Jahn-Teller-Effect Induced Superconductivity in Copper Oxides: Theoretical Developments

This paper is dedicated to Professor K. Alex Muller. In 1986, 30 years ago, high temperature superconductivity was discovered by George Bednorz and Alex Muller in copper oxides, by their epoch-making insight that higher superconducting transition temperature could be achieved by strong interactions with Jahn-Teller (JT) distortion of the constituting CuO6 octahedron unit. Paying special attention to the experimental fact that the doped holes induce a new kind of deformation of CuO6 octahedrons in copper oxide superconductors called the anti-JT effect, we developed a non-rigid band theory for calculating the energy bands of holes traversing in the deformed potential field. Thus, we obtained a complete picture of the doping-induced alteration in the electronic structure of Sr-doped La2CuO4, La2−xSrxCuO4 (LSCO), the simplest copper oxide superconductor. This description has brought the following major advances: The Fermi surface structure of this cuprate in the underdoped region consists of Fermi pockets in the antinodal region and Fermi arcs in the nodal region, and thus that the origin of a so-called pseudogap is closely related to the existence of Fermi pockets. Moreover, we have shown that the carriers on the Fermi pockets contribute to the phonon mechanism in d-wave superconductivity. Further it is shown that the strong electron–phonon coupling in copper oxides is due to the orbital near- degeneracy, and this gives rise to high temperature superconductivity [The present paper is based on our recent paper uploaded in arXiv 1608.08338].