High-Tc Superconductivity Driven by Overscreening of Coulomb Interaction

The recent neutron scattering experiments in high-Tc superconductors show that the frequencies of the longitudinal optical (LO) phonon of a stretching mode is strongly softened and the frequencies of the LO and transverse optical (TO) phonons are inverted in certain wave number regions. This fact suggests that the electronic dielectric function becomes negative because of some correlation effect in these regions, and that the Coulomb interaction in the LO phonon is overscreened. In this case, theory suggests that the attractive interaction mediated by optical phonons is highly enhanced. This fact will be a capable of mechanism of high-Tc superconductivity. There the flat bands appear just below the Fermi level along the Kx and Ky axes as observed by angle-resolved photoemissions. A calculation taking account of the above effects shows that the symmetry of the order parameter is of the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$d_{x^2 - y^2 } $$ \end{document} wave and the superconducting transition temperature can exceed 200 K.