Anomalously large damping of long-wavelength quasiparticles caused by long-range interaction

TL;DR

This paper shows that long-range interactions in 3D ferromagnets cause significant damping of long-wavelength magnons, challenging traditional quasiparticle assumptions and suggesting new experimental directions.

Contribution

It reveals that long-range dipolar forces induce anomalously large damping of magnons at long wavelengths in 3D ferromagnets, a novel insight into quasiparticle behavior.

Findings

Long-range interactions cause strong magnon damping.

Damping ratio G_k/e_k can reach 0.3 for certain wavevectors.

Effect observed in both quantum and classical ferromagnets.

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<<T_C. We obtain that a fraction of long-wavelength magnons with energies e_k<T has anomalously large damping G_k (ratio G_k/e_k 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.