Optical signatures of type-II Weyl fermions in the noncentrosymmetric semimetals $R$AlSi ($R$=La, Ce, Pr, Nd, Sm)

TL;DR

This study identifies optical signatures of type-II Weyl fermions in noncentrosymmetric $R$AlSi semimetals through infrared spectroscopy, revealing their metallic behavior and the presence of Weyl nodes near the Fermi energy.

Contribution

It provides the first optical evidence of Weyl fermions in $R$AlSi compounds, highlighting their type-II Weyl state and symmetry-breaking origins, expanding understanding of topological semimetals.

Findings

Optical conductivity shows linear-in-frequency behavior indicating Weyl nodes.

All compounds exhibit metallic character with significant free charge carriers.

$R$AlSi compounds are likely to host type-II Weyl states with overtilted cones.

Abstract

Weyl semimetals with magnetic ordering provide a promising platform for the investigation of rare topological effects such as the anomalous Hall effect, resulting from the interplay of nontrivial bands with various spin configurations. The materials $R$AlSi, where $R$ represents a rare-earth element, are prominent representatives of Weyl semimetals, where the Weyl states are induced by space inversion symmetry breaking and in addition, for several rare-earth elements $R$, enhanced by time-reversal symmetry breaking through the formation of a magnetic order at low temperature. We report optical signatures of Weyl fermions in the magnetic compounds CeAlSi, PrAlSi, NdAlSi, and SmAlSi as well as the non-magnetic family member LaAlSi by broad-frequency infrared spectroscopy at room temperature, i.e., in the paramagnetic phase. A similar profile of the optical conductivity spectrum and a metallic character are observed for all compounds, with LaAlSi showing the strongest free charge carrier contribution. Furthermore, the linear-in-frequency behavior of the optical conductivity of all investigated compounds indicates the presence of Weyl nodes in close vicinity of the Fermi energy, resulting from inversion symmetry breaking in noncentrosymmetric structures. According to the characteristics of these linear slopes, the $R$AlSi compounds are expected to host mainly type-II Weyl states with overtilted Weyl cones. The results are compared to the optical response of the closely related $R$AlGe materials, which are considered as potential hybridization-driven Weyl-Kondo systems.