Origin of the Type-II Weyl state in topological antiferromagnetic YbMnBi2

TL;DR

This study clarifies the topological nature of YbMnBi2, showing it is a Dirac semimetal with collinear antiferromagnetism, but can transition to a Type-II Weyl semimetal through magnetic moment canting induced by Bi vacancies.

Contribution

The paper demonstrates that magnetic moment canting, induced by Bi vacancies, can turn YbMnBi2 from a Dirac semimetal into a Type-II Weyl semimetal, revealing a mechanism for topological phase transition.

Findings

YbMnBi2 is a collinear antiferromagnetic Dirac semimetal.

Magnetic moment canting above 2° induces a Type-II Weyl state.

Bi vacancies can promote magnetic canting and topological transition.

Abstract

Recently, the topological nature of an antiferromagnet YbMnBi2 has been controversial. YbMnBi2 is regarded as a candidate of Type-II Weyl semimetals with magnetic moments of Mn atoms canting about 10{\deg} in some studies but as a Dirac semimetal without canting in others. By means of systematical density functional theory calculations, we show the perfect YbMnBi2 bulk has a collinear antiferromagnetic ordering and, naturally, it is a Dirac semimetal. Considering the vital role of magnetic moment canting in generating the Type-II Weyl state, we artificially cant the magnetic moments of Mn atoms and find that YbMnBi2 enters into the Type-II Weyl state from about 2{\deg}. Inspired by this and taking into account the possible defects in experiments, we suggest that Bi vacancies in Mn-Bi-Mn bonds, which produce sizable Dzyaloshinskii-Moriya interactions and thereby cant the magnetic moments of Mn atoms, can tune the topological nature of YbMnBi2 from Dirac semimetals to Type-II Weyl semimetals. Our work unveils the possible underlying mechanism for the Type-II Weyl state in YbMnBi2, providing insights into the Weyl state in other magnetic topological materials.