Theory of Spiral Magnetism in Weyl semimetal SmAlSi

TL;DR

This paper models the spiral magnetic order in Weyl semimetal SmAlSi by analyzing long-range exchange interactions mediated by itinerant electrons, revealing the importance of antiferromagnetic exchange and complex spin textures.

Contribution

It introduces a material-specific tight-binding model and RKKY-based analysis to explain the magnetic modulation and spin structure in SmAlSi, advancing understanding of Weyl electron-mediated magnetism.

Findings

Fermi-surface nesting alone does not explain the magnetic wave vector.

Antiferromagnetic exchange interactions are crucial for correct magnetic modulation.

The spin texture is close to a cycloidal structure with symmetry preservation.

Abstract

Recent neutron scattering and thermodynamic measurements suggest that Weyl electrons in the emergent Weyl semimetal SmAlSi mediate unconventional magnetic interactions and induce spiral magnetic order. In this work, we investigate the nature of these interactions by modelling long-range $f-f$ exchange mediated by itinerant $d$ electrons via the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism, employing a material-specific tight-binding Hamiltonian obtained from first-principles calculations. The magnetic susceptibility is derived from the spin-spin correlation function based on the random phase approximation. Our results demonstrate that Fermi-surface nesting alone cannot account for the experimentally observed magnetic modulation at the wave vector (1/3, 1/3, 0); however, incorporating appropriate antiferromagnetic exchange interactions among the $d$ electrons yields the correct propagation vector. The spin-texture analysis reveals a configuration that is close to a cycloidal spin structure, preserving combined glide and time-reversal symmetry, and reflecting an intricate competition between inter- and intra-sublattice interactions in SmAlSi.