Anomalously large damping of long-wavelength quasiparticles caused by long-range interaction
Abstract
We demonstrate that long-range interaction in a system can lead to a very strong interaction between long-wavelength quasiparticles and make them heavily damped. In particular, we discuss magnon spectrum using 1/S expansion in 3D Heisenberg ferromagnet (FM) with arbitrary small dipolar forces at T<<TC. We obtain that a fraction of long-wavelength magnons with energies ek<T has anomalously large damping Gk (ratio Gk/ek reaches 0.3 for certain k). This effect is observed both in quantum and classical FMs. Remarkably, this result contradicts expectation of the quasiparticle concept according which a weakly excited state of a many-body system can be represented as a collection of weakly interacting elementary excitations. Particular materials are pointed out which are suitable for corresponding experiments.