Anisotropic magnetism and electronic structure of trigonal EuAl$_2$Ge$_2$ single crystals

TL;DR

This study investigates the magnetic and electronic properties of EuAl$_2$Ge$_2$, revealing antiferromagnetic order, metallic behavior, and topologically nontrivial electronic states through neutron diffraction, ARPES, and first-principles calculations.

Contribution

It provides a comprehensive analysis of EuAl$_2$Ge$_2$'s magnetic structure and electronic topology, highlighting the interplay between magnetism and electronic states in this layered compound.

Findings

EuAl$_2$Ge$_2$ exhibits A-type antiferromagnetic order below 27.5 K.

The compound remains metallic across the magnetic transition.

First-principles calculations predict nontrivial topological states near the Fermi level.

Abstract

The magnetic and electronic properties of the layered Zintl-phase compound EuAl$_2$Ge$_2$ crystallizing in the trigonal CaAl$_2$Si$_2$-type structure are reported. Our neutron-diffraction measurements show that EuAl$_2$Ge$_2$ undergoes A-type antiferromagnetic (AFM) ordering below $T_{\rm N} = 27.5(5)$~K, with the Eu moments (Eu$^{2+}$, $S = 7/2$) aligned ferromagnetically in the $ab$ plane. The $H = 0$ magnetic structure consists of trigonal AFM domains associated with $ab$-plane magnetic anisotropy and a field-induced reorientation of the Eu spins in the domains is evident at $T = 2$~K below the critical field $H_{c1} = 2.5(1)$ kOe. Electrical resistivity and ARPES measurements show that EuAl$_2$Ge$_2$ is metallic both above and below $T_{\rm N}$. In the AFM phase, we directly observe folded bands in ARPES due to the doubling of the magnetic unit cell along the $c$ axis with an enhancement of quasiparticle weight due to the complex change in the coupling between the magnetic moments and itinerant electrons on cooling below $T_{\rm N}$. The observed electronic structure is well reproduced by first-principle calculations, which also predict the presence of nontrivial electronic states near the Fermi level in the AFM phase with $Z_2$ topological numbers 1;(000).