632 research outputs found
Dielectric properties of Granodiorite partially saturated with water and its correlation to the detection of seismic electric signals
Transient electric signals emitted prior to earthquake occurrence are
recorded at certain sites in the Earth's crust termed sensitive. These field
observations enforce the laboratory investigation of the dielectric response of
rocks forming these localities. The dielectric relaxation of granodiorite rock
coming from such a sensitive locality (Keratea, Greece) reveals, through
complex impedance spectroscopy, that the activation volume for relaxation of
this rock is negative which so far has been reported only rarely. This result,
however, supports a theoretical model on the pre-seismic electric signals and
is likely to be correlated with the sensitivity of the site and hence with the
selectivity
Negative activation volume for dielectric relaxation in hydrated rocks
Negative defect activation volumes are extremely rare in solids. Here, we
report for the first time that this holds in a couple of hydrated rocks for
dielectric relaxation by exploring the complex impedance spectra at various
pressures and temperatures. The present findings mean that the relaxation time
of the relevant relaxation mechanisms decreases upon increasing pressure, thus
it may become too short at higher pressure and hence lead to the emission of
transient electric signals before fracture. This may constitute the
long-standing laboratory confirmation for the explanation of the generation of
electric signals prior to an earthquake, as recently pointed out by Uyeda et al
[Tectonophysics 470 (2009) 205-213]
Pressure dependence of the dielectric loss in semi-conducting polypyrrole aged at room temperature
The effect of physical aging of semi-conducting polypyrrole at ambient
temperature for two years duration on the dielectric loss at various pressures
is investigated. Changes of the dielectric loss spectra and the modification of
the values of the activation volume for relaxation are interpreted through the
division of chain clusters into smaller components and the reduction of the
size of the conductive grains.Comment: Synthetic Metals (in print
Sparse Exploratory Factor Analysis
Sparse principal component analysis is a very active research area in the last decade. It produces component loadings with many zero entries which facilitates their interpretation and helps avoid redundant variables. The classic factor analysis is another popular dimension reduction technique which shares similar interpretation problems and could greatly benefit from sparse solutions. Unfortunately, there are very few works considering sparse versions of the classic factor analysis. Our goal is to contribute further in this direction. We revisit the most popular procedures for exploratory factor analysis, maximum likelihood and least squares. Sparse factor loadings are obtained for them by, first, adopting a special reparameterization and, second, by introducing additional [Formula: see text]-norm penalties into the standard factor analysis problems. As a result, we propose sparse versions of the major factor analysis procedures. We illustrate the developed algorithms on well-known psychometric problems. Our sparse solutions are critically compared to ones obtained by other existing methods
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