arXiv subjects

C. Grimaldi

Publications and source records attributed to C. Grimaldi.

63 records · Page 4Linked to original sources

arXiv1999

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 $χ$ one of such quantities and show that the nonadiabatic corrections reduce $χ$ with respect to its adiabatic limit. We show also that the nonadiabatic regime induces an isotope dependence of $χ$, which in principle could be measured.

cond-mat.str-el↗

arXiv1999

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.

cond-mat.supr-con↗

arXiv1999

Spin-orbit scattering in d-wave superconductors

When non-magnetic impurities are introduced in a d-wave superconductor, both thermodynamic and spectral properties are strongly affected if the impurity potential is close to the strong resonance limit. In addition to the scalar impurity potential, the charge carriers are also spin-orbit coupled to the impurities. Here it is shown that (i) close to the unitarity limit for the impurity scattering, the spin-orbit contribution is of the same order of magnitude than the scalar scattering and cannot be neglected, (ii) the spin-orbit scattering is pair-breaking and (iii) induces a small id_xy component to the off-diagonal part of the self-energy.

cond-mat.supr-con↗

arXiv1998

The Small Polaron Crossover: Role of Dimensionality

The crossover from quasi free electron to small polaron in the Holstein model for a single electron is studied by means of both exact and self-consistent calculations in one dimension and on an infinite coordination lattice, in order to understand the role of dimensionality in such a crossover. We show that a small polaron ground-state occurs when both strong coupling ($λ>1$) and multiphonon ($α^2 >1$) conditions are fulfilled leading to different relevant coupling constants ($λ$) in adiabatic and ($α^2$) anti adiabatic region of the parameters space. We also show that the self-consistent calculations obtained by including the first electron-phonon vertex correction give accurate results in a sizeable region of the phase diagram well separated from the polaronic crossover.

cond-mat.str-el↗

arXiv1998

The small polaron crossover: comparison between exact results and vertex correction approximation

We study the crossover from quasi free electron to small polaron in the Holstein model for a single electron by means of both exact and self-consistent calculations in one dimension and on an infinite coordination lattice. We show that the crossover occurs when both strong coupling and multiphonon conditions are fulfilled leading to different relevant coupling constants in adiabatic and anti-adiabatic region of the parameters space. We also show that the self-consistent calculations obtained by including the first electron-phonon vertex correction give accurate results in a sizeable region of the phase diagram well separated from the polaronic crossover.

cond-mat.str-el↗

arXiv1998

Isotope effect on $m^*$ in high-T_c materials due to the breakdown of Migdal's theorem

We show that the inclusion of effects beyond Migdal's limit in the electron-phonon interaction naturally leads to an isotope effect for the effective mass $m^{*}$ of the charge carriers even much before reaching the polaron limit. This is the situation already considered in our approach to nonadiabatic superconductivity. Such a result provides a scenario different from the polaronic one for the interpretation of the recently observed isotope effect on $m^{*}$ in YBa$_{2}$Cu$_{3}$O$_{6+x}$ and La$_{2-x}$Sr$_{x}$CuO$_{4}$.

cond-mat.supr-con↗

arXiv1998

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.

cond-mat.supr-con↗

arXiv1998

Nonadiabatic pairing effects for tight-binding electrons interacting with phonons

The nonadiabatic electron-phonon corrections for the superconducting pairing are investigated for a specific tight-binding model corresponding to a 2d square lattice. This permits to investigate the role of various specific properties like the band filling, nesting effects and a realistic van Hove singularity on the superconducting effective pairing beyond Migdal's limit. The main results are: (i) Starting from a momentum independent electron-phonon coupling the nonadiabatic effects lead to an effective pairing which is strongly dependent on frequency and momentum. (ii) If instead the electron-phonon coupling is mainly forward (as due to correlation effects) the resulting pairing results to be strongly enhanced. These results confirm but also extend the simplified scheme used up to now to compute these properties. In this respect our results link the nonadiabatic effects to the specific properties of realistic materials.

cond-mat.supr-con↗

arXiv1998

Role of the superconducting gap opening on vertex corrections

The nonadiabatic effects due to the breakdown of Migdal's theorem in high-$T_c$ superconductors are strongly affected by the opening of the superconducting gap. Here we report how the momentum-frequency dependence of vertex corrections is modified by the gap. A general effect is that the positive region of the vertex corrections is increased leading to an enhancement of their relevance. This has a number of physical consequences that we discuss in details in relation to specific experiments.

cond-mat.supr-con↗