Generation of Weyl points and a nodal line by magnetization reorientation in Co$_3$Sn$_2$S$_2$

TL;DR

This study demonstrates how magnetization reorientation in Co$_3$Sn$_2$S$_2$ can control its topological electronic states, including the creation of Weyl points and a nodal line, using magneto-optical spectroscopy and theoretical modeling.

Contribution

It provides the first experimental and theoretical evidence that external magnetic fields can manipulate topological band structures in collinear magnets by breaking crystal symmetries.

Findings

Creation of 26 Weyl points with in-plane magnetization.

Prediction of a gapless nodal loop with orthogonal in-plane magnetization.

Observation of a redshift in the nodal loop resonance during magnetization rotation.

Abstract

Topological magnets exhibit fascinating properties like topologically protected surface states or anomalous transport phenomena. While these properties can be significantly altered by manipulating the magnetic state, the experimental verification of such predictions remains challenging. Here, we demonstrate the efficient magnetic field control of the Weyl semimetallic state of the collinear ferromagnet Co$_3$Sn$_2$S$_2$ by magneto-optical spectroscopy. We resolve a redshift of the nodal loop resonance as the magnetization is rotated into the kagome plane by the magnetic field. Our material-specific theory, capturing the observed field-induced spectral reconstruction, shows the creation of 26 Weyl points for one in-plane magnetization direction and predicts the emergence of a gapless nodal loop for the orthogonal in-plane magnetization orientation. These findings demonstrate that while topological band structures are generally considered robust, breaking underlying crystal symmetries with external fields provides an efficient way to manipulate them, even in collinear magnets. This approach opens exciting avenues to control band topology also in materials with more complex magnetic structures and even to study the interplay of real- and momentum-space topological states, e.g. in skyrmion-lattice systems.