Electron Correlation and the c-axis Dispersion of Cu d_z^2: a New Band Structure for High Temperature Superconductors

TL;DR

This paper explores how electron correlation and c-axis dispersion influence the band structure of cuprate superconductors, revealing new band crossings and potential interband pairing mechanisms relevant to high-temperature superconductivity.

Contribution

It introduces c-axis dispersion into the Hubbard model, showing the emergence of a 3D z^2 band and band crossings that support interband pairing theories.

Findings

z^2 band exhibits anisotropic 3D character

A new hole-like surface appears in the z^2 band with doping

Band crossing at the Fermi level enables interband Cooper pairing

Abstract

Previously we showed the major effect of electron correlation in the cuprate superconductors is to lower the energy of the Cu d_x^2-y^2/O p_sigma (x^2-y^2) band with respect to the Cu d_z^2/O' p_z (z^2) band. In our 2D Hubbard model for La_1.85Sr_0.15CuO_4 (LaSCO), the z^2 band is narrow and crosses the standard x^2-y^2 band just below the Fermi level. In this work, we introduce c-axis dispersion to the model and find the z^2 band to have considerable anisotropic 3D character. An additional hole-like surface opens up in the z^2 band at (0,0,2pi/c) which expands with doping. At sufficient doping levels, a symmetry allowed x^2-y^2/z^2 band crossing along the (0,0)-(pi,pi) direction of the Brillouin zone appears at the Fermi level. At this point, Cooper pairs between the two bands (e.g. (k uparrow x^2-y^2/k downarrow z^2)) can form, providing the basis for the Interband Pairing Theory of superconductivity in these materials.