4,676 research outputs found
s- and d-wave Symmetries in Nonadiabatic Theory of Superconductivity
High- superconductors have Fermi energies much smaller than
conventional metals comparable to phonon frequencies. In such a situation
nonadiabatic effects are important. A generalization of Eliashberg theory in
the nonadiabatic regime has previously been shown to reproduce some anomalous
features of the high- superconductors as for istance the enhancement of
or the isotopic effects on and . In this contribution we
address the issue of the symmetry of the gap in the context of nonadiabatic
superconductivity. We show that vertex corrections have a momentum structure
which favours d-wave superconductivity when forward scattering is predominant.
An additional increase of is also found.Comment: 6 pages, 3 eps figure, ijmpb-macros, proceeding of SATT10, to appear
on Int. Journ. Mod. Phys.
A model of transport nonuniversality in thick-film resistors
We propose a model of transport in thick-film resistors which naturally
explains the observed nonuniversal values of the conductance exponent t
extracted in the vicinity of the percolation transition. Essential ingredients
of the model are the segregated microstructure typical of thick-film resistors
and tunneling between the conducting grains. Nonuniversality sets in as
consequence of wide distribution of interparticle tunneling distances.Comment: 3 pages, 1 figur
Zeeman response of d-wave superconductors: Born approximation for impurity and spin-orbit scattering potentials
The effects of impurity and spin-orbit scattering potentials can strongly
affect the Zeeman response of a d-wave superconductor. Here, both the phase
diagram and the quasiparticle density of states are calculated within the Born
approximation and it is found that the spin-orbit interaction influences in a
qualitatively different way the Zeeman response of d-wave and s-wave
superconductors.Comment: 19 pages, 6 eps figures, submitted to Physica
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.
Nonadiabatic high-Tc superconductivity in hole-doped fullerenes
In this paper we address the possibility of high-T-c superconductivity (T(c)similar to100 K) in hypothetical hole doped C-60 within the context of the nonadiabatic theory of superconductivity. Our analysis shows that electron doped fullerenes, represented by the A(3)C(60) family, are characterized by relatively small values of the electron-phonon coupling constant lambda, which can thus be further increased by hole doping before lattice instabilities occur. In particular we show that T-c larger than 100 K are compatible in the nonadiabatic context with microscopic parameters lambda(h)similar or equal to0.5-1.0, mu(*)similar or equal to0.3-0.5 and phonon frequencies omega(ph)similar or equal to1500-2000 K. These results provide a stimulus for material engineering and optimization along the lines indicated
Relevance of multiband Jahn-Teller effects on the electron-phonon interaction in C
Assessing the effective relevance of multiband effects in the fullerides is
of fundamental importance to understand the complex superconducting and
transport properties of these compounds. In this paper we investigate in
particular the role of the multiband effects on the electron-phonon (el-ph)
properties of the bands coupled with the Jahn-Teller intra-molecular
vibrational modes in the C compounds. We show that, assuming
perfect degeneracy of the electronic bands, vertex diagrams arising from the
breakdown of the adiabatic hypothesis, are one order of magnitude smaller than
the non-crossing terms usually retained in the Migdal-Eliashberg (ME) theory.
These results permit to understand the robustness on ME theory found by
numerical calculations. The effects of the non degeneracy of the in
realistic systems are also analyzed. Using a tight-binding model we show that
the el-ph interaction is mainly dominated by interband scattering within a
single electronic band. Our results question the reliability of a degenerate
band modeling and show the importance of these combined effects in the
C family.Comment: 5 pages, 3 eps figure
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