Observation of non-conventional spin waves in composite fermion ferromagnets

TL;DR

This paper reports the discovery of unconventional low-energy spin wave excitations in composite fermion ferromagnets within fractional quantum Hall systems, revealing complex topological and magnetic interactions.

Contribution

It uncovers novel spin-flip excitonic modes with negative dispersion and roton minima, highlighting intricate mode mixing in composite fermion ferromagnets.

Findings

Unconventional negative dispersion of spin waves at small momenta

Deep roton minima at larger momenta in spin-flip modes

Highly tunable mode couplings affecting topological states

Abstract

We find unexpected low energy excitations of fully spin-polarized composite-fermion ferromagnets in the fractional quantum Hall liquid, resulting from a complex interplay between a topological order manifesting through new energy levels and a magnetic order due to spin polarization. The lowest energy modes, which involve spin reversal, are remarkable in displaying unconventional negative dispersion at small momenta followed by a deep roton minimum at larger momenta. This behavior results from a nontrivial mixing of spin-wave and spin-flip modes creating a spin-flip excitonic state of composite-fermion particle-hole pairs. The striking properties of spin-flip excitons imply highly tunable mode couplings that enable fine control of topological states of itinerant two-dimensional ferromagnets.