THEORY OF HIGH-TEMPERATURE SUPERCONDUCTIVITY IN CUPRATES - V - MECHANISM OF SUPERCONDUCTIVITY

A possibility of phonon-mediated interaction as a mechanism of high-T- c superconductivity is discussed. Because of different spatial distrib ution of Bloch wave function for up-spin particle and down-spin partic le in the present model, the effective pair interaction which is deriv ed from electron-phonon interaction is shown to have significant momen tum-transfer dependence that is quite different from ordinary phonon-m ediated interaction. This characteristic of pair interaction together with the geometric feature of Fermi-surface derived in a previous pape r III leads to a d(x)2(-y)2 symmetric gap state which is consistent wi th experimental results. Since d-wave state has much smaller Coulomb r epulsion term than s-wave state, strong electron-phonon coupling in th e present model is expected to cause the occurrence of high-temperatur e superconductivity. However, in the present model, it is shown that a factor that reduces superconducting transition temperature appear as a result of a fluctuation effect of antiferromagnetic (AF) order of lo calized Cu spins. The factor lambda(s) which should be proportional to AF correlation length lambda(AF), determines the time scale tau(s) wh ich eliminates the contribution from the retarded effective pair inter action of time argument larger than tau(s). Existence of length scale lambda(s) explains why T-c goes to zero as increasing hole carrier to some extent, and why nonmagnetic impurity such as Pb reduces T-c stron gly. lambda(s) also explains the finite density of zero-gap states obs erved in overdoped regime or samples with non-magnetic impurities.