Small-q electron-phonon scattering in all crystalline 'unconventional' superconductors

All unconventional superconductors belong to families of materials of which the majority are insulators. We argue that, they 'survive' in the metallic regime because they are close to instabilities at small-q (phase separatio n) contrary to the insulators which are 'victims' of instabilities at large -q. The proximity to a small-q instability is shown to be a plausible gener ic feature of Fermi-Liquid systems with short-ranged Coulomb correlations ( Hubbard-like), coupled adiabatically to phonons. The presence of antiferrom agnetic phases in the insulator regions of the phase diagrams illustrates t he short-range character of the Coulomb repulsion, a necessary ingredient f or our picture. We show that the resulting small-q electron-phonon scatteri ng may naturally lead to unconventional superconductivity of d-wave or othe r gap symmetry. The superconducting gap symmetry is controlled by the Coulo mb pseudopotential. This provides a generic mechanism for phonon mediated u nconventional superconductivity that could be relevant for all crystalline unconventional superconductors. What could differentiate cuprates and lead to high-ir, is possibly the addition of the Anderson interlayer tunneling p rocesses which correspond effectively to q = 0 pairing. (C) 1999 Elsevier S cience B.V. All rights reserved.