112 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.
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
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
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
Theory of proximity effect in superconductor/ferromagnet heterostructures
We present a microscopic theory of proximity effect in the
ferromagnet/superconductor/ferromagnet (F/S/F) nanostructures where S is s-wave
low-T_c superconductor and F's are layers of 3d transition ferromagnetic metal.
Our approach is based on the solution of Gor'kov equations for the normal and
anomalous Green's functions together with a self-consistent evaluation of the
superconducting order parameter. We take into account the elastic
spin-conserving scattering of the electrons assuming s-wave scattering in the S
layer and s-d scattering in the F layers. In accordance with the previous
quasiclassical theories, we found that due to exchange field in the ferromagnet
the anomalous Green's function F(z) exhibits the damping oscillations in the
F-layer as a function of distance z from the S/F interface. In the given model
a half of period of oscillations is determined by the length \xi_m^0 = \pi
v_F/E_ex, where v_F is the Fermi velocity and E_ex is the exchange field, while
damping is governed by the length l_0 = (1/l_{\uparrow} +
1/l_{\downarrow})^{-1} with l_{\uparrow} and l_{\downarrow} being
spin-dependent mean free paths in the ferromagnet. The superconducting
transition temperature T_c(d_F) of the F/S/F trilayer shows the damping
oscillations as a function of the F-layer thickness d_F with period \xi_F =
\pi/\sqrt{m E_ex}, where m is the effective electron mass. We show that strong
spin-conserving scattering either in the superconductor or in the ferromagnet
significantly suppresses these oscillations. The calculated T_c(d_F)
dependences are compared with existing experimental data for Fe/Nb/Fe trilayers
and Nb/Co multilayers.Comment: 13 pages, REVTeX4, 8 PS-figures; improved version, submitted to PR
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
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
- …