Origin of the maximal critical temperature disparities in one-layer cuprate superconductors

Abstract: Recently a phonon exchange d - wave pairing mechanism in cuprates was proposed. The phonons are the lateral apical oxygen atoms vibrations. They generate the attractive pairing potential peaked at X point of the Brillouin zone, V (k) = exp [-2kda], where da is distance from the CuO planes. The model explains a rather paradoxical well known negative correlation of the optimal doping critical temperature Tcmax with da and increase of Tcmax with pressure. However the large disparities in Tcmax, especially in one - layer cuprate superconductors, from 39K for La2-xSrxCuO4, to 95K for HgBa2CuO4+x, cannot be attributed by differencies in da. Other important material parameters include the hopping amplitudes t; t0 and the on site Coulomb repulsion U. It is shown (within weak coupling)that Tcmax is highest for materials close to the topological (Lifshitz) transition from an open to a close Fermi surface. The transition occurss for t0 = -0.19t and rather small values of effective value of U = 2t at optimal doping. Analytic expressions for Tcmax are derived both near criticality and away from it.