2,625 research outputs found
Coexistence of antiferrodistortive and ferroelectric distortions at the PbTiO (001) surface
The c(22) reconstruction of (001) PbTiO surfaces is studied by
means of first principles calculations for paraelectric (non-polar) and
ferroelectric ([001] polarized) films. Analysis of the atomic displacements in
the near-surface region shows how the surface modifies the antiferrodistortive
(AFD) instability and its interaction with ferroelectric (FE) distortions. The
effect of the surface is found to be termination dependent. The AFD instability
is suppressed at the TiO termination while it is strongly enhanced,
relative to the bulk, at the PbO termination resulting in a c(2x2) surface
reconstruction which is in excellent agreement with experiments. We find that,
in contrast to bulk PbTiO, in-plane ferroelectricity at the PbO termination
does not suppress the AFD instability. The AFD and the in-plane FE distortions
are instead concurrently enhanced at the PbO termination. This leads to a novel
surface phase with coexisting FE and AFD distortions which is not found in
PbTiO bulk
High sensitivity of 17O NMR to p-d hybridization in transition metal perovskites: first principles calculations of large anisotropic chemical shielding
A first principles embedded cluster approach is used to calculate O chemical
shielding tensors, sigma, in prototypical transition metal oxide ABO_3
perovskite crystals. Our principal findings are 1) a large anisotropy of sigma
between deshielded sigma_x ~ sigma_y and shielded sigma_z components (z along
the Ti-O bond); 2) a nearly linear variation, across all the systems studied,
of the isotropic sigma_iso and uniaxial sigma_ax components, as a function of
the B-O-B bond asymmetry. We show that the anisotropy and linear variation
arise from large paramagnetic contributions to sigma_x and sigma_y due to
virtual transitions between O(2p) and unoccupied B(nd) states. The calculated
isotropic delta_iso and uniaxial delta_ax chemical shifts are in good agreement
with recent BaTiO_3 and SrTiO_3 single crystal 17O NMR measurements. In PbTiO_3
and PbZrO_3, calculated delta_iso are also in good agreement with NMR powder
spectrum measurements. In PbZrO_3, delta_iso calculations of the five
chemically distinct sites indicate a correction of the experimental
assignments. The strong dependence of sigma on covalent O(2p)-B(nd)
interactions seen in our calculations indicates that 17O NMR spectroscopy,
coupled with first principles calculations, can be an especially useful tool to
study the local structure in complex perovskite alloys.Comment: 12 pages, 3 figures, and 3 Table
Effect of placental transfusion on neonatal resuscitation attempts
Objective:
Overall, neonatal mortality has been shown to be reduced by: placental transfusion (the transfer of blood from the placenta to the neonatal circulation after birth); delayed cord clamping (DCM) (waiting for the umbilical cord to stop pulsating before clamping and cutting the cord); and umbilical cord milking (UCM) (clamping and cutting the cord immediately before milking the cord towards the neonate to expel remaining volume). This systematic review aimed to determine whether placental transfusion negatively impacts resuscitation by delaying it or has any effect on infant mortality, and to identify any barriers to performing it.
Methods:
CINAHL, MEDLINE, AMED and the British Nursing Index were searched using key terms to identify relevant English language publications between 2017 and 2019.
Results:
Five papers were selected for critical analysis—three randomised control trials and two cohort studies.
Conclusion:
Placental transfusion was not found to have a negative impact on neonatal resuscitation but, equally, had no significant effect on Apgar at 5 minutes; however, Apgar is a crude measure of infant mortality. The question remains around the proven multifaceted benefit of placental transfusion in the prehospital environment, which requires further research. There is evidence to suggest prehospital clinicians should be looking to change practice. Further research, considerations and consultations are required to ascertain the best way to implement the procedure with a balanced and proportionate approach considering neonatal thermoregulation and maternal management. The main reported barrier to placental transfusion was a lack of appropriate equipment
d0 Perovskite-Semiconductor Electronic Structure
We address the low-energy effective Hamiltonian of electron doped d0
perovskite semiconductors in cubic and tetragonal phases using the k*p method.
The Hamiltonian depends on the spin-orbit interaction strength, on the
temperature-dependent tetragonal distortion, and on a set of effective-mass
parameters whose number is determined by the symmetry of the crystal. We
explain how these parameters can be extracted from angle resolved
photo-emission, Raman spectroscopy, and magneto-transport measurements and
estimate their values in SrTiO3
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
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
Suppressed dependence of polarization on epitaxial strain in highly polar ferroelectrics
A combined experimental and computational investigation of coupling between
polarization and epitaxial strain in highly polar ferroelectric
PbZr_0.2Ti_0.8O_3 (PZT) thin films is reported. A comparison of the properties
of relaxed (tetragonality c/a = 1.05) and highly-strained (c/a = 1.09)
epitaxial films shows that polarization, while being amongst the highest
reported for PZT or PbTiO_3 in either film or bulk forms (P_r = 82
microC/cm^2), is almost independent of the epitaxial strain. We attribute this
behavior to a suppressed sensitivity of the A-site cations to epitaxial strain
in these Pb-based perovskites, where the ferroelectric displacements are
already large, contrary to the case of less polar perovskites, such as BaTiO_3.
In the latter case, the A-site cation (Ba) and equatorial oxygen displacements
can lead to substantial polarization increases.Comment: 4 pages, 3 figure
Point force manipulation and activated dynamics of polymers adsorbed on structured substrates
We study the activated motion of adsorbed polymers which are driven over a
structured substrate by a localized point force.Our theory applies to
experiments with single polymers using, for example, tips of scanning force
microscopes to drag the polymer.We consider both flexible and semiflexible
polymers,and the lateral surface structure is represented by double-well or
periodic potentials. The dynamics is governed by kink-like excitations for
which we calculate shapes, energies, and critical point forces. Thermally
activated motion proceeds by the nucleation of a kink-antikink pair at the
point where the force is applied and subsequent diffusive separation of kink
and antikink. In the stationary state of the driven polymer, the collective
kink dynamics can be described by an one-dimensional symmetric simple exclusion
process.Comment: 7 pages, 2 Figure
Parents' experiences and satisfaction with care during the birth of their very preterm baby: a qualitative study
Objective: To assess parents' experiences and satisfaction with care during very preterm birth and to identify domains associated with positive and negative experiences of care.
Design: Qualitative study using semi-structured interviews.
Setting: Three neonatal units in tertiary care hospitals in South-East England.
Population: Thirty-two mothers and seven fathers who had a baby born before 32 weeks of gestation and spoke English well.
Methods: Semi-structured interviews were conducted. Results were analysed using thematic analysis.
Main outcome measures
Participants' experiences and satisfaction with care during the birth of their preterm baby.
Results: Overall, 80% of participants were extremely satisfied with the care during the birth of their preterm baby, seven were generally satisfied but felt some things could be improved and one was dissatisfied. Four key determinants of experiences of care were identified: staff professionalism, which included information and explanation, being calm in a crisis, appearing confident and in control, and conversely not listening to the woman; staff empathy, which included caring and emotional support, and encouragement and reassurance; involvement of the father; and birth environment.
Conclusions: Although the determinants of experiences of care are generally consistent with previous research on term births, unique factors to preterm birth were identified. These were the importance of the staff appearing calm during the birth, and the staff portraying confidence and taking control during the birth. Women valued being listened to, and both they and their partners valued staff helping fathers to feel involved during the birth
Temperature-dependent Raman scattering of DyScO3 and GdScO3 single crystals
We report a temperature-dependent Raman scattering investigation of DyScO3
and GdScO3 single crystals from room temperature up to 1200 {\deg}C. With
increasing temperature, all modes decrease monotonously in wavenumber without
anomaly, which attests the absence of a structural phase transition. The high
temperature spectral signature and extrapolation of band positions to higher
temperatures suggest a decreasing orthorhombic distortion towards the ideal
cubic structure. Our study indicates that this orthorhombic-to-cubic phase
transition is close to or higher than the melting point of both rare-earth
scandates (\approx 2100 {\deg}C), which might exclude the possibility of the
experimental observation of such a phase transition before melting. The
temperature-dependent shift of Raman phonons is also discussed in the context
of thermal expansion
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