4,066 research outputs found
Dimensional effects on the tunneling conductivity of gold-implanted nanocomposite films
We study the dependence of the electrical conductivity on the gold
concentration of Au-implanted polymethylmethacrylate (PMMA) and alumina
nanocomposite thin films. For Au contents larger than a critical concentration,
the conductivity of Au-PMMA and Au-alumina is well described by percolation in
two dimensions, indicating that the critical correlation length for percolation
is larger than the thickness of the films. Below the critical loading, the
conductivity is dominated by tunneling processes between isolated Au particles
dispersed in PMMA or alumina continuous matrices. Using an effective medium
analysis of the tunneling conductivity, we show that Au-PMMA behaves as a
tunneling system in two dimensions, as the film thickness is comparable to the
mean Au particle size. On the contrary, the conductivity of Au-alumina films is
best described by tunneling in three dimensions, although the film thickness is
only a few times larger than the particle size. We interpret the enhancement of
the effective dimensionality of Au-alumina films in the tunneling regime as due
to the larger film thickness as compared to the mean interparticle distances.Comment: 7 pages, 7 figure
The small polaron crossover: comparison between exact results and vertex correction approximation
We study the crossover from quasi free electron to small polaron in the
Holstein model for a single electron by means of both exact and self-consistent
calculations in one dimension and on an infinite coordination lattice. We show
that the crossover occurs when both strong coupling and multiphonon conditions
are fulfilled leading to different relevant coupling constants in adiabatic and
anti-adiabatic region of the parameters space. We also show that the
self-consistent calculations obtained by including the first electron-phonon
vertex correction give accurate results in a sizeable region of the phase
diagram well separated from the polaronic crossover.Comment: 6 pages, revtex (europhys.sty,euromacr.tex); 3 postscript figure
The physical origin of the electron-phonon vertex correction
The electron-phonon vertex correction has a complex structure both in
momentum and frequency. We explain this structure on the basis of physical
considerations and we show how the vertex correction can be decomposed into two
terms with different physical origins. In particular, the first term describes
the lattice polarization induced by the electrons and it is essentially a
single-electron process whereas the second term is governed by the
particle-hole excitations due to the exchange part of the phonon-mediated
electron-electron interaction. We show that by weakening the influence of the
exchange interaction the vertex takes mostly positive values giving rise to an
enhanced effective coupling in the scattering with phonons. This weakening of
the exchange interaction can be obtained by lowering the density of the
electrons, or by considering only long-ranged (small q) electron-phonon
couplings. These findings permit to understand why in the High-Tc materials the
small carrier density and the long ranged electron-phonon interaction may play
a positive role in enhancing Tc.Comment: 11 pages, 5 postscript figure
Anomalous impurity effects in nonadiabatic superconductors
We show that, in contrast with the usual electron-phonon Migdal-Eliashberg
theory, the critical temperature Tc of an isotropic s-wave nonadiabatic
superconductor is strongly reduced by the presence of diluted non-magnetic
impurities. Our results suggest that the recently observed Tc-suppression
driven by disorder in K3C60 [Phys. Rev. B vol.55, 3866 (1997)] and in
Nd(2-x)CexCuO(4-delta) [Phys. Rev. B vol.58, 8800 (1998)] could be explained in
terms of a nonadiabatic electron-phonon coupling. Moreover, we predict that the
isotope effect on Tc has an impurity dependence qualitatively different from
the one expected for anisotropic superconductors.Comment: 10 pages, euromacr.tex, europhys.sty, 6 figures. Replaced with
accepted version (Europhysics Letters
Anisotropic random resistor networks: a model for piezoresistive response of thick-film resistors
A number of evidences suggests that thick-film resistors are close to a
metal-insulator transition and that tunneling processes between metallic grains
are the main source of resistance. We consider as a minimal model for
description of transport properties in thick-film resistors a percolative
resistor network, with conducting elements governed by tunneling. For both
oriented and randomly oriented networks, we show that the piezoresistive
response to an applied strain is model dependent when the system is far away
from the percolation thresold, while in the critical region it acquires
universal properties. In particular close to the metal-insulator transition,
the piezoresistive anisotropy show a power law behavior. Within this region,
there exists a simple and universal relation between the conductance and the
piezoresistive anisotropy, which could be experimentally tested by common
cantilever bar measurements of thick-film resistors.Comment: 7 pages, 2 eps figure
Pauli susceptibility of nonadiabatic Fermi liquids
The nonadiabatic regime of the electron-phonon interaction leads to behaviors
of some physical measurable quantities qualitatively different from those
expected from the Migdal-Eliashberg theory. Here we identify in the Pauli
paramagnetic susceptibility one of such quantities and show that the
nonadiabatic corrections reduce with respect to its adiabatic limit. We
show also that the nonadiabatic regime induces an isotope dependence of ,
which in principle could be measured.Comment: 7 pages, 3 figures, euromacr.tex, europhys.sty. Replaced with
accepted version (Europhysics Letters
Isotope effects in the Hubbard-Holstein model within dynamical mean-field theory
We study the isotope effects arising from the coupling of correlated
electrons with dispersionless phonons by considering the Hubbard-Holstein model
at half-filling within the dynamical mean-field theory. In particular we
calculate the isotope effects on the quasi-particle spectral weight , the
renormalized phonon frequency, and the static charge and spin susceptibilities.
In the weakly correlated regime , where is the Hubbard
repulsion and is the bare electron half-bandwidth, the physical properties
are qualitatively similar to those characterizing the Holstein model in the
absence of Coulomb repulsion, where the bipolaronic binding takes place at
large electron-phonon coupling, and it reflects in divergent isotope responses.
On the contrary in the strongly correlated regime , where the
bipolaronic metal-insulator transition becomes of first order, the isotope
effects are bounded, suggesting that the first order transition is likely
driven by an electronic mechanism, rather then by a lattice instability. These
results point out how the isotope responses are extremely sensitive to phase
boundaries and they may be used to characterize the competition between the
electron-phonon coupling and the Hubbard repulsion.Comment: 10 pages, 8 figures. The paper has been already accepted on Phys.
Rev.
Longitudinal and transversal piezoresistive response of granular metals
In this paper, we study the piezoresistive response and its anisotropy for a
bond percolation model of granular metals. Both effective medium results and
numerical Monte Carlo calculations of finite simple cubic networks show that
the piezoresistive anisotropy is a strongly dependent function of bond
probability p and of bond conductance distribution width \Delta g. We find that
piezoresistive anisotropy is strongly suppressed as p is reduced and/or \Delta
g is enhanced and that it vanishes at the percolation thresold p=p_c. We argue
that a measurement of the piezoresistive anisotropy could be a sensitive tool
to estimate critical metallic concentrations in real granular metals.Comment: 14 pages, 7 eps figure
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