OUT-OF-PLANE INSTABILITY AND ELECTRON-PHONON CONTRIBUTION TO S-WAVE AND D-WAVE PAIRING IN HIGH-TEMPERATURE SUPERCONDUCTORS - LDA LINEAR-RESPONSE CALCULATION FOR DOPED CACUO2 AND A GENERIC TIGHT-BINDING MODEL

The equilibrium structure, energy bands, phonon dispersions, and s- an d d-channel electron-phonon interactions (EPIs) are calculated for the infinite-layer superconductor CaCuO2 doped with 0.12 holes per CuO2. The LDA and the linear-response full-potential LMTO method were used. In the equilibrium structure, oxygen is found to buck le slightly out Of the plane and, as a result, the characters of the energy bands near EF are found to be similar to those of other optimally doped HTSCs. F or the EPI we find lambda(s) similar to 0.4, in accord with previous L DA calculations for YBa2Cu3O7. This supports the common belief that th e EPI mechanism alone is insufficient to explain HTSC. lambda(x2-y2) i s found to be positive and nearly as large as lambda(s). This is surpr ising and indicates that the EPI could enhance some other d-wave pairi ng mechanism. Like in YBa2Cu3O7. the buckling modes contribute signifi cantly to the EPI, although these contributions are proportional to th e static buckling and would vanish for flat planes. These numerical re sults can be understood from a generic tight-binding model originally derived from the LDA bands of YBa2Cu3O7. In the future, the role of an harmonicity of the buckling-modes and the influence of the spin-fluctu ations should be investigated.