A SINGLE OXYGEN HOLE IN THE COPPER-OXIDE PLANES

The strong-coupling behaviour of a single oxygen hole in the copper-oxide planes of the high-Tc superconductors is determined by exact diagonalisation on a finite cluster. The only energy scales are those determined by the hybridisation of the hole via Cu+ or Cu3+ excitations. As is already known for hole motion via virtual Cu3+ excitations the relevant two-band model ('d-p' model) may be mapped onto an effective one-band model, and the constant phase Nagaoka 'ferromagnetism' (S=(N-1)/2, where N is the number of lattice sites) is the groundstate. However, hole motion via virtual Cu+ excitations is more interesting and it is shown that the one-band model is not a useful approximation: the magnetic phase coherence preferred by the propagating hole is a total spin singlet. The stability of the total spin singlet state with respect to the ferromagnetic' state is discussed as the oxygen energy level is raised from the lower Hubbard copper band to the upper Hubbard band. Finally, the consequences of this calculation if super-exchange between the copper spins were included are considered.