585 research outputs found
The boson mediators of high-Tc superconductivity: phonons versus composite bosons from the superconducting phenomenology
We address the question of whether boson mediators of high-
superconductivity are composite (electronic) or independent phonons. For s-wave
superconductivity we show from the available experiments that the hypothesis of
composite bosons is rather unlikely. Our analysis points naturally towards
phonon mediators. In addition we point out that the eventual presence of a peak
in the temperature dependence of the microwave conductivity while the
Hebel-Slichter peak is absent in the temperature dependence of the NMR
relaxation rate, can be understood within a phonon mechanism if one takes into
account the modulation of the electron-phonon coupling (predominance of forward
scattering) induced by Coulomb correlation of the carriers.Comment: Accepted for publication in the Rapid Communications section of
Physical Review B, 4 pages latex (revtex),4 figures available upon request to
[email protected]
Cross-over from BCS superconductivity to Bose condensation and High-Tc Superconductors
We consider the Eliashberg theory in the coupling region where some
fundamental qualitative deviations from the conventional BCS-like behaviour
begin to appear. These deviations are identified as the onset of a cross-over
from BCS superconductivity to Bose condensation. We point out that the
beginning of this cross-over occurs when the gap becomes comparable
to the boson energies . This condition is equivalent to the
condition of Ref. \cite{Strinati} and traduces the
physical constraint that the distance the paired electron covers during the
absorbtion of the virtual boson, cannot be larger than the coherence length.
The frontier region of couplings is of the order of , and
high- materials are concerned. A clear qualitative indication of the
occurence of a cross-over regime should be a dip structure above the gap in the
density of states of excitations. Comparing our results with tunneling and
photoemission experiments we conclude that high- materials (cuprates and
fullerides) are indeed at the beginning of a cross-over from BCS
superconductivity to Bose condensation, even though the fermionic nature still
prevails. Taking into account the analysis of Ref. \cite{Strinati}, we predict
a dip structure in heavy fermion and organic superconductors. Non-adiabatic
effects beyond Migdal's theory are considered and give insight on the
robustness of Eliashberg theory in describing qualitatively this cross-over
regime, although for the quantitative interpretation of the results the
inclusion of non-adiabatic corrections can be important.Comment: 37 pages, latex , 16 figures available upon request to
[email protected]
Spin susceptibility in small Fermi energy systems: effects of nonmagnetic impurities
In small Fermi energy metals, disorder can deeply modify superconducting
state properties leading to a strong suppression of the critical temperature
. In this paper, we show that also normal state properties can be
seriously influenced by disorder when the Fermi energy is
sufficiently small. We calculate the normal state spin susceptibility
for a narrow band electron-phonon coupled metal as a function of the
non-magnetic impurity scattering rate . We find that as soon
as is comparable to , is strongly reduced
with respect to its value in the clean limit. The effects of the
electron-phonon interaction including the nonadiabatic corrections are
discussed. Our results strongly suggest that the recent finding on irradiated
MgB samples can be naturally explained in terms of small values
associated with the -bands of the boron plane, sustaining therefore the
hypothesis that MgB is a nonadiabatic metal.Comment: 7 pages, 6 eps figures, to appear on Eur. Phys. J.
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
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