121 research outputs found
Vanishing Loss Effect on the Effective ac Conductivity behavior for 2D Composite Metal-Dielectric Films At The Percolation Threshold
We study the imaginary part of the effective conductivity as well as its
distribution probability for vanishing losses in 2D composites. This
investigation showed that the effective medium theory provides only
informations about the average conductivity, while its fluctuations which
correspond to the field energy in this limit are neglected by this theory.Comment: 6 pages, 2 figures, submitted to Phys.Rev.
Restoration of A P657-647 form from P645-638 in extracts of etiolated primary bean leaves
Measurements of absorption, fluorescence and fluorescence excitation spectra at liquid nitrogen temperature show that it is possible to restore in vitro a P657-647 protochlorophyllide lipoprotein complex (PLC) starting from preparations which essentially contain the P645-638 complex. The restoration is obtained by drying according to an experimental procedure which is described. Evidence for the attribution to the restored P657-647 of a Soret absorption band at 460 nm is given. © 1975
Localization and Absorption of Light in 2D Composite Metal-Dielectric Films at the Percolation Threshold
We study in this paper the localization of light and the dielectric
properties of thin metal-dielectric composites at the percolation threshold and
around a resonant frequency where the conductivities of the two components are
of the same order. In particular, the effect of the loss in metallic components
are examined. To this end, such systems are modelized as random networks,
and the local field distribution as well as the effective conductivity are
determined by using two different methods for comparison: an exact resolution
of Kirchoff equations, and a real space renormalization group method. The
latter method is found to give the general behavior of the effective
conductivity but fails to determine the local field distribution. It is also
found that the localization still persists for vanishing losses. This result
seems to be in agreement with the anomalous absorption observed experimentally
for such systems.Comment: 14 page latex, 3 ps figures. submitte
On random symmetric matrices with a constraint: the spectral density of random impedance networks
We derive the mean eigenvalue density for symmetric Gaussian random N x N
matrices in the limit of large N, with a constraint implying that the row sum
of matrix elements should vanish. The result is shown to be equivalent to a
result found recently for the average density of resonances in random impedance
networks [Y.V. Fyodorov, J. Phys. A: Math. Gen. 32, 7429 (1999)]. In the case
of banded matrices, the analytical results are compared with those extracted
from the numerical solution of Kirchhoff equations for quasi one-dimensional
random impedance networks.Comment: 4 pages, 5 figure
Fluctuations in random networks: non-linear model description
Disordered networks are known to be an adequate model for describing
fluctuations of electric fields in a random metal-dielectric composite. We show
that under appropriate conditions the statistical properties of such a system
can be studied in the framework of the Efetov's non-linear model.
This fact provides a direct link to the theory of Anderson localization.Comment: 4 pages, latex, no figure
Theory of the temperature and doping dependence of the Hall effect in a model with x-ray edge singularities in d=oo
We explain the anomalous features in the Hall data observed experimentally in
the normal state of the high-Tc superconductors. We show that a consistent
treatment of the local spin fluctuations in a model with x-ray edge
singularities in d=oo reproduces the temperature and the doping dependence of
the Hall constant as well as the Hall angle in the normal state. The model has
also been invoked to justify the marginal-Fermi-liquid behavior, and provides a
consistent explanation of the Hall anomalies for a non-Fermi liquid in d=oo.Comment: 5 pages, 4 figures, To appear in Phys. Rev. B, title correcte
Intermediate Valence Model for the Colossal Magnetoresistance in Tl_{2}Mn_{2}O_{7}
The colossal magnetoresistance exhibited by Tl_{2}Mn_{2}O_{7} is an
interesting phenomenon, as it is very similar to that found in perovskite
manganese oxides although the compound differs both in its crystalline
structure and electronic properties from the manganites. At the same time,
other pyrochlore compounds, though sharing the same structure with
Tl_{2}Mn_{2}O_{7}, do not exhibit the strong coupling between magnetism and
transport properties found in this material. Mostly due to the absence of
evidence for significant doping into the Mn-O sublattice, and the tendency of
Tl to form conduction bands, the traditional double exchange mechanism
mentioned in connection with manganites does not seem suitable to explain the
experimental results in this case. We propose a model for Tl_{2}Mn_{2}O_{7}
consisting of a lattice of intermediate valence ions fluctuating between two
magnetic configurations, representing Mn-3d orbitals, hybridized with a
conduction band, which we associate with Tl. This model had been proposed
originally for the analysis of intermediate valence Tm compounds. With a
simplified treatment of the model we obtain the electronic structure and
transport properties of Tl_{2}Mn_{2}O_{7}, with good qualitative agreement to
experiments. The presence of a hybridization gap in the density of states seems
important to understand the reported Hall data.Comment: 8 pages + 5 postscript fig
Quantum Size Effect transition in percolating nanocomposite films
We report on unique electronic properties in Fe-SiO2 nanocomposite thin films
in the vicinity of the percolation threshold. The electronic transport is
dominated by quantum corrections to the metallic conduction of the Infinite
Cluster (IC). At low temperature, mesoscopic effects revealed on the
conductivity, Hall effect experiments and low frequency electrical noise
(random telegraph noise and 1/f noise) strongly support the existence of a
temperature-induced Quantum Size Effect (QSE) transition in the metallic
conduction path. Below a critical temperature related to the geometrical
constriction sizes of the IC, the electronic conductivity is mainly governed by
active tunnel conductance across barriers in the metallic network. The high 1/f
noise level and the random telegraph noise are consistently explained by random
potential modulation of the barriers transmittance due to local Coulomb
charges. Our results provide evidence that a lowering of the temperature is
somehow equivalent to a decrease of the metal fraction in the vicinity of the
percolation limit.Comment: 21 pages, 8 figure
Electronic Transport in Metallic Systems and Generalized Kinetic Equations
This paper reviews some selected approaches to the description of transport
properties, mainly electroconductivity, in crystalline and disordered metallic
systems. A detailed qualitative theoretical formulation of the electron
transport processes in metallic systems within a model approach is given.
Generalized kinetic equations which were derived by the method of the
nonequilibrium statistical operator are used. Tight-binding picture and
modified tight-binding approximation (MTBA) were used for describing the
electron subsystem and the electron-lattice interaction correspondingly. The
low- and high-temperature behavior of the resistivity was discussed in detail.
The main objects of discussion are nonmagnetic (or paramagnetic) transition
metals and their disordered alloys. The choice of topics and the emphasis on
concepts and model approach makes it a good method for a better understanding
of the electrical conductivity of the transition metals and their disordered
binary substitutional alloys, but the formalism developed can be applied (with
suitable modification), in principle, to other systems. The approach we used
and the results obtained complements the existent theories of the electrical
conductivity in metallic systems. The present study extends the standard
theoretical format and calculation procedures in the theories of electron
transport in solids.Comment: 100 pages, 3 figures, Refs.37
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