# An optical investigation of the strong spin-orbital coupled magnetic semimetal YbMnBi$_2$

## TL;DR

This study investigates the optical properties of YbMnBi$_2$, a magnetic semimetal with potential Weyl semimetal characteristics, revealing a gapped Dirac dispersion and challenging the necessity of magnetic canting for its topological features.

## Contribution

The paper provides the first optical conductivity measurements of YbMnBi$_2$, combining experimental data with DFT calculations to analyze its electronic structure and topological properties.

## Key findings

- Optical conductivity is explained by intra-band and inter-band transitions.
- Magnetic canting is not necessary to account for the observed optical response.
- No definitive evidence of bulk Weyl nodes, instead indicating a gapped Dirac dispersion.

## Abstract

Strong spin-orbit coupling (SOC) can result in ground states with non-trivial topological properties. The situation is even richer in magnetic systems where the magnetic ordering can potentially have strong influence over the electronic band structure. The class of AMnBi$_2$ (A = Sr, Ca) compounds are important in this context as they are known to host massive Dirac fermions with strongly anisotropic dispersion that is believed to be due to the interplay between strong SOC and magnetic degrees of freedom. We report the optical conductivity of YbMnBi$_2$, a newly discovered member of this family and a proposed Weyl semi-metal (WSM) candidate with broken time reversal symmetry. Together with density functional theory (DFT) band structure calculations, we show how the complete conductivity can be interpreted as the sum of a intra-band Drude response and inter-band transitions. We argue that the canting of the magnetic moments that has been proposed to be essential for the realization of the WSM in an otherwise antiferromagnetically ordered system is not necessary to explain the optical conductivity. We believe our data is explained qualitatively by the uncanted magnetic structure with a small offset of the chemical potential from strict stochiometry. There is no definite evidence of bulk Weyl nodes, instead we see signatures of a gapped Dirac dispersion, common in other members of AMnBi$_2$ family or compounds with similar 2D network of Bi atoms.