Magneto-optical spectroscopy on Weyl nodes for anomalous and topological Hall effects in chiral MnGe

Y Hayashi,Y Takahashi,Y Okamura,N Kanazawa,T Koretsune,M Kawasaki,T Yu,A Tsukazaki,R Arita,Y Tokura,M Ichikawa

doi:10.1038/s41467-021-25276-1

Abstract

Physics of Weyl electrons has been attracting considerable interests and further accelerated by recent discoveries of giant anomalous Hall effect (AHE) and topological Hall effect (THE) in several magnetic systems including non-coplanar magnets with spin chirality or small-size skyrmions. These AHEs/THEs are often attributed to the intense Berry curvature generated around the Weyl nodes accompanied by band anti-crossings, yet the direct experimental evidence still remains elusive. Here, we demonstrate an essential role of the band anti-crossing for the giant AHE and THE in MnGe thin film by using the terahertz magneto-optical spectroscopy. The low-energy resonance structures around ~ 1.2 meV in the optical Hall conductivity show the enhanced AHE and THE, indicating the emergence of at least two distinct anti-crossings near the Fermi level. The theoretical analysis demonstrates that the competition of these resonances with opposite signs is a cause of the strong temperature and magnetic-field dependences of observed DC Hall conductivity. These results lead to the comprehensive understanding of the interplay among the transport phenomena, optical responses and electronic/spin structures.

Highlights

Physics of Weyl electrons has been attracting considerable interests and further accelerated by recent discoveries of giant anomalous Hall effect (AHE) and topological Hall effect (THE) in several magnetic systems including non-coplanar magnets with spin chirality or small-size skyrmions
The enhanced Berry curvature owing to the crossing point, i.e., Weyl node[6,7], is considered to cause the large AHE; this character is contrasted to the extrinsic origin such as skew scattering and side-jump mechanisms, which are irrelevant to the electronic band structure[8,9,10]
Upon the application of external magnetic field, the emergent monopole and anti-monopole are forced to move in opposite directions; these movements of emergent magnetic monopoles/anti-monopoles are accompanied by deformation of the bridging skyrmion-strings (Fig. 1c), giving rise to the uncancelled emergent magnetic field or the relevant Berry-curvature generation in the momentum space

Summary

Introduction

Physics of Weyl electrons has been attracting considerable interests and further accelerated by recent discoveries of giant anomalous Hall effect (AHE) and topological Hall effect (THE) in several magnetic systems including non-coplanar magnets with spin chirality or small-size skyrmions These AHEs/THEs are often attributed to the intense Berry curvature generated around the Weyl nodes accompanied by band anti-crossings, yet the direct experimental evidence still remains elusive. The theoretical analysis demonstrates that the competition of these resonances with opposite signs is a cause of the strong temperature and magnetic-field dependences of observed DC Hall conductivity These results lead to the comprehensive understanding of the interplay among the transport phenomena, optical responses and electronic/spin structures. We demonstrate that the terahertz resonances of the band anti-crossing points (Weyl nodes) close to the Fermi level are assigned distinctly to the origins of AHE and THE and that the competition of these resonances causes the strong temperature and magnetic-field dependence of DC Hall conductivity as observed

Methods

Results

Conclusion