257 research outputs found
Is LaOFFeAs an electron-phonon superconductor ?
In this paper we calculate the electron-phonon coupling of the
newly-discovered superconductor LaOFFeAs from first-principles,
using Density Functional Perturbation Theory. For pure LaOFeAs, the calculated
electron-phonon coupling constant and logarithmic-averaged
frequency , give a maximum of 0.8 K, using the
standard Migdal-Eliashberg theory.
For the doped compounds, we predict even smaller coupling constants, due
to the strong suppression of the electronic Density of States at the Fermi
level. To reproduce the experimental , a 5-6 times larger coupling
constant would be needed.
Our results indicate that electron-phonon coupling is not sufficient to
explain superconductivity in the newly-discovered LaOFFeAs
superconductor, probably due to the importance of strong correlation effects
Quasiparticle interference in multiband superconductors with strong coupling
We develop a theory of the quasiparticle interference (QPI) in multiband
superconductors based on strong-coupling Eliashberg approach within the Born
approximation. In the framework of this theory, we study dependencies of the
QPI response function in the multiband superconductors with nodeless s-wave
superconductive order parameter. We pay a special attention to the difference
of the quasiparticle scattering between the bands having the same and opposite
signs of the order parameter. We show that, at the momentum values close to the
momentum transfer between two bands, the energy dependence of the quasiparticle
interference response function has three singularities. Two of these correspond
to the values of the gap functions and the third one depends on both the gaps
and the transfer momentum. We argue that only the singularity near the smallest
band gap may be used as an universal tool to distinguish between and
order parameters. The robustness of the sign of the response function
peak near the smaller gap value, irrespective of the change in parameters, in
both the symmetry cases is a promising feature that can be harnessed
experimentally.Comment: 16 pages, 16 figure
Recommended from our members
Employment Protection and Takeovers
Labor restructuring is a key driver of takeovers and the associated synergy gains worldwide. In a difference-in-differences research design, we show that major increases in employment protection reduce takeover activity by 14-27% and the combined firm gains (synergies) by over half. Consistent with the labor channel behind these effects, deals with greater potential for workforce restructuring show a greater reduction in volume, number, and synergies. The reforms do impede layoffs, and the associated wage costs match the magnitude of synergy losses. Offer prices are not fully adjusted, with both bidders and targets exhibiting lower returns following the reforms
Strong electron-phonon interaction in multiband superconductors
We discuss the effects of anisotropy on superconducting critical temperature
and order parameter in a strongly coupled regime. The multiband representation
is used as a model for anisotropy. We show that strong coupling effects in
multiband superconductors lead to pair-breaking due to interband coupling
because soft phonon modes play the same role as usual impurities. This effect
makes the order parameters in different bands equal to each other and limits
the upper bound on critical temperature.Comment: 5 pages, 1 figur
Disorder induced transition between s_+- and s_++ states in two-band superconductors
We have reexamined the problem of disorder in two-band superconductors, and
shown within the framework of the T-matrix approximation, that the suppression
of T_c can be described by a single parameter depending on the intraband and
interband impurity scattering rates. T_c is shown to be more robust against
nonmagnetic impurities than would be predicted in the trivial extension of
Abrikosov-Gor'kov theory. We find a disorder-induced transition from the
s_{\pm} state to a gapless and then to a fully gapped s_{++} state, controlled
by a single parameter -- the sign of the average coupling constant .
We argue that this transition has strong implications for experiments.Comment: 5 pages, 4 figures; suppl. material: 3 pages, 2 figures; published
versio
- …