Fermi blockade of the strong electron-phonon interaction in optimally doped high temperature superconductors

We study how manifestations of strong electron-phonon interaction (EPI) depend on the carrier concentration by solving the two-dimensional Holstein model for the spin-polarized fermions using an approximation free bold-line diagrammatic Monte Carlo (BDMC) method. We show that the strong EPI, obviously present at very small Fermion concentration, is masked by the Fermi blockade effects and Migdal's theorem to the extent that it manifests itself as moderate one at large carriers densities. Suppression of strong EPI fingerprints is in agreement with experimental observations in doped high temperature superconductors.

Fermi Blockade of the Electron-phonon Interaction: Why Strong Coupling Effects May Not Be Seen in Optimally Doped High Temperature Superconductors

We study how manifestations of strong electron-phonon interaction (EPI) depend on the carrier concentration by solving the two-dimensional Holstein model for the spin-polarized fermions using an approximation free bold-line diagrammatic Monte Carlo (BDMC) method. We show that the strong EPI, obviously present at very small Fermion concentration, is masked by the Fermi blockade effects and Migdal's theorem to the extent that it manifests itself as moderate one at large carriers densities. Suppression of strong EPI fingerprints is in agreement with experimental observations in doped high temperature superconductors.

A SURVEY OF NONADIABATIC SUPERCONDUCTIVITY IN CUPRATES AND FULLERIDES

High-Tc 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.

The Effects of the Breakdown of Migdal's Theorem in Various Superconductors

In this work we investigate some effects of the breakdown of Migdal's theorem in high and low-Tc superconductors. In the cuprates the electronic bandwidth has a very low value compared to the characteristic phonon energy and, in order to reproduce the experimental Tc and tunneling data in the framework of the Eliashberg theory for strong electron-boson interaction it is necessary to use large values of the coupling constant. Our purpose is to numerically quantify the correlation between the amount of violation of the Migdal's theorem and the renormalization of the coupling constant λ, of the gap Δ and of Tc by examining different kinds of superconducting materials. For this task we use the theoretical approach developed by Grimaldi, Pietronero and Strässler [PRB 52, 10516 & 10530, 1995], together with the real-axis direct solution of the Eliahsberg equations in s -wave pair symmetry.