70,819 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
Wormwholes: A Commentary On K.F. Schaffer\u27s Genes, Behavior, And Developmental Emergentism
Although Caenorhabditis elegans was chosen and modified to be an organism that would facilitate a reductionist program for neurogenetics, recent research has provided evidence for properties that are emergent from the neurons. While neurogenetic advances have been made using C. elegans which may be useful in explaining human neurobiology, there are severe limitations on C. elegans to explain any significant human behavior
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
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
The COBE Normalization for Standard CDM
The COBE detection of CMB anisotropies provides the best way of fixing the
amplitude of fluctuations on the largest scales. This normalization is usually
given for an n=1 spectrum, including only the anisotropy caused by the Sachs-
Wolfe effect. This is certainly not a good approximation for a model containing
any reasonable amount of baryonic matter. In fact, even tilted S-W spectra are
not a good fit to models like CDM. Here we normalize standard CDM (sCDM) to the
2-year COBE data, and quote the best amplitude in terms of the conventionally
used measures of power. We also give normalizations for some specific variants
of this standard model, and we indicate how the normalization depends on the
assumed values of n, Omega_B and H_0. For sCDM we find =19.9\pm1.5uK,
corresponding to sigma_8=1.34\pm0.10, with the normalization at large scales
being B=(8.16\pm1.04)\times10^5 (Mpc/h)^4, and other numbers given in the
Table. The measured rms temperature fluctuation smoothed on 10deg is a little
low relative to this normalization. This is mainly due to the low quadrupole in
the data: when the quadrupole is removed, the measured value of sigma(10) is
quite consistent with the best-fitting . The use of should be preferred
over sigma(10), when its value can be determined for a particular theory, since
it makes full use of the data.Comment: 4 pages compressed uuencoded postscript. We have corrected an error
in our analysi
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
Coherency in space of lake and river temperature and water quality records
Environmental time series observed over 100’s of monitoring locations usually possess some spatial structure in terms of common patterns throughout time, commonly described as temporal coherence. This paper will apply, develop and compare two methods for clustering time series on the basis of their patterns over time. The first approach treats the time series as functional data and applies hierarchical clustering while the second uses a state-space model based clustering approach. Both methods are developed to incorporate spatial correlation and stopping criteria are investigated to identify an appropriate number of clusters. The methods are applied to Total Organic Carbon data from river sites across Scotland
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