Conventional Superconductivity in Fe-Based Pnictides: the Relevance of Intra-Band Electron-Boson Scattering

Various recent experimental data and especially the large Fe-isotope effect point against unconventional pairings, since the large intra-band impurity scattering is strongly pair-breaking for them. The strength of the inter-band impurity scattering in some single crystals may be strong and probably beyond the Born scattering limit. In that case the proposed s(+-) pairing (hole(h)- and electron(el)-gaps are of opposite signs) is suppressed but possibly not completely destroyed. The data imply that the intra-band pairing in the h- and in the el-band, which are inevitably due to some nonmagnetic el-boson interaction (EBI), must be taken into account. EBI is either due to phonons (EPI) or possibly due to excitons (EEI), or both are simultaneously operative. We discuss their interplay briefly. The large Fe-isotope effect favors the EPI and the s(+) pairing (the h- and el-gaps are in-phase).Comment: 7 pages, no figures, explanations and argumentations improved, references adde

Bosonic Spectral Function and The Electron-Phonon Interaction in HTSC Cuprates

In Part I we discuss accumulating experimental evidence related to the structure and origin of the bosonic spectral function in high-temperature superconducting (HTSC) cuprates at and near optimal doping. Some global properties of the spectral function, such as number and positions of peaks, are extracted by combining optics, neutron scattering, ARPES and tunnelling measurements. These methods give convincing evidence for strong electron-phonon interaction (EPI) with the coupling constant between 1-3 in cuprates near optimal doping. Here we clarify how these results are in favor of the Eliashberg-like theory for HTSC cuprates near optimal doping. In Part II we discuss some theoretical ingredients - such as strong EPI, strong correlations - which are necessary to explain the experimental results related to the mechanism of d-wave pairing in optimally doped cuprates. These comprise the Migdal-Eliashberg theory for EPI in strongly correlated systems which give rise to the forward scattering peak. The latter is further supported by the weakly screened Madelung interaction in the ionic-metallic structure of layered cuprates. In this approach EPI is responsible for the strength of pairing while the residual Coulomb interaction (by including spin fluctuations) triggers the d-wave pairing.Comment: 59 pages, 38 figures, review articl

Forward Electron-Phonon Scattering in Normal and Superconducting States

The sharp forward electron-phonon $(FEP)$ and impurity $(FIS)$ scattering change the normal and superconducting properties significantly. The pseudo-gap like features are present in the density of states for $\omega <\Omega$, where $\Omega$ is the phonon frequency. The superconducting critical temperature $T_c$, due to the $FEP$ pairing, is linear with respect to the electron-phonon coupling constant. The $FIS$ impurities are pair weakening for $s-$ and $d-wave$ pairing.Comment: 3 pages, 1 figur

Elastic forward scattering in the cuprate superconducting state

We investigate the effect of elastic forward scattering on the ARPES spectrum of the cuprate superconductors. In the normal state, small angle scattering from out-of-plane impurities is thought to broaden the ARPES spectral response with minimal effect on the resistivity or the superconducting transition temperature $T_c$. Here we explore how such forward scattering affects the ARPES spectrum in the d-wave superconducting state. Away from the nodal direction, the one-electron impurity scattering rate is found to be suppressed as $\omega$ approaches the gap edge by a cancellation between normal and anomalous scattering processes, leading to a square-root-like feature in the spectral weight as $\omega$ approaches -\Delta_\k from below. For momenta away from the Fermi surface, our analysis suggests that a dirty optimally or overdoped system will still display a sharp but nondispersive peak which could be confused with a quasiparticle spectral feature. Only in cleaner samples should the true dispersing quasiparticle peak become visible. At the nodal point on the Fermi surface, the contribution of the anomalous scattering vanishes and the spectral weight exhibits a Lorentzian quasiparticle peak in both energy and momentum. Our analysis, including a treatment of unitary scatterers and inelastic spin fluctuation scattering, suggests explanations for the sometimes mysterious lineshapes and temperature dependences of the peak structures observed in the \BSCCO system.Comment: 12 pages, 14 figure

Unconventional superconducting pairing by conventional phonons

The common wisdom that the phonon mechanism of electron pairing in the weak-coupling Bardeen-Cooper-Schrieffer (BCS) superconductors leads to conventional s-wave Cooper pairs is revised. An inevitable anisotropy of sound velocity in crystals makes the phonon-mediated attraction of electrons non-local in space providing unconventional Cooper pairs with a nonzero orbital momentum in a wide range of electron densities. As a result of this anisotropy quasi-two dimensional charge carriers undergo a quantum phase transition from an unconventional d-wave superconducting state to a conventional s-wave superconductor with more carriers per unit cell. In the opposite strong-coupling regime rotational symmetry breaking appears as a result of a reduced Coulomb repulsion between unconventional bipolarons dismissing thereby some constraints on unconventional pairing in the Bose-Einstein condensation (BEC) limit. The conventional phonons, and not superexchange, are shown to be responsible for the d-wave symmetry of cuprate superconductors, where the on-site Coulomb repulsion is large.Comment: 4 pages, 4 figures, more references adde