42 research outputs found
High superconductivity in MgB by nonadiabatic pairing
The evidence for the key role of the bands in the electronic
properties of MgB points to the possibility of nonadiabatic effects in the
superconductivity of these materials. These are governed by the small value of
the Fermi energy due to the vicinity of the hole doping level to the top of the
bands. We show that the nonadiabatic theory leads to a coherent
interpretation of K and the boron isotope coefficient without invoking very large couplings and it naturally explains the
role of the disorder on . It also leads to various specific predictions
for the properties of MgB and for the material optimization of these type
of compounds.Comment: 4 revtex pages, 3 eps figures, to appear on Phys. Rev. Let
Superconductivity of RbC: breakdown of the Migdal-Eliashberg theory
In this paper, through an exhaustive analysis within the Migdal-Eliashberg
theory, we show the incompatibility of experimental data of RbC with
the basic assumptions of the standard theory of superconductivity. For
different models of the electron-phonon spectral function
we solve numerically the Eliashberg equations to find which values of the
electron-phonon coupling , of the logarithmic phonon frequency
and of the Coulomb pseudopotential reproduce the
experimental data of RbC. We find that the solutions are essentially
independent of the particular shape of and that, to explain
the experimental data of RbC, one has to resort to extremely large
couplings: . This results differs from the usual partial
analyses reported up to now and we claim that this value exceeds the maximum
allowed compatible with the crystal lattice stability. Moreover, we
show quantitatively that the obtained values of and
strongly violate Migdal's theorem and consequently are incompatible with the
Migdal-Eliashberg theory. One has therefore to consider the generalization of
the theory of superconductivity in the nonadiabatic regime to account for the
experimental properties of fullerides.Comment: 9 pages, 8 eps figure encloses, epjb style, to appear on Eur. Phys.
J.
Tunneling-percolation origin of nonuniversality: theory and experiments
A vast class of disordered conducting-insulating compounds close to the
percolation threshold is characterized by nonuniversal values of transport
critical exponent t, in disagreement with the standard theory of percolation
which predicts t = 2.0 for all three dimensional systems. Various models have
been proposed in order to explain the origin of such universality breakdown.
Among them, the tunneling-percolation model calls into play tunneling processes
between conducting particles which, under some general circumstances, could
lead to transport exponents dependent of the mean tunneling distance a. The
validity of such theory could be tested by changing the parameter a by means of
an applied mechanical strain. We have applied this idea to universal and
nonuniversal RuO2-glass composites. We show that when t > 2 the measured
piezoresistive response \Gamma, i. e., the relative change of resistivity under
applied strain, diverges logarithmically at the percolation threshold, while
for t = 2, \Gamma does not show an appreciable dependence upon the RuO2 volume
fraction. These results are consistent with a mean tunneling dependence of the
nonuniversal transport exponent as predicted by the tunneling-percolation
model. The experimental results are compared with analytical and numerical
calculations on a random-resistor network model of tunneling-percolation.Comment: 13 pages, 12 figure
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
Gor'kov and Eliashberg Linear Response Theory: Rigorous Derivation and Limits of Applicability
A rigorous microscopic calculation of the polarizability of disordered
mesoscopic particles within the grand canonical ensemble is given in terms of
the supersymmetry method. The phenomenological result of Gor'kov and Eliashberg
is confirmed. Thus the underlying assumptions of their method are justified.
This encourages application of RMT in the Gor'kov--Eliashberg style to more
complicated situations.Comment: Final published versio
Magnetic pair-breaking in superconducting (Ba,K)BiO_3 investigated by magnetotunneling
The de Gennes and Maki theory of gapless superconductivity for dirty
superconductors is used to interpret the tunneling measurements on the strongly
type-II high-Tc oxide-superconductor Ba1-xKxBiO3 in high magnetic fields up to
30 Tesla. We show that this theory is applicable at all temperatures and in a
wide range of magnetic fields starting from 50 percent of the upper critical
field Bc2. In this magnetic field range the measured superconducting density of
states (DOS) has the simple energy dependence as predicted by de Gennes from
which the temperature dependence of the pair-breaking parameter alpha(T), or
Bc2(T), has been obtained. The deduced temperature dependence of Bc2(T) follows
the Werthamer-Helfand-Hohenberg prediction for classical type-II
superconductors in agreement with our previous direct determination. The
amplitudes of the deviations in the DOS depend on the magnetic field via the
spatially averaged superconducting order parameter which has a square-root
dependence on the magnetic field. Finally, the second Ginzburg-Landau parameter
kappa2(T) has been determined from the experimental data.Comment: 11 pages, 5 figure
Segregated tunneling-percolation model for transport nonuniversality
We propose a theory of the origin of transport nonuniversality in disordered
insulating-conducting compounds based on the interplay between microstructure
and tunneling processes between metallic grains dispersed in the insulating
host. We show that if the metallic phase is arranged in quasi-one dimensional
chains of conducting grains, then the distribution function of the chain
conductivities g has a power-law divergence for g -> 0 leading to nonuniversal
values of the transport critical exponent t. We evaluate the critical exponent
t by Monte Carlo calculations on a cubic lattice and show that our model can
describe universal as well nonuniversal behavior of transport depending on the
value of few microstructural parameters. Such segregated tunneling-percolation
model can describe the microstructure of a quite vast class of materials known
as thick-film resistors which display universal or nonuniversal values of t
depending on the composition.Comment: 8 pages, 5 figures (Phys. Rev. B - 1 August 2003)(fig1 replaced
Electron-Electron Interaction in Disordered Mesoscopic Systems: Weak Localization and Mesoscopic Fluctuations of Polarizability and Capacitance
The weak localization correction and the mesoscopic fluctuations of the
polarizability and the capacitance of a small disordered sample are studied
systematically in 2D and 3D geometries. While the grand canonical ensemble
calculation gives the positive magnetopolarizability, in the canonical ensemble
(appropriate for isolated samples) the sign of the effect is reversed. The
magnitude of mesoscopic fluctuations for a single sample exceeds considerably
the value of the weak localization correction.Comment: 13 pages Latex, 3 .eps figures included. To appear in Phys. Rev. B.
Minor corrections, in particular in formulae; new references adde
Piezoresistivity and conductance anisotropy of tunneling-percolating systems
Percolating networks based on interparticle tunneling conduction are shown to
yield a logarithmic divergent piezoresistive response close to the critical
point as long as the electrical conductivity becomes nonuniversal. At the same
time, the piezoresistivity or, equivalently, the conductivity anisotropy
exponent remains universal also when the conductive exponent is not,
suggesting a purely geometric origin of . We discuss our results in
relation to the nature of transport for a variety of materials such as
carbon-black--polymer composites and RuO_2-glass systems which show
nonuniversal transport properties and coexistence between tunneling and
percolating behaviors.Comment: 6 pages, 3 figures, Added discussion on experiment