2,632 research outputs found
Enhancement of piezoelectricity in a mixed ferroelectric
We use first-principles density-functional total energy and polarization
calculations to calculate the piezoelectric tensor at zero temperature for both
cubic and simple tetragonal ordered supercells of Pb_3GeTe_4. The largest
piezoelectric coefficient for the tetragonal configuration is enhanced by a
factor of about three with respect to that of the cubic configuration. This can
be attributed to both the larger strain-induced motion of cations relative to
anions and higher Born effective charges in the tetragonal case. A normal mode
decomposition shows that both cation ordering and local relaxation weaken the
ferroelectric instability, enhancing piezoelectricity.Comment: 5 pages, revtex, 2 eps figure
Predicting polarization enhancement in multicomponent ferroelectric superlattices
Ab initio calculations are utilized as an input to develop a simple model of
polarization in epitaxial short-period CaTiO3/SrTiO3/BaTiO3 superlattices grown
on a SrTiO3 substrate. The model is then combined with a genetic algorithm
technique to optimize the arrangement of individual CaTiO3, SrTiO3 and BaTiO3
layers in a superlattice, predicting structures with the highest possible
polarization and a low in-plane lattice constant mismatch with the substrate.
This modelling procedure can be applied to a wide range of layered
perovskite-oxide nanostructures providing guidance for experimental development
of nanoelectromechanical devices with substantially improved polar properties.Comment: 4 pages, submitted to PR
Polar phonons and intrinsic dielectric response of the ferromagnetic insulating spinel CdCrS from first principles
We have studied the dielectric properties of the ferromagnetic spinel
CdCrS from first principles. Zone-center phonons and Born effective
charges were calculated by frozen-phonon and Berry phase techniques within
LSDA+U. We find that all infrared-active phonons are quite stable within the
cubic space group. The calculated static dielectric constant agrees well with
previous measurements. These results suggest that the recently observed
anomalous dielectric behavior in CdCrS is not due to the softening of a
polar mode. We suggest further experiments to clarify this point
Ab initio study of the phase diagram of epitaxial BaTiO3
Using a combination of first-principles and effective-Hamiltonian approaches,
we map out the structure of BaTiO3 under epitaxial constraints applicable to
growth on perovskite substrates. We obtain a phase diagram in temperature and
misfit strain that is qualitatively different from that reported by Pertsev et
al. [Phys. Rev. Lett. 80, 1988 (1998)], who based their results on an empirical
thermodynamic potential with parameters fitted at temperatures in the vicinity
of the bulk phase transitions. In particular, we find a region of `r phase' at
low temperature where Pertsev et al. have reported an `ac phase'. We expect our
results to be relevant to thin epitaxial films of BaTiO3 at low temperatures
and experimentally-achievable strains.Comment: 4 pages, with 4 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/od_epi/index.htm
First-principles theory of ferroelectric phase transitions for perovskites: The case of BaTiO3
We carry out a completely first-principles study of the ferroelectric phase
transitions in BaTiO. Our approach takes advantage of two features of these
transitions: the structural changes are small, and only low-energy distortions
are important. Based on these observations, we make systematically improvable
approximations which enable the parameterization of the complicated energy
surface. The parameters are determined from first-principles total-energy
calculations using ultra-soft pseudopotentials and a preconditioned
conjugate-gradient scheme. The resulting effective Hamiltonian is then solved
by Monte Carlo simulation. The calculated phase sequence, transition
temperatures, latent heats, and spontaneous polarizations are all in good
agreement with experiment. We find the transitions to be intermediate between
order-disorder and displacive character. We find all three phase transitions to
be of first order. The roles of different interactions are discussed.Comment: 33 pages latex file, 9 figure
Structural, electronic and magnetic properties of SrRuO under epitaxial strain
Using density functional theory within the local spin density approximation,
structural, electronic and magnetic properties of SRO are investigated. We
examine the magnitude of the orthorhombic distortion in the ground state and
also the effects of applying epitaxial constraints, whereby the influence of
large (in the range of ) in-plane strain resulting from coherent
epitaxy, for both [001] and [110] oriented films, have been isolated and
investigated. The overall pattern of the structural relaxations reveal coherent
distortions of the oxygen octahedra network, which determine stability of the
magnetic moment on the Ru ion. The structural and magnetic parameters exhibit
substantial changes allowing us to discuss the role of symmetry and
possibilities of magneto-structural tuning of \SRO-based thin film structures.Comment: 11 page
Localized Basis for Effective Lattice Hamiltonians: Lattice Wannier Functions
A systematic method is presented for constructing effective Hamiltonians for
general phonon-related structural transitions. The key feature is the
application of group theoretical methods to identify the subspace in which the
effective Hamiltonian acts and construct for it localized basis vectors, which
are the analogue of electronic Wannier functions. The results of the symmetry
analysis for the perovskite, rocksalt, fluorite and A15 structures and the
forms of effective Hamiltonians for the ferroelectric transition in
and , the oxygen-octahedron rotation transition in , the
Jahn-Teller instability in and the
antiferroelectric transition in are discussed. For the oxygen-
octahedron rotation transition in , this method provides an
alternative to the rotational variable approach which is well behaved
throughout the Brillouin zone. The parameters appearing in the Wannier basis
vectors and in the effective Hamiltonian, given by the corresponding invariant
energy expansion, can be obtained for individual materials using first-
principles density-functional-theory total energy and linear response
techniques, or any technique that can reliably calculate force constants and
distortion energies. A practical approach to the determination of these
parameters is presented and the application to ferroelectric
discussed.Comment: extensive revisions in presentation, 32 pages, Revtex, 7 Postscript
figure
First principles investigation of ferroelectricity in epitaxially strained PbTiO
The structure and polarization of the as-yet hypothetical Ruddlesden-Popper
compound PbTiO are investigated within density-functional theory. Zone
enter phonons of the high-symmetry KNiF-type reference structure, space
group , were calculated. At the theoretical ground-state lattice
constants, there is one unstable infrared-active phonon. This phonon freezes in
to give the ferroelectric state. As a function of epitaxial strain, two
additional ferroelectric phases are found, with space groups and
at compressive and tensile strains, respectively.Comment: 4 pages, 4 figure
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
Successful private–public funding of paediatric medicines research: lessons from the EU programme to fund research into off-patent medicines
The European Paediatric Regulation mandated the European Commission to fund research on off-patent medicines with demonstrated therapeutic interest for children. Responding to this mandate, five FP7 project calls were launched and 20 projects were granted. This paper aims to detail the funded projects and their preliminary results. Publicly
available sources have been consulted and a descriptive
analysis has been performed. Twenty Research Consortia
including 246 partners in 29 European and non-European
countries were created (involving 129 universities or public funded research organisations, 51 private companies with 40 SMEs, 7 patient associations). The funded projects investigate 24 medicines, covering 10 therapeutic areas in all paediatric age groups. In response to the Paediatric Regulation and to apply for a Paediatric Use Marketing Authorisation, 15 Paediatric Investigation Plans have been granted by the EMAPaediatric Committee, including 71 studies of whom 29 paediatric clinical trials, leading to a total of 7,300 children to be recruited in more than 380 investigational centres.
Conclusion: Notwithstanding the EU contribution for each
study is lower than similar publicly funded projects, and also considering the complexity of paediatric research, these projects are performing high-quality research and are progressing towards the increase of new paediatric medicines on the market. Private–public partnerships have been effectively implemented, providing a good example for future collaborative actions. Since these projects cover a limited number of offpatent drugs and many unmet therapeutic needs in paediatrics remain, it is crucial foreseeing new similar initiatives in forthcoming European funding programmes
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