SELF-TRAPPING AT A NONLINEAR IMPURITY IN A HEISENBERG FERROMAGNETIC CHAIN

We study the problem of a linearized Heisenberg ferromagnetic chain in the presence of a nonlinear impurity. The exchange coupling of the im purity to its nearest-neighbour spins is anisotropic and expanded to t he fourth order of the magnon operators, while the rest of the couplin gs are isotropic and linearized. We show that, in a parameter region, a magnon bound state trapped at the impurity exists with energy higher than the magnon band of the linearized chain. The calculation of the transmission coefficient of plane waves through such an impurity revea ls the existence of completely unscattered modes with transmission equ al to unity. We also demonstrate a corresponding dynamical self-trappi ng phenomenon at the non-linear impurity by a numerical study of the t ime-integrated autocorrelation function.