70,784 research outputs found
Negative Differential Resistivity and Positive Temperature Coefficient of Resistivity effect in the diffusion limited current of ferroelectric thin film capacitors
We present a model for the leakage current in ferroelectric thin- film
capacitors which explains two of the observed phenomena that have escaped
satisfactory explanation, i.e. the occurrence of either a plateau or negative
differential resistivity at low voltages, and the observation of a Positive
Temperature Coefficient of Resistivity (PTCR) effect in certain samples in the
high-voltage regime. The leakage current is modelled by considering a
diffusion-limited current process, which in the high-voltage regime recovers
the diffusion-limited Schottky relationship of Simmons already shown to be
applicable in these systems
Topology of the polarization field in ferroelectric nanowires from first principles
The behaviour of the cross-sectional polarization field is explored for thin
nanowires of barium titanate from first-principles calculations. Topological
defects of different winding numbers have been obtained, beyond the known
textures in ferroelectric nanostructures. They result from the inward
accommodation of the polarization patterns imposed at the surface of the wire
by surface and edge effects. Close to a topological defect the polarization
field orients out of the basal plane in some cases, maintaining a close to
constant magnitude, whereas it virtually vanishes in other cases.Comment: 4 pages, 3 figure
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
Space Charge Effects in Ferroelectric Thin Films
The effects of space charges on hysteresis loops and field distributions in
ferroelectrics have been investigated numerically using the phenomenological
Landau-Ginzburg-Devonshire theory. Cases with the ferroelectric fully and
partially depleted have been considered. In general, increasing the number of
charged impurities results in a lowering of the polarization and coercive field
values. Squarer loops were observed in the partially depleted cases and a
method was proposed to identify fully depleted samples experimentally from
dielectric and polarization measurements alone. Unusual field distributions
found for higher dopant concentrations have some interesting implications for
leakage mechanisms and limit the range of validity of usual semiconductor
equations for carrier transport.Comment: 23 pages, 11 figure
A Symbiotic View Of Life: We Have Never Been Individuals
The notion of the biological individual is crucial to studies of genetics, immunology, evolution, development, anatomy, and physiology. Each of these biological subdisciplines has a specific conception of individuality, which has historically provided conceptual contexts for integrating newly acquired data. During the past decade, nucleic acid analysis, especially genomic sequencing and high-throughput RNA techniques, has challenged each of these disciplinary definitions by finding significant interactions of animals and plants with symbiotic microorganisms that disrupt the boundaries that heretofore had characterized the biological individual. Animals cannot be considered individuals by anatomical or physiological criteria because a diversity of symbionts are both present and functional in completing metabolic pathways and serving other physiological functions. Similarly, these new studies have shown that animal development is incomplete without symbionts. Symbionts also constitute a second mode of genetic inheritance, providing selectable genetic variation for natural selection. The immune system also develops, in part, in dialogue with symbionts and thereby functions as a mechanism for integrating microbes into the animal-cell community. Recognizing the holobiont -the multicellular eukaryote plus its colonies of persistent symbionts-as a critically important unit of anatomy, development, physiology, immunology, and evolution opens up new investigative avenues and conceptually challenges the ways in which the biological subdisciplines have heretofore characterized living entities
Multiferroic behavior of Aurivillius Bi4Mn3O12 from first-principles
The multiferroic behavior of the hypothetical Aurivillius compound Bi4Mn3O12
has been explored on the basis of density functional calculations. We find that
the tetragonal paraelectric phase of this material is ferromagnetic, showing
ferroelectric and antiferrodistortive instabilities similar to the ones
observed in its ferroelectric parent compound Bi4Ti3O12 . Our results indicate,
however, that the presence of Mn+4 ions at the B-sites shrinks the cell volume
and consequently the unstable polar mode, associated with the ferroelectric
polarization, is overcame by an antiferrodistortive distortion. In this way,
Bi4Mn3O12 exhibits incipient ferroelectricity at its equilibrium volume. We
show that the ferroelectric state can be favored by strain or partial
substitution of Mn with Ti.Comment: 6 pages, 5 figure
Physics of thin-film ferroelectric oxides
This review covers the important advances in recent years in the physics of
thin film ferroelectric oxides, the strongest emphasis being on those aspects
particular to ferroelectrics in thin film form. We introduce the current state
of development in the application of ferroelectric thin films for electronic
devices and discuss the physics relevant for the performance and failure of
these devices. Following this we cover the enormous progress that has been made
in the first principles computational approach to understanding ferroelectrics.
We then discuss in detail the important role that strain plays in determining
the properties of epitaxial thin ferroelectric films. Finally, we look at the
emerging possibilities for nanoscale ferroelectrics, with particular emphasis
on ferroelectrics in non conventional nanoscale geometries.Comment: This is an invited review for Reviews of Modern Physics. We welcome
feedback and will endeavour to incorporate comments received promptly into
the final versio
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