Conditions under which localized states and resonances are absent in a Heisenberg ferromagnet

The problem of localized bound states of quasiparticles in crystals has aroused interest recently. The relatively simple case of a system of weakly interacting quasiparticles, the number of which remains unchanged, has not yet been examined. An example of such a system is given by magnons in a three-dimensional Heisenberg ferromagnet with large spin at lattice sites (S≫1, N≪2S). In a previous investigation of Schrodinger equations for N particles which interact by weak pair short-range potentials, it has been shown that there are no bound states (the theorem of Jorrio and Carroll. In this work the author gives the analogous result for a system of weakly interacting quasiparticles in a three-dimensional crystal.