Zero-magnon sound in quantum Heisenberg ferromagnets

TL;DR

This paper demonstrates the existence of zero-magnon sound as a well-defined quasi-particle in low-dimensional quantum Heisenberg ferromagnets at low temperatures, using a functional renormalization group approach.

Contribution

It introduces the concept of zero-magnon sound in quantum ferromagnets and analyzes its behavior across different dimensions with a novel theoretical approach.

Findings

Zero-magnon sound exists in $D\leq ext{2}$ dimensions with a linear dispersion.

In $D>2$, the spectral feature broadens into a zero-frequency hump.

The study uses a functional renormalization group method to analyze the dynamic structure factor.

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 $D\leq 2$ exhibits a well-defined quasi-particle 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.