249 research outputs found
Comparing the Weighted Density Approximation with the LDA and GGA for Ground State Properties of Ferroelectric Perovskites
First-principles calculations within the weighted density approximation (WDA)
were performed for ground state properties of ferroelectric perovskites
PbTiO, BaTiO, SrTiO, KNbO and KTaO. We used the plane-wave
pseudopotential method, a pair distribution function based on the uniform
electron gas, and shell partitioning. Comparing with the local density
approximation (LDA) and the general gradient approximation (GGA), we found that
the WDA significantly improves the equilibrium volume of these materials in
cubic symmetry over both the LDA and GGA; Ferroelectric instabilities
calculated by the WDA agree with the LDA and GGA very well; At the experimental
ferroelectric lattice, optimized atom positions by the WDA are in good
agreement with measured data; However the WDA overestimates the strain of
tetragonal PbTiO at experimental volume; The WDA overestimates the volume
of fully relaxed structures, but the GGA results are even worse. Some
calculations were also done with other models for . It is found that a
with longer range behavior yields improved relaxed structures. Possible avenues
for improving the WDA are discussed.Comment: 19 pages, 3 figures, submitted to PR
Impact of layer defects in ferroelectric thin films
Based on a modified Ising model in a transverse field we demonstrate that
defect layers in ferroelectric thin films, such as layers with impurities,
vacancies or dislocations, are able to induce a strong increase or decrease of
the polarization depending on the variation of the exchange interaction within
the defect layers. A Green's function technique enables us to calculate the
polarization, the excitation energy and the critical temperature of the
material with structural defects. Numerically we find the polarization as
function of temperature, film thickness and the interaction strengths between
the layers. The theoretical results are in reasonable accordance to
experimental datas of different ferroelectric thin films.Comment: 17 pages, 8 figure
Nanoelectromechanics of Piezoresponse Force Microscopy
To achieve quantitative interpretation of Piezoresponse Force Microscopy
(PFM), including resolution limits, tip bias- and strain-induced phenomena and
spectroscopy, analytical representations for tip-induced electroelastic fields
inside the material are derived for the cases of weak and strong indentation.
In the weak indentation case, electrostatic field distribution is calculated
using image charge model. In the strong indentation case, the solution of the
coupled electroelastic problem for piezoelectric indentation is used to obtain
the electric field and strain distribution in the ferroelectric material. This
establishes a complete continuum mechanics description of the PFM contact
mechanics and imaging mechanism. The electroelastic field distribution allows
signal generation volume in PFM to be determined. These rigorous solutions are
compared with the electrostatic point charge and sphere-plane models, and the
applicability limits for asymptotic point charge and point force models are
established. The implications of these results for ferroelectric polarization
switching processes are analyzed.Comment: 81 pages, 19 figures, to be published in Phys. Rev.
First-principles study of (BiScO3){1-x}-(PbTiO3){x} piezoelectric alloys
We report a first-principles study of a class of (BiScO3)_{1-x}-(PbTiO3)_x
(BS-PT) alloys recently proposed by Eitel et al. as promising materials for
piezoelectric actuator applications. We show that (i) BS-PT displays very large
structural distortions and polarizations at the morphotropic phase boundary
(MPB) (we obtain a c/a of ~1.05-1.08 and P_tet of ~1.1 C/m^2); (ii) the
ferroelectric and piezoelectric properties of BS-PT are dominated by the onset
of hybridization between Bi/Pb-6p and O-2p orbitals, a mechanism that is
enhanced upon substitution of Pb by Bi; and (iii) the piezoelectric responses
of BS-PT and Pb(Zr_{1-x}Ti_x)O3 (PZT) at the MPB are comparable, at least as
far as the computed values of the piezoelectric coefficient d_15 are concerned.
While our results are generally consistent with experiment, they also suggest
that certain intrinsic properties of BS-PT may be even better than has been
indicated by experiments to date. We also discuss results for PZT that
demonstrate the prominent role played by Pb displacements in its piezoelectric
properties.Comment: 6 pages, with 3 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/ji_bi/index.htm
Anomalous Sliding Friction and Peak Effect near the Flux Lattice Melting Transition
Recent experiments have revealed a giant "peak effect" in ultrapure high
superconductors. Moreover, the new data show that the peak effect
coincides exactly with the melting transition of the underlying flux lattice.
In this work, we show using dynamical scaling arguments that the friction due
to the pinning centers acting on the flux lattice develops a singularity near a
continuous phase transition and can diverge for many systems. The magnitude of
the nonlinear sliding friction of the flux lattice scales with this atomistic
friction. Thus, the nonlinear conductance should diverge for a true continuous
transition in the flux lattice or peak at a weakly first order transition or
for systems of finite size.Comment: 4 pages, to appear in Phys. Rev.
Lattice Dynamics of II-VI materials using adiabatic bond charge model
We extend the adiabatic bond charge model, originally developed for group IV
semiconductors and III-V compounds, to study phonons in more ionic II-VI
compounds with a zincblende structure. Phonon spectra, density of states and
specific heats are calculated for six II-VI compounds and compared with both
experimental data and the results of other models. We show that the 6-parameter
bond charge model gives a good description of the lattice dynamics of these
materials. We also discuss trends in the parameters with respect to the
ionicity and metallicity of these compounds.Comment: 16 pages of RevTex with 3 figures submitted as a uuencode compressed
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Global assessment of marine plastic exposure risk for oceanic birds
Plastic pollution is distributed patchily around the world's oceans. Likewise, marine organisms that are vulnerable to plastic ingestion or entanglement have uneven distributions. Understanding where wildlife encounters plastic is crucial for targeting research and mitigation. Oceanic seabirds, particularly petrels, frequently ingest plastic, are highly threatened, and cover vast distances during foraging and migration. However, the spatial overlap between petrels and plastics is poorly understood. Here we combine marine plastic density estimates with individual movement data for 7137 birds of 77 petrel species to estimate relative exposure risk. We identify high exposure risk areas in the Mediterranean and Black seas, and the northeast Pacific, northwest Pacific, South Atlantic and southwest Indian oceans. Plastic exposure risk varies greatly among species and populations, and between breeding and non-breeding seasons. Exposure risk is disproportionately high for Threatened species. Outside the Mediterranean and Black seas, exposure risk is highest in the high seas and Exclusive Economic Zones (EEZs) of the USA, Japan, and the UK. Birds generally had higher plastic exposure risk outside the EEZ of the country where they breed. We identify conservation and research priorities, and highlight that international collaboration is key to addressing the impacts of marine plastic on wide-ranging species
Global assessment of marine plastic exposure risk for oceanic birds
Plastic pollution is distributed patchily around the world’s oceans. Likewise, marine organisms that are vulnerable to plastic ingestion or entanglement have uneven distributions. Understanding where wildlife encounters plastic is crucial for targeting research and mitigation. Oceanic seabirds, particularly petrels, frequently ingest plastic, are highly threatened, and cover vast distances during foraging and migration. However, the spatial overlap between petrels and plastics is poorly understood. Here we combine marine plastic density estimates with individual movement data for 7137 birds of 77 petrel species to estimate relative exposure risk. We identify high exposure risk areas in the Mediterranean and Black seas, and the northeast Pacific, northwest Pacific, South Atlantic and southwest Indian oceans. Plastic exposure risk varies greatly among species and populations, and between breeding and nonbreeding seasons. Exposure risk is disproportionately high for Threatened species. Outside the Mediterranean and Black seas, exposure risk is highest in the high seas and Exclusive Economic Zones (EEZs) of the USA, Japan, and the UK. Birds generally had higher plastic exposure risk outside the EEZ of the country where they breed. We identify conservation and research priorities, and highlight that international collaboration is key to addressing the impacts of marine plastic on wide-ranging speciespublishedVersio
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