Holes in Two-Dimensional CuO₂System –Pairing Mechanism Emerging from Small-Cluster Studies–

The strongly correlated two-dimensional CuO 2 model is studied to identify the pairing mechanism of holes. Detailed studies of the ground state of clusters up to 30 atoms (10 Cu and 20 O atoms) reveal a binding of two holes doped on oxygen sites for particular values of parameters, which appear relevant to high- T c superconductors. The wave function for the ground state shows that a doped hole on oxygen couples strongly with two neighboring Cu spins, which in turn causes a large magnetic distortion in the surrounding Cu spin system. The interaction between two holes becomes attractive through a constructive interference of the magnetic distortions.