905 research outputs found
Ferroelectric Phase Transitions from First Principles
An effective Hamiltonian for the ferroelectric transition in is
constructed from first-principles density-functional-theory total-energy and
linear-response calculations through the use of a localized, symmetrized basis
set of ``lattice Wannier functions.'' Preliminary results of Monte Carlo
simulations for this system show a first-order cubic-tetragonal transition at
660 K. The involvement of the Pb atom in the lattice instability and the
coupling of local distortions to strain are found to be particularly important
in producing the behavior characteristic of the transition. A
tentative explanation for the presence of local distortions experimentally
observed above is suggested. Further applications of this method to a
variety of systems and structures are proposed for first-principles study of
finite-temperature structural properties in individual materials.Comment: 14 pages, harvmac, 4 uuencoded figure
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
Magnetic-induced phonon anisotropy in ZnCrO from first principles
We have studied the influence of magnetic order on the optical phonons of the
geometrically frustrated spinel ZnCrO from first-principles. By mapping
the first-principles phonon calculations onto a Heisenberg-like model, we
developed a method to calculate exchange derivatives and subsequently the
spin-phonon couping parameter from first-principles. All calculations were
performed within LSDA+U
Ab initio statistical mechanics of the ferroelectric phase transition in PbTiO<SUB>3</SUB>
An effective Hamiltonian for the ferroelectric transition in PbTiO3 is constructed from ab initio pseudopotential local-density-functional total-energy and linear-response calculations through the use of a localized, symmetrized basis set of "lattice Wannier functions''. Explicit parametrization of the polar lattice Wannier functions is used for subspace projection, addressing the issues of LO-TO splitting and coupling to the complementary subspace. In contrast with ferroelectric BaTiO3 and KNbO3, we find significant involvement of the Pb atom in the lattice instability. Monte Carlo simulations for this Hamiltonian show a first-order cubic-tetragonal transition at 660 K. The resulting temperature dependence of spontaneous polarization, c/a ratio, and unit-cell volume near the transition are in good agreement with experiment. Comparison of Monte Carlo results with mean-field theory analysis shows that both strain and fluctuations are necessary to produce the first-order character of this transition
Comparison of the lossy image data compressions for the MESUR Pathfinder and for the Huygens Titan Probe
The commercial JPEG standard complies well with the specific requirements of exploratory space missions. Therefore, JPEG has been chosen to be the baseline for a series of spaceborne image data compressions (e.g. MARS94-HRSC, -WAOSS, HUYGENS-DISR, MESUR-IMP). One S/W-implementation (IMP) and one H/W-implementation (DISR) of image data compression are presented. Details of the modifications applied to standard JPEG are outlined. Finally, a performance comparison of the two implementations is given
Materials Characterization Using High-Frequency Atomic Force Microscopy and Friction Force Microscopy
During the last decade, Atomic Force Microscopy (AFM) has been widely used to image the topography of various surfaces with corrugations down to the atomic scale [1,2]. Since then, development of new techniques based on AFM has been conducted to evaluate physical, chemical or mechanical surface properties [3]. We describe the use of near-field acoustic microscopy, based on AFM and hereafter referred to as Acoustic Microscopy by Atomic Force Microscopy (AFAM), as it has been developed earlier [4]. The relevance of this new scanning probe microscopy for high-resolution nondestructive testing and evaluation purposes is pointed out. It is shown that AFAM is capable of measuring elasticity on surfaces with a spatial resolution of less than 100 nm. Subsurface elastic properties and subsurface microdefect characterization can be performed by this technique. The high frequency Friction Force Microscopy (FFM) image, hereafter called Acoustic Friction Force Micropscopy (AFFM), reveals information different from the conventionally taken friction force image. We describe experimental and theoretical aspects of high-frequency atomic force and friction force microscopy
Dynamic Behavior in Piezoresponse Force Microscopy
Frequency dependent dynamic behavior in Piezoresponse Force Microscopy (PFM)
implemented on a beam-deflection atomic force microscope (AFM) is analyzed
using a combination of modeling and experimental measurements. The PFM signal
comprises contributions from local electrostatic forces acting on the tip,
distributed forces acting on the cantilever, and three components of the
electromechanical response vector. These interactions result in the bending and
torsion of the cantilever, detected as vertical and lateral PFM signals. The
relative magnitudes of these contributions depend on geometric parameters of
the system, the stiffness and frictional forces of tip-surface junction, and
operation frequencies. The dynamic signal formation mechanism in PFM is
analyzed and conditions for optimal PFM imaging are formulated. The
experimental approach for probing cantilever dynamics using frequency-bias
spectroscopy and deconvolution of electromechanical and electrostatic contrast
is implemented.Comment: 65 pages, 15 figures, high quality version available upon reques
Applications of Radio-Frequency Heating in Environmental Technology
AbstractDirect dielectric heating with radio-frequency (RF) electromagnetic fields is used to optimize a variety of processes in environmental technology, namely soil remediation by soil vapor extraction or biodegradation, adsorptive-catalytic off-gas treatment, thermal regeneration of drying agents in biogas or natural gas treatment and decontamination and/or drying of brickwork
Does roflumilast decrease exacerbations in severe COPD patients not controlled by inhaled combination therapy? the REACT study protocol.
Many patients with chronic obstructive pulmonary disease (COPD) continue to suffer exacerbations, even when treated with maximum recommended therapy (eg, inhaled combinations of long-acting β(2)-agonist and high dose inhaled corticosteroids, with or without a long-acting anticholinergic [long-acting muscarinic antagonist]). Roflumilast is approved to treat severe COPD in patients with chronic bronchitis - and a history of frequent exacerbations - as an add-on to bronchodilators.PURPOSE:The REACT (Roflumilast in the Prevention of COPD Exacerbations While Taking Appropriate Combination Treatment) study (identification number RO-2455-404-RD, clinicaltrials. gov identifier NCT01329029) will investigate whether roflumilast further reduces exacerbations when added to inhaled combination therapy in patients still suffering from frequent exacerbations.PATIENTS AND METHODS:REACT is a 1-year randomized, double-blind, multicenter, phase III/IV study of roflumilast 500 μg once daily or placebo on top of a fixed long-acting β(2)-agonist/inhaled corticosteroid combination. A concomitant long-acting muscarinic antagonist will be allowed at stable doses. The primary outcome is the rate of moderate or severe COPD exacerbations. Using a Poisson regression model with a two-sided significance level of 5%, a sample size of 967 patients per treatment group is needed for 90% power. COPD patients with severe to very severe airflow limitation, symptoms of chronic bronchitis, and at least two exacerbations in the previous year will be recruited.CONCLUSION:It is hypothesized that because roflumilast (a phosphodiesterase-4 inhibitor) has a different mode of action to bronchodilators and inhaled corticosteroids, it may provide additional benefits when added to these treatments in frequent exacerbators. REACT will be important to determine the role of roflumilast in COPD management. Here, the design and rationale for this important study is described
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