Magnetic topological semimetals provide a fertile ground for exploring how long-range magnetic order can alter electronic band structures and generate novel quasiparticles such as Weyl fermions. Here, we investigate the coupled magnetic and electronic structure of single-crystalline EuAgAs, a hexagonal pnictide whose magnetic ground state has remained elusive. Using neutron diffraction and resonant elastic X-ray scattering, we identify an unusual magnetic ordering sequence with two successive phase transitions at TN1=12 K and TN2=8 K. We observe two slightly different magnetic propagation vectors, one associated with TN1 and the other with TN2. Spherical neutron polarimetry reveals that the magnetic structure is a transverse helix aligned along the c axis with a period that is approximately twice the c lattice parameter. First-principles calculations for the helical phase predict subtle band folding effects and the emergence of effective higher-order Weyl nodes. These topological features appear near the calculated Fermi energy EF which, however, lies above the position of EF obtained from angle-resolved photoemission spectroscopy so could not be probed in this study.
@article{arxiv.2605.19347,
title = {Higher-order Weyl nodes driven by helical magnetic order in EuAgAs},
author = {Jian-Rui Soh and Ziming Zhu and Louis Withers and J. Alberto Rodríguez-Velamazán and Timur K. Kim and Oscar Fabelo and Anne Stunault and Daniil Yevtushynsky and Dharmalingam Prabhakaran and Shengyuan A. Yang and Andrew T. Boothroyd},
journal= {arXiv preprint arXiv:2605.19347},
year = {2026}
}