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arXiv · 2605.19347

Higher-order Weyl nodes driven by helical magnetic order in EuAgAs

Jian-Rui Soh·Ziming Zhu·Louis Withers·J. Alberto Rodríguez-Velamazán·Timur K. Kim·Oscar Fabelo·Anne Stunault·Daniil Yevtushynsky·Dharmalingam Prabhakaran·Shengyuan A. Yang·Andrew T. Boothroyd
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Abstract

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 $T_\mathrm{N1} = 12$ K and $T_\mathrm{N2} = 8$ K. We observe two slightly different magnetic propagation vectors, one associated with $T_\mathrm{N1}$ and the other with $T_\mathrm{N2}$. 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 $E_{\mathrm{F}}$ which, however, lies above the position of $E_{\mathrm{F}}$ obtained from angle-resolved photoemission spectroscopy so could not be probed in this study.

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Jian-Rui Soh, Ziming Zhu, Louis Withers, J. Alberto Rodríguez-Velamazán, Timur K. Kim, Oscar Fabelo, Anne Stunault, Daniil Yevtushynsky, Dharmalingam Prabhakaran, Shengyuan A. Yang, Andrew T. Boothroyd. 2026-05-19. Higher-order Weyl nodes driven by helical magnetic order in EuAgAs. https://arxiv.org/abs/2605.19347

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