Fermion-Magnon Bound State in a Spin-Fermion Model

Abstract

Spin-fermion model, or a generalized Kondo lattice model, in the phase where the localized spins are antiferromagnetically ordered, is studied. We obtain the bound state wave function of a single fermion and a single magnon with corrections of spin polaron analogous to the ordinary polaron, under some simplifying conditions analogous to Cooper's problem of the superconductivity. It is found that each fermion-magnon bound state extends over the whole space unlike the localized bound state in the single impurity Kondo model. It is also shown that there exists a finite critical value of the Kondo coupling for the bound state formation in the presense of the three dimensional interactions. We discuss the instability of the fermi surface.