3,609 research outputs found
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
Anomalous impurity effects in nonadiabatic superconductors
We show that, in contrast with the usual electron-phonon Migdal-Eliashberg
theory, the critical temperature Tc of an isotropic s-wave nonadiabatic
superconductor is strongly reduced by the presence of diluted non-magnetic
impurities. Our results suggest that the recently observed Tc-suppression
driven by disorder in K3C60 [Phys. Rev. B vol.55, 3866 (1997)] and in
Nd(2-x)CexCuO(4-delta) [Phys. Rev. B vol.58, 8800 (1998)] could be explained in
terms of a nonadiabatic electron-phonon coupling. Moreover, we predict that the
isotope effect on Tc has an impurity dependence qualitatively different from
the one expected for anisotropic superconductors.Comment: 10 pages, euromacr.tex, europhys.sty, 6 figures. Replaced with
accepted version (Europhysics Letters
Nonadiabatic Superconductivity and Vertex Corrections in Uncorrelated Systems
We investigate the issue of the nonadiabatic superconductivity in
uncorrelated systems. A local approximation is employed coherently with the
weak dependence on the involved momenta. Our results show that nonadiabatic
vertex corrections are never negligible, but lead to a strong suppression of
with respect to the conventional theory. This feature is understood in
terms of the momentum-frequency dependence of the vertex function. In contrast
to strongly correlated systems, where the small -selection probes the
positive part of vertex function, vertex corrections in uncorrelated systems
are essentially negative resulting in an effective reduction of the
superconducting pairing. Our analysis shows that vertex corrections in
nonadiabatic regime can be never disregarded independently of the degree of
electronic correlation in the system.Comment: 4 pages, 3 eps fig
Pauli susceptibility of nonadiabatic Fermi liquids
The nonadiabatic regime of the electron-phonon interaction leads to behaviors
of some physical measurable quantities qualitatively different from those
expected from the Migdal-Eliashberg theory. Here we identify in the Pauli
paramagnetic susceptibility one of such quantities and show that the
nonadiabatic corrections reduce with respect to its adiabatic limit. We
show also that the nonadiabatic regime induces an isotope dependence of ,
which in principle could be measured.Comment: 7 pages, 3 figures, euromacr.tex, europhys.sty. Replaced with
accepted version (Europhysics Letters
Polaronic and nonadiabatic phase diagram from anomalous isotope effects
Isotope effects (IEs) are powerful tool to probe directly the dependence of
many physical properties on the lattice dynamics. In this paper we invenstigate
the onset of anomalous IEs in the spinless Holstein model by employing the
dynamical mean field theory. We show that the isotope coefficients of the
electron effective mass and of the dressed phonon frequency are sizeable also
far away from the strong coupling polaronic crossover and mark the importance
of nonadiabatic lattice fluctuations in the weak to moderate coupling region.
We characterize the polaronic regime by the appearence of huge IEs. We draw a
nonadiabatic phase diagram in which we identify a novel crossover, not related
to polaronic features, where the IEs attain their largest anomalies.Comment: 5 pages, 4 figure
Crowdsourcing and open innovation: a systematic literature review, an integrated framework and a research agenda
The source of competitive advantage in University spin-offs: a case study
University Spin-Offs are incorporated to exploit the knowledge and skills achieved within Universities. Often, their competitive advantage is represented by specific know-how that may be hardly imitated by competitors. In this article we present an analysis of the intellectual capital assets owned by a University Spin-Off using a framework recently introduced
in literature. The framework resorts to a series of structured interviews to key figures within the organization. The interviews are synthesized through the Analytic Network Process and the results are compared using graphical and cost/
benefit analyses. The implementation of the framework creates a useful panel for the planning of investments in intellectual capital assets in order to create value. Moreover, it may emphasize possible discrepancies among interviewees about the
importance of each intellectual capital asset
Electron Spin Dynamics in Impure Quantum Wells for Arbitrary Spin-Orbit Coupling
Strong interest has arisen recently on low-dimensional systems with strong
spin-orbit interaction due to their peculiar properties of interest for some
spintronic applications. Here, the time evolution of the electron spin
polarization of a disordered two-dimensional electron gas is calculated exactly
within the Boltzmann formalism for arbitrary couplings to a Rashba spin-orbit
field. The classical Dyakonov-Perel mechanism of spin relaxation is shown to
fail for sufficiently strong Rashba fields, in which case new regimes of spin
decay are identified. These results suggest that spin manipulation can be
greatly improved in strong spin-orbit interaction materials.Comment: 5 pages, 2 figures -revised versio
ARPES kink is a "smoking gun" for the theory of high-Tc superconductors: dominance of the electron-phonon interaction with forward scattering peak
The ARPES spectra in high-Tc superconductors show four distinctive features
in the quasiparticle self-energy. All of them can be explained consistently by
the theory in which the electron phonon interaction (EPI) with the forward
scattering peak dominates over the Coulomb scattering. In particular, this
theory explains why there is no shift of the nodal kink at 70 meV in the
superconducting state, contrary to the clear shift of the anti-nodal
singularity at 40 meV. The theory predicts a ``knee''-like structure of the
imaginary part of the self-energy, which is phonon dominated for , and shows linear behavior for - due to the Coulomb scattering. Recent ARPES spectra give
that the EPI coupling constant is much larger than the Coulomb one. The
dip-hump structure in the spectral function comes out naturally from the
proposed theory.Comment: 5 pages, 3 figure
How can crowdsourcing help tackle the COVID-19 pandemic? An explorative overview of innovative collaborative practices
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