[Paper Review] Observation of multiple Dirac states in a magnetic topological material EuMg2Bi2

Firoza Kabir, M. Mofazzel Hosen+12 more|arXiv (Cornell University)|Dec 18, 2019

Topological Materials and Phenomena46 references4 citations

TL;DR

This study identifies multiple Dirac states in the intrinsic magnetic topological material EuMg2Bi2 using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. It reveals two distinct Dirac cones at the Γ and K points of the Brillouin zone, with Dirac nodes located at 375 meV and 340 meV below the Fermi level, respectively, establishing a platform to explore the interplay between nontrivial topology and magnetism in a stoichiometric system without magnetic doping-induced inhomogeneity.

ABSTRACT

Initiated by the discovery of topological insulators, topologically non-trivial materials, more specifically topological semimetals and metals have emerged as new frontiers in the field of quantum materials. In this work, we perform a systematic measurement of EuMg2Bi2, a compound with antiferromagnetic transition temperature at 6.7 K, observed via electrical resistivity, magnetization and specific heat capacity measurements. By utilizing angle-resolved photoemission spectroscopy in concurrence with first-principles calculations, we observe Dirac cones at the corner and the zone center of the Brillouin zone. From our experimental data, multiple Dirac states at G and K points are observed, where the Dirac nodes are located at different energy positions from the Fermi level. Our experimental investigations of detailed electronic structure as well as transport measurements of EuMg2Bi2 suggest that it could potentially provide a platform to study the interplay between topology and magnetism.

Motivation & Objective

• To investigate the electronic structure of the intrinsic magnetic topological material EuMg2Bi2, which exhibits antiferromagnetic order at 6.7 K.
• To determine whether multiple Dirac states exist in this system, given its potential to host nontrivial topology and magnetism in a stoichiometric compound.
• To explore the interplay between topology and magnetism in a system free from magnetic impurities that often plague doped topological materials.
• To establish EuMg2Bi2 as a robust platform for studying topological quantum phenomena such as the quantum anomalous Hall effect and axion insulator states.

Proposed method

• Angle-resolved photoemission spectroscopy (ARPES) was performed at the ALS beamline 10.0.1 using a Scienta R4000 hemispherical electron analyzer with energy resolution <20 meV and angular resolution <0.2°.
• In situ sample cleaving under ultra-high vacuum (5×10⁻¹¹ Torr) at 15 K ensured surface stability and high-quality measurements over 20-hour periods.
• First-principles electronic structure calculations were performed using VASP with the PBE functional, spin-orbit coupling, and an effective U=11.0 eV for localized Eu 4f electrons.
• X-ray diffraction and energy dispersive X-ray spectroscopy confirmed the crystal structure and chemical composition of the single crystals.
• Transport and thermodynamic measurements, including electrical resistivity, heat capacity, and magnetic susceptibility, were conducted using a Quantum Design PPMS system with up to 9 T magnetic field.
• A 9x9x3 Monkhorst-Pack k-point mesh and 520 eV kinetic energy cutoff were used in DFT calculations to ensure convergence and accuracy.

Experimental results

Research questions

• RQ1 Do multiple Dirac states exist in the intrinsic magnetic topological material EuMg2Bi2, and if so, where are they located in momentum space?
• RQ2 What is the energy position of the Dirac nodes relative to the Fermi level, and how do they differ between high-symmetry points such as Γ and K?
• RQ3 How does the presence of antiferromagnetic order at 6.7 K influence the electronic structure and topological nature of the Dirac states?
• RQ4 Can the Dirac fermions in EuMg2Bi2 be tuned via chemical doping or electrical gating, given their proximity to the Fermi level?
• RQ5 To what extent does the f-electron band structure, located ~4.5 eV below the Fermi level, affect the topological properties of the system?

Key findings

• Multiple Dirac states were experimentally observed at the Γ and K points of the surface Brillouin zone in EuMg2Bi2, confirmed by high-resolution ARPES measurements.
• The Dirac node at the Γ point lies 375 meV below the Fermi level, while the node at the K point is located 340 meV below the Fermi level, indicating distinct energy dispersions.
• The Dirac dispersion at the K point is well-separated from other bands near the Fermi level, suggesting minimal hybridization and high topological fidelity.
• The system exhibits a clear antiferromagnetic transition at 6.7 K, as confirmed by anomalies in heat capacity and magnetic susceptibility measurements.
• First-principles calculations support the ARPES observations, showing consistent Dirac-like dispersions at both Γ and K points with spin-orbit coupling and U=11.0 eV for Eu 4f electrons.
• The high-quality crystal stability under UHV conditions enabled reproducible ARPES data over extended measurement times, confirming the reliability of the observed Dirac states.