Origin of multiple skyrmion phases in EuAl4

TL;DR

This study reveals that electronic Fermi-surface nesting, rather than Dzyaloshinskii-Moriya interaction, drives the formation of multiple skyrmion phases in EuAl4, a centrosymmetric material.

Contribution

The paper demonstrates that nesting-induced RKKY interactions explain the complex magnetic phases in EuAl4, challenging the traditional view of DM interaction's role in skyrmion formation.

Findings

Identified an x-dependent Lifshitz transition leading to a new Fermi-surface pocket.

Found multiple nesting vectors matching the periodicities of observed skyrmion lattices.

Suggested a common electronic origin for various magnetic orders in EuAl4.

Abstract

The Dzyaloshinskii-Moriya (DM) interaction has been considered essential for skyrmion formation, however, the discovery of skyrmion lattices (SkLs) in nominally centrosymmetric materials where the DM interaction is forbidden, such as Eu(Ga$_{1-x}$Al$_x$)$_4$, has challenged this established view. Recent structural investigations of Eu(Ga$_{1-x}$Al$_x$)$_4$ have further complicated this issue by revealing that the charge-density wave breaks local symmetry, theoretically allowing DM interaction. This raises a fundamental question: are the complex magnetic phases driven by the DM interaction or by alternative mechanisms? Here, using soft-x-ray angle-resolved photoemission spectroscopy, we determine the three-dimensional bulk electronic structure of Eu(Ga$_{1-x}$Al$_x$)$_4$, and elucidate the electronic origins of its rich magnetic orders. We directly observe an x-dependent Lifshitz transition leading to the emergence of a Fermi-surface pocket. Importantly, multiple nesting vectors derived from this pocket match the symmetries and periodicities of the multiple SkLs. Moreover, these nesting vectors can also account for other magnetic orders, such as the zero-field helical magnetism, suggesting a common electronic origin of the complex magnetic phases. These findings suggest that competing nesting-induced Ruderman-Kittel-Kasuya-Yosida interactions and their engineering can generate and control various SkLs and related topological spin textures.