Title: GAP SYMMETRY AND CHARGE DENSITY EXCITATIONS IN HIGH-T_c SUPERCONDUCTORS WITH EXTENDED SADDLE POINTS IN ELECTRON SPECTRUM
Authors: E.A.Pashitskii, V.I.Pentegov, A.V.Semenov (Institute of Physics, National Academy of Sciences of Ukraine, 252650, Kiev, Ukraine)

It is shown that the strong anisotropy of the one-particle electron spectrum, due to the presence of extended saddle-point features (ESPF) close to the Fermi level in the hole-type cuprates YBCO and BSCCO, leads to the occurrence of a low-frequency peak in the spectral function of the charge density fluctuations due to the presence of acoustic plasmon branch in the collective electron spectrum. The retarded anisotropic electron-plasmon interaction leads to the suppression of the static screened Coulomb repulsion for small transferred momenta and, consequently, to the effective attraction between electrons in the $d_{x^2-y^2}$-wave channel of the Cooper pairing of current carriers. Breaking of $C_{4v}$ symmetry in YBCO crystals leads to a possibility of a change of $ d_{x^2-y^2} $-wave symmetry of the gap to a mixed $s-d$ gap symmetry for singlet Cooper pairs or to a $p$-wave gap symmetry for triplet pairs.

Comments: Figs. 5, Refs. 13, Tabs. 0.