Pairing of Fermions Coupled with Spin-1/2 Heisenberg system —exact Diagonalization Study for Mechanism of High-T_c Superconductivity—

A coupled spin-fermion system is investigated as a model of high-T_c superconductivity. By the exact diagonalization technique, the binding energy of two fermions and several correlation functions in the ground state are calculated to study possible pairings of holes in orbitals of p_πp_σ d3z²-r² etc. The bound states of fermions are suggested in two regions in the parameter space. In the small transfer region, it is caused by local defects induced in the antiferromagnet at the fermion sites. The larger transfer region is characterized by the relatively extended disturbance of the antifer-romagnetic correlation of the substrate spins, where the fermion forms singlet-like cloud around it. The latter is a characteristic feature of the antiferromagnetic fermion-spin coupling. The relevance of the latter region to high-T_c superconductivity is discussed. The possibilities of bound states of fermions in Kondo lattices are also investigated.