Emergence of Nontrivial Low‐Energy Dirac Fermions in Antiferromagnetic EuCd₂As₂

Abstract

Parity‐time symmetry plays an essential role for the formation of Dirac states in Dirac semimetals. So far, all of the experimentally identified topologically nontrivial Dirac semimetals (DSMs) possess both parity and time reversal symmetry. The realization of magnetic topological DSMs remains a major issue in topological material research. Here, combining angle‐resolved photoemission spectroscopy with density functional theory calculations, it is ascertained that band inversion induces a topologically nontrivial ground state in EuCd2As2. As a result, ideal magnetic Dirac fermions with simplest double cone structure near the Fermi level emerge in the antiferromagnetic (AFM) phase. The magnetic order breaks time reversal symmetry, but preserves inversion symmetry. The double degeneracy of the Dirac bands is protected by a combination of inversion, time‐reversal, and an additional translation operation. Moreover, the calculations show that a deviation of the magnetic moments from the c‐axis leads to the breaking of C3 rotation symmetry, and thus, a small bandgap opens at the Dirac point in the bulk. In this case, the system hosts a novel state containing three different types of topological insulator: axion insulator, AFM topological crystalline insulator (TCI), and higher order topological insulator. The results provide an enlarged platform for the quest of topological Dirac fermions in a magnetic system.

Unconventional Transverse Transport above and below the Magnetic Transition Temperature in Weyl Semimetal EuCd2As2

“…It has been demonstrated recently that the antiferromagnet EuCd 2 As 2 with T N ∼ 9.5 K would be an ideal candidate for the study of the interplay between magnetism and topology, as it exhibits various topological states in both the antiferromagnetic (AFM) and paramagnetic (PM) phases [23][24][25][26][27][28]. In the AFM phase, depending on the direction of the Eu magnetic moments, various nontrivial topological ground states have been predicted, such as magnetic topological Dirac semimetal, or axion insulator, AFM topological crystalline insulator, and higher order topological insulator [27]. When the spins are aligned along the c axis by external magnetic field, a single pair of Weyl points appears near the Fermi level [25,27].…”

“…In the AFM phase, depending on the direction of the Eu magnetic moments, various nontrivial topological ground states have been predicted, such as magnetic topological Dirac semimetal, or axion insulator, AFM topological crystalline insulator, and higher order topological insulator [27]. When the spins are aligned along the c axis by external magnetic field, a single pair of Weyl points appears near the Fermi level [25,27]. In the PM phase, EuCd 2 As 2 turns out to be the first discovered centrosymmetric Weyl semimetal where ferromagnetic (FM) spin fluctuations, instead of long-range magnetic order, lift the Kramers degeneracy [24].…”

“…In the PM phase, EuCd 2 As 2 turns out to be the first discovered centrosymmetric Weyl semimetal where ferromagnetic (FM) spin fluctuations, instead of long-range magnetic order, lift the Kramers degeneracy [24]. A rich magnetic phase diagram is thus expected for EuCd 2 As 2 [23,[25][26][27]. Profound insights into the interplay between magnetism and topology, from the exploration of this phase diagram in the context of unconventional transport, can be reasonably foreseen and, thus, such a transport study is highly desired.…”

Dimensionality Concept in Solid‐State Reactions: A Way to Control Synthesis of Functional Materials at the Nanoscale

“…SrAl 2 Si 2 and SrAl 2 Ge 2 are candidate topological materials, [ 27 ] EuAl 2 Si 2 is an antiferromagnet, structurally similar to the established magnetic Weyl and Dirac systems EuA 2 X 2 . [ 30–33 ] The 1D nature of the reactions is best illustrated by structural studies of SrSi 2 and SrAl 2 Si 2 using reflection high‐energy electron diffraction (RHEED) and X‐ray diffraction (XRD). RHEED probes lateral lattice parameters: Figure 2b shows that [ a (SrAl 2 Si 2 ) = 4.19(3) Å, a (SrSi 2 ) = 3.98(3) Å].…”

“…Here, we introduce 1D reactions with extreme transformation of the 2D‐Xene covalent framework and recruit them to illustrate the concept of reaction dimensionality and its use for synthesis control at the nanoscale. Starting with MX 2 , we produce films of layered MA 2 X 2 phases—a numerous class of materials best known for thermoelectric properties [ 26 ] but also boasting topological properties established by both theory [ 27–29 ] and recent experiments on magnetically induced Weyl [ 30–32 ] and Dirac [ 33 ] states. We prove that 1D reactions make a synthetic route to produce MA 2 X 2 for a selection of M and X, probe different templates, establish the reaction mechanism.…”