How a skyrmion can appear both massive and massless

TL;DR

This paper explores how magnetic skyrmions can exhibit both massless and massive behaviors depending on the modeling approach, revealing conditions where Larmor oscillations are suppressed and proposing a new coupled-particle model.

Contribution

It introduces a novel mechanical model of skyrmions as two coupled massless particles, clarifying the conditions under which skyrmions appear massive or massless.

Findings

Guiding-center skyrmion dynamics are massless.

Certain external forces suppress Larmor oscillations.

A coupled massless particle model explains skyrmion behavior.

Abstract

When a magnetic skyrmion is modeled as a point particle, its dynamics depends on the precise definition of the skyrmion center. The guiding-center position, defined as the first moment of the skyrmion density, exhibits Thiele's massless dynamics; position based on the first moment of magnetization component $m_z$ shows Larmor oscillations characteristic of a massive particle. We show that, even with the latter definition, the Larmor oscillations may be absent for certain types of external forces such as adiabatic spin torque. We offer an alternative mechanical model of a skyrmion featuring two coupled massless particles.