calculations, we argue that it is more appropriate to begin with a ensemble of
models that overfit the data. Within a Bayesian framework, a most likely model
can be defined that incorporates physical knowledge, provides error estimates
for systems not included in the fit, and reproduces the original data exactly.
We apply this approach to obtain a cluster expansion model for the Ca[Zr,Ti]O3
solid solution.Comment: 10 pages, 3 figures, submitted to Physical Review Letter
Exact Solution of an Octagonal Random Tiling Model
We consider the two-dimensional random tiling model introduced by Cockayne,
i.e. the ensemble of all possible coverings of the plane without gaps or
overlaps with squares and various hexagons. At the appropriate relative
densities the correlations have eight-fold rotational symmetry. We reformulate
the model in terms of a random tiling ensemble with identical rectangles and
isosceles triangles. The partition function of this model can be calculated by
diagonalizing a transfer matrix using the Bethe Ansatz (BA). The BA equations
can be solved providing {\em exact} values of the entropy and elastic
constants.Comment: 4 pages,3 Postscript figures, uses revte
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