Topological asymmetry in the damping-pairing contribution of electron-boson scattering

We make a harmonic analysis of the pairing and damping contribution of a finite $k$ range isotropic electron-phonon (or other boson) scattering in an anisotropic two-dimensional electronic system. We show that the pairing contribution of the anisotropic part of the electronic system can be much larger than its damping contribution enhancing significantly T_c. The higher is the order of the harmonic of the electronic anisotropy, smaller is its damping contribution and higher can be the asymmetry in its damping-pairing contribution. This could explain the puzzle of a much broader quasiparticle peak in the n-doped than in the p-doped cuprates, their smaller T_c's being also attributed to larger damping effects.Comment: LATEX file and 3 Postscript figure

Localization corrections and small-q phonon-mediated unconventional superconductivity in the cuprates

Taking into account the first localization corrections in the electron-impurity self-energy we study the effect of non-magnetic impurities on unconventional superconductivity (SC) mediated by small-q electron-phonon scattering. We show that when van Hove singularities are close to the Fermi level making the electronic system anisotropic as in the high-$T_c$ oxides, both the d-wave and s-wave states are sensitive to non-magnetic impurities and beyond a critical impurity concentration SC disappears in {\it both gap symmetry channels}. Impurity doping may induce a first order transition from d-wave to s-wave SC, but no saturation of the impuruty effect is reported due to the intrinsically anisotropic character of the localization corrections in this context.Comment: 4 pages and 6 figure

Cross-over from BCS superconductivity to Bose condensation and High-Tc Superconductors

We consider the Eliashberg theory in the coupling region where some fundamental qualitative deviations from the conventional BCS-like behaviour begin to appear. These deviations are identified as the onset of a cross-over from BCS superconductivity to Bose condensation. We point out that the beginning of this cross-over occurs when the gap $\Delta_g$ becomes comparable to the boson energies $\Omega_{ph}$. This condition is equivalent to the condition of Ref. \cite{Strinati} $k_F\xi\approx 2\pi$ and traduces the physical constraint that the distance the paired electron covers during the absorbtion of the virtual boson, cannot be larger than the coherence length. The frontier region of couplings is of the order of $\lambda\approx 3$, and high-$T_c$ materials are concerned. A clear qualitative indication of the occurence of a cross-over regime should be a dip structure above the gap in the density of states of excitations. Comparing our results with tunneling and photoemission experiments we conclude that high-$T_c$ materials (cuprates and fullerides) are indeed at the beginning of a cross-over from BCS superconductivity to Bose condensation, even though the fermionic nature still prevails. Taking into account the analysis of Ref. \cite{Strinati}, we predict a dip structure in heavy fermion and organic superconductors. Non-adiabatic effects beyond Migdal's theory are considered and give insight on the robustness of Eliashberg theory in describing qualitatively this cross-over regime, although for the quantitative interpretation of the results the inclusion of non-adiabatic corrections can be important.Comment: 37 pages, latex , 16 figures available upon request to [email protected]

Small-q electron-phonon scattering and linear dc resistivity in high-T_c oxides

We examine the effect on the DC resistivity of small-q electron-phonon scattering, in a system with the electronic topology of the high-T_c oxides. Despite the fact that the scattering is dominantly forward, its contribution to the transport can be significant due to ``ondulations'' of the bands in the flat region and to the umpklapp process. When the extended van-Hove singularities are sufficiently close to $E_F$ the acoustic branch of the phonons contribute significantly to the transport. In that case one can obtain linear $T$ dependent resistivity down to temperatures as low as 10 K, even if electrons are scattered also by optical phonons of about 500 K as reported by Raman measurements.Comment: LATEX file and 4 Postscript figure

Chirality Induced Tilted-Hill Giant Nernst Signal

We reveal a novel source of giant Nernst response exhibiting strong non-linear temperature and magnetic field dependence including the mysterious tilted-hill temperature profile observed in a pleiad of materials. The phenomenon results directly from the formation of a chiral ground state, e.g. a chiral d-density wave, which is compatible with the eventual observation of diamagnetism and is distinctly different from the usual quasiparticle and vortex Nernst mechanisms. Our picture provides a unified understanding of the anomalous thermoelectricity observed in materials as diverse as hole doped cuprates and heavy-fermion compounds like URu2Si2.Comment: 5 pages and 4 figures, Final version accepted by Phys. Rev. Let

The boson mediators of high-Tc superconductivity: phonons versus composite bosons from the superconducting phenomenology

We address the question of whether boson mediators of high-$T_c$ superconductivity are composite (electronic) or independent phonons. For s-wave superconductivity we show from the available experiments that the hypothesis of composite bosons is rather unlikely. Our analysis points naturally towards phonon mediators. In addition we point out that the eventual presence of a peak in the temperature dependence of the microwave conductivity while the Hebel-Slichter peak is absent in the temperature dependence of the NMR relaxation rate, can be understood within a phonon mechanism if one takes into account the modulation of the electron-phonon coupling (predominance of forward scattering) induced by Coulomb correlation of the carriers.Comment: Accepted for publication in the Rapid Communications section of Physical Review B, 4 pages latex (revtex),4 figures available upon request to [email protected]