Zero-magnon sound in quantum Heisenberg ferromagnets

Abstract

Using a functional renormalization-group approach, we show that at low temperatures and in a certain range of magnetic fields, the longitudinal dynamic structure factor of quantum Heisenberg ferromagnets in dimensions $D\ensuremath{\le}2$ exhibits a well-defined quasiparticle peak with linear dispersion that we identify as zero-magnon sound. In $D>2$, the larger phase space available for the decay into transverse spin waves leads only to a broad hump centered at zero frequency whose width scales linearly in momentum.

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