Non-linearity and anisotropy of electron-phonon coupling in high-T-c oxides

The calculated electronic structures of YBa2Cu3O7, HgBa2CuO4 and Bi2Sr2CaCu 2O8 are found to be sensitive to lattice distortions. For large distortion amplitudes of some modes, the calculations show that the electron-phonon co upling (lambda) will be much enhanced, so that the coupling behaves non-lin early with the distortion. The c-axis transport properties vary with the di stortion from being almost insulating and thereby weakly screened, to being more conducting with rather metallic screening. The calculated anisotropy lambda(k, k') over the Fermi surface (FS) of the Hg-compound, is found to b e of "forward scattering" type, which is favorable for d-wave superconducti vity.