Spin Excitations and Spin Wave Gap in the Ferromagnetic Weyl Semimetal Co$_3$Sn$_2$S$_2$

TL;DR

This study uses neutron scattering to explore spin excitations in the ferromagnetic Weyl semimetal Co$_3$Sn$_2$S$_2$, revealing a large spin wave gap and the influence of Weyl fermions on spin dynamics.

Contribution

It provides the first detailed measurement of spin wave dispersions and the spin wave gap in Co$_3$Sn$_2$S$_2$, linking magnetic excitations with topological electronic states.

Findings

Observation of dispersive spin waves both in-plane and out-of-plane.

Detection of a significant spin wave gap of 2.3 meV at low temperature.

Temperature dependence of the gap suggests Weyl fermion contribution.

Abstract

We report a comprehensive neutron scattering study on the spin excitations in the magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$ with quasi-two-dimensional structure. Both in-plane and out-of-plane dispersions of the spin waves are revealed in the ferromagnetic state, similarly dispersive but damped spin excitations persist into the paramagnetic state. The effective exchange interactions have been estimated by a semi-classical Heisenberg model to consistently reproduce the experimental $T_C$ and spin stiffness. However, a full spin wave gap below $E_g=2.3$ meV is observed at $T=4$ K, much larger than the estimated magnetic anisotropy energy ($\sim0.6$ meV), while its temperature dependence indicates a significant contribution from the Weyl fermions. These results suggest that Co$_3$Sn$_2$S$_2$ is a three-dimensional correlated system with large spin stiffness, and the low-energy spin dynamics could interplay with the topological electron states.