Topological character of the antiferromagnetic EuMg$_{2}$Bi$_{2}$

TL;DR

This study resolves the topological character of EuMg$_{2}$Bi$_{2}$ by combining high-resolution ARPES, magnetic/thermodynamic measurements, and state-of-the-art first-principles calculations. It identifies linearly dispersing hole-like bands near $E_F$ and leverages Wannier-based analysis to predict a strong topological insulator with a bulk invariant $ u_0=1$, implying metallic surface states, though the experimental $E_F$ is offset by ≈$0.1$ eV from theory, hindering direct observation of these states. The work also demonstrates a magnetic-field-induced anomalous Hall conductivity arising from spin-splitting and potential canting of Eu moments, linking magnetism and topology. Overall, EuMg$_{2}$Bi$_{2}$ emerges as a compelling platform to explore the interplay between antiferromagnetism and nontrivial band topology, with tunable surface states via Fermi-level engineering.

Abstract

Antiferromagnetic EuM$_{2}$Pn$_{2}$ compounds, where M is a metal element and Pn is a pnictogen element, have been recognized as candidates for realizing a topologically nontrivial electronic structure. In this paper, we focus on EuMg$_2$Bi$_2$, whose topological nature still remains unclear. We present a comprehensive study based on several experimental and theoretical techniques. Magnetic susceptibility, electrical resistivity, and specific heat capacity measurements confirm the existence of an antiferromagnetic ordering. The electronic band structure was investigated by high-resolution angle-resolved photoemission spectroscopy (ARPES), supported by ab initio calculations. ARPES measurement reveals that the electronic structure of this system is dominated by linearly dispersive hole-like bands near the Fermi level. Theoretical analyses of the electronic band structure indicates that EuMg$_2$Bi$_2$ is a strong topological insulator, which should be reflected in the presence of a metallic surface state. We also theoretically examine the magnetic-field-induced anomalous Hall conductivity, confirming previously reported observations.

Figures (15)

• Figure 1: (a) Crystal structure of EuMg$_{2}$Bi$_{2}$ with P$\bar{3}$m1 symmetry, and (b) the corresponding bulk Brillouin zone with its high-symmetry points. (c) The phonon dispersion curves and the corresponding density of states.
• Figure 2: Phonon optical modes at the $\Gamma$ point.
• Figure 3: (a) Heat capacity of EuMg$_{2}$Bi$_{2}$ measured in zero magnetic field (blue curve) and in a high field of $9$ T (red curve). A pronounced anomaly marks a phase transition at $6.7$ K. The inset shows the temperature dependence of the magnetic susceptibility of EuMg$_{2}$Bi$_{2}$, with the antiferromagnetic transition at $T_{N} = 6.7$ K. Magnetic susceptibility measurements were performed under $0.5$ T (violet curve) and $9$ T (green curve) manetic fields. (b) Temperature variation of the electrical resistivity of EuMg$_{2}$Bi$_{2}$, measured within the trigonal plane.
• Figure 4: (a) Measured core levels spectrum of EuMg$_{2}$Bi$_{2}$. Sharp peaks correspond to the Eu $5s$, Eu $4f$, SOC split Bi $5d$, and Mg $2p$ levels (as labeled). (b) Calculated electronic band structure along high-symmetry directions. Results are shown with spin--orbit coupling included, for Eu $4f$ electrons treated as core states (blue lines) and valence states (pink lines).
• Figure 5: Fermi surface map and constant-energy contours of EuMg$_{2}$Bi$_{2}$. (a) Experimentally measured Fermi surface maps acquired at photon energy of $60$ eV and constant-energy contour plots at various binding energies The corresponding binding energies are noted in the plots. All measurements were performed at the SSRL beamline 5-2 at a temperature of $10$ K in the Paramagnetic (PM) phase. (b) Corresponding theoretically obtained results (bottom panels).