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

TL;DR

This study investigates EuAgAs, revealing a complex magnetic order that predicts higher-order Weyl nodes near the Fermi energy, advancing understanding of magnetic topological semimetals.

Contribution

It uncovers a unique magnetic helix in EuAgAs and predicts the emergence of higher-order Weyl nodes through first-principles calculations.

Findings

Magnetic ordering sequence with two phase transitions at 12 K and 8 K.

Identification of a transverse helical magnetic structure along the c axis.

Prediction of higher-order Weyl nodes near the Fermi energy.

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.