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Stability of a skyrmion and interaction of magnons

The stability of a single Belavin-Polyakov (BP) skyrmion in isotropic Heisenberg ferromagnet is studied. Such skyrmion is higher in energy than the uniform ferromagnetic state and is thus metastable. Starting from the lattice model in two spatial dimensions and using Maleyev-Dyson representation for spin operators, we examine the effects of magnon-magnon interaction for two quantities at $T=0$. First we discuss the self-energy corrections to magnon energy. Second we analyze the two-particle Green's function and possible bound states of two magnons. The simplicity of the model makes possible full analytic treatment of all relevant processes. We found that the magnons remain well-defined quasiparticles with a finite lifetime. The bound states of two magnons are suppressed near the skyrmion, although they are not excluded far away from it. A resonance for the magnons of dilational mode in the vicinity of BP skyrmion is also found, which leads to a redistribution of the spectral weight. We conclude that the BP skyrmion as the classical topological object is not destroyed by quantum fluctuations.