On competition between s- and d-symmetry types of the order parameter in high-temperature superconductors

Abstract

It is shown that strong anisotropy of the quasi-two-dimensional electron spectrum in high-temperature superconductors and the competition between attraction and repulsion in the electron–electron interaction can lead either to anisotropic s-, or d-type of Cooper pairing depending on the system parameters. In the case when attraction prevails on the entire Fermi surface (e.g., due to electron–phonon interaction), the sxy- or s*-symmetry in the superconducting order parameter is advantageous from the energy point of view. At the same time, the dx2−y2-symmetry of the gap is observed in the case of repulsion in the entire Brillouin zone (which is typical, for example, of the electron–magnon interaction) as well as in the case of effective attraction on the regions of Fermi surface with the maximum density of states. In the latter case, the superconducting transition temperature Tc is always higher than in the former case, indicating the important role of additional attraction in the mechanism of high-temperature superconductivity.