4,310 research outputs found
Large polaron formation induced by Rashba spin-orbit coupling
Here the electron-phonon Holstein model with Rashba spin-orbit interaction is
studied for a two dimensional square lattice in the adiabatic limit. It is
demonstrated that a delocalized electron at zero spin-orbit coupling localizes
into a large polaron state as soon as the Rashba term is nonzero. This
spin-orbit induced polaron state has localization length inversely proportional
to the Rashba coupling , and it dominates a wide region of the
- phase diagram, where is the electron-phonon
interaction.Comment: 5 pages, 3 figures, version as publishe
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.
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 survey of nonadiabatic superconductivity in cuprates and fullerides
High- superconductors are characterized by very low carrier densities.
This feature leads to two fundamental consequences: on one hand the Fermi
energies are correspondingly small and they can be of the same order of phonon
frequencies. In such a situation nonadiabatic corrections arising from the
breakdown of Migdal's theorem can not be longer neglected. In addition, small
carrier densities imply poor screening and correlation effects have to be taken
into account. We present a comprehensive overview of the theory of
superconductivity generalized into the nonadiabatic regime which is
qualitatively different from the conventional one. In this framework some of
the observed properties of the cuprates and the fullerene compounds can be
naturally accounted for, and a number of theoretical predictions are proposed
that can be experimentally tested.Comment: 1 eps figure, ijmpb-macros, proceeding of SATT10, to appear on Int.
Journ. Mod. Phys.
Electron-phonon effects on spin-orbit split bands of two dimensional systems
The electronic self-energy is studied for a two dimensional electron gas
coupled to a spin-orbit Rashba field and interacting with dispersionless
phonons. For the case of a momentum independent electron-phonon coupling
(Holstein model) we solve numerically the self-consistent non-crossing
approximation for the self-energy and calculate the electron mass enhancement
and the spectral properties. We find that, even for nominal weak
electron-phonon interaction, for strong spin-orbit couplings the electrons
behave as effectively strongly coupled to the phonons. We interpret this result
by a topological change of the Fermi surface occurring at sufficiently strong
spin-orbit coupling, which induces a square-root divergence in the electronic
density of states at low energies. We provide results for and for the
density of states of the interacting electrons for several values of the
electron filling and of the spin-orbit interaction.Comment: 9 pages, 6 figures. Version as printe
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
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