Pairing of Fermious Coupled with Spin-1/2 Heisenberg System –Exact Diagonalization Study for Mechanism of High-Tc Superconductivity–

Abstract

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 _π , p _σ , d _{3 z ² - r ²} 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 antiferomagnetic 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.