High-field Studies on Layered Magnetic and Polar Dirac Metals

TL;DR

This paper reviews high-field studies on layered Dirac metals with magnetic and polar properties, highlighting how design of the block layers influences the realization of various Dirac fermions and their coupling with magnetic order and lattice polarization.

Contribution

It demonstrates how the layered $A$Mn$X_2$ materials can be systematically engineered to realize different types of Dirac fermions with coupled magnetic and polar properties.

Findings

EuMnBi$_2$ shows large magnetoresistance linked to magnetic order changes.

BaMn$X_2$ exhibits spin-valley coupling evidenced by quantum Hall effect.

Design principles enable control over Dirac fermion properties in layered materials.

Abstract

Recently, the interplay between the Dirac/Weyl fermion and various bulk properties, such as magnetism, has attracted considerable attention, since unconventional transport and optical phenomena were discovered. However, the design principles for such materials have not been established well. Here, we propose that the layered material $A$Mn$X_2$ ($A$: alkaline and rare-earth ions, $X$: Sb, Bi) is a promising platform for systematically exploring strongly correlated Dirac metals, which consists of the alternative stack of the $X^-$ square net layer hosting a 2D Dirac fermion and the $A^{2+}$-Mn$^{2+}$-$X^{3-}$ magnetic block layer. In this article, we shall review recent high-field studies on this series of materials to demonstrate that various types of Dirac fermions are realized by designing the block layer. First, we give an overview of the Dirac fermion coupled with the magnetic order in EuMnBi$_2$ ($A$=Eu). This material exhibits large magnetoresistance by the field-induced change in the magnetic order of Eu layers, which is associated with the strong exchange interaction between the Dirac fermion and the local Eu moment. Second, we review the Dirac fermion coupled with the lattice polarization in BaMn$X_2$ ($A$=Ba). There, spin-valley coupling manifests itself owing to the Zeeman-type spin-orbit interaction, which is experimentally evidenced by the bulk quantum Hall effect observed at high fields.