Anisotropic superconducting states in a copper peroxide layer.

摘要

The high-temperature superconductors share CuO{dollar}sb2{dollar} layers as structural elements in which the Coulomb repulsion between holes on the Cu site is very large. As a result, we choose to model the system with a three-band Hubbard model including the Cu 3d(x{dollar}sp2{dollar}-y{dollar}sp2{dollar}) and the O 2p(x,y) orbitals. The Coulomb repulsion on the Cu site is treated in the U {dollar}to{dollar} {dollar}infty{dollar} limit using the slave boson formalism. We expand this model's Lagrangian in a large N expansion considering terms up to order 1/N. 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 an interaction between neighboring Cu spins only, the lowest energy solution possesses d(x{dollar}sp2{dollar}-y{dollar}sp2{dollar}) symmetry. The use of a three-band model leads to the possibility of the addition of an interaction between Cu and O spins. The resulting novel "d + idp" superconducting state involves pairing of carriers in Cu orbitals both with themselves and with holes on the O orbitals. This additional pairing removes the node in the d-wave state at T = 0 by an amount which depends on the Cu-O coupling parameter; however, the mixed symmetry state occurs only for a narrow range of coupling parameters. The angle-resolved photo-emission and tunneling results are calculated and compared to experimental findings. As a function of decreasing temperature, symmetry arguments require transition to a d-wave state before transition to the d + idp state. A BCS analysis is performed on a tight-binding model to examine the effect of this transition on tunneling, specific heat, penetration depth, Knight shift and NMR measurements.