Calculation of angle-resolved photoemission and tunneling for a CuO₂ layer in the normal and superconducting states

We represent the normal-state electronic structure of a CuO₂ layer in terms of a three-band model having an infinite Cu intrasite Coulomb repulsion. We express the Lagrangian for this model using a slave-boson formalism and approximate it in a large-N expansion to order 1/N in the zero-temperature limit. The angle-resolved spectral weight determined from the resulting Green’s functions suggests that within this picture higher-order corrections in 1/N are necessary for good agreement with the corresponding angle-resolved photoemission data. We phenomenologically add spin-dependent Heisenberg interactions between neighboring Cu sites and neighboring Cu and O sites. These interactions form the basis of a nonretarded calculation of the superconducting state. For the case of an interaction between neighboring Cu spins only, the lowest-energy solution possesses d(x²-y²) symmetry. The use of a three-band model leads to the possibility of the addition of the interaction between Cu and O spins. The resulting d+idp superconducting state involves pairing of carriers in Cu orbitals both with themselves and with holes on the O orbitals. This additional pairing will remove the node in the d-wave state at T=0 by an amount that depends on the Cu-O coupling parameter; however, the mixed-symmetry state occurs only for a narrow range of coupling parameters. The angle-resolved photoemission and tunneling results are calculated and compared to experimental findings.