Noninteger-spin magnonic excitations in untextured magnets

TL;DR

This paper theoretically shows that dipole-dipole interactions in magnets can produce bosonic excitations with non-integer average spins, revealing exotic spin states and quasiparticles beyond traditional magnon models.

Contribution

It introduces the concept of non-integer-spin magnonic excitations caused by dipolar interactions in uniformly magnetized systems, expanding understanding of magnetic quasiparticles.

Findings

Dipole-dipole interactions lead to eigen-excitations with average spin from zero to above .

Eigenmodes in easy-axis antiferromagnets are spin-zero quasiparticles.

Large magnetic fields restore the conventional  magnons.

Abstract

Interactions are responsible for intriguing physics, e.g. emergence of exotic ground states and excitations, in a wide range of systems. Here we theoretically demonstrate that dipole-dipole interaction leads to bosonic eigen-excitations with average spin ranging from zero to above $\hbar$ in magnets with uniformly magnetized ground states. These exotic excitations can be interpreted as quantum coherent conglomerates of spin $\hbar$ magnons, the eigen-excitations when the dipolar interactions are disregarded. We further find that the eigenmodes in an easy-axis antiferromagnet are spin-zero quasiparticles instead of the widely believed spin $\pm \hbar$ magnons. The latter re-emerge when the symmetry is broken by a sufficiently large applied magnetic field. The average spin greater than $\hbar$ is accompanied by vacuum fluctuations and may be considered to be a weak form of frustration.