Magnetic crystalline-symmetry-protected axion electrodynamics and field-tunable unpinned Dirac cones in EuIn2As2

Abstract

Knowledge of magnetic symmetry is vital for exploiting nontrivial surface states of magnetic topological materials. EuIn2As2 is an excellent example, as it is predicted to have collinear antiferromagnetic order where the magnetic moment direction determines either a topological-crystalline-insulator phase supporting axion electrodynamics or a higher-order-topological-insulator phase with chiral hinge states. Here, we use neutron diffraction, symmetry analysis, and density functional theory results to demonstrate that EuIn2As2 actually exhibits low-symmetry helical antiferromagnetic order which makes it a stoichiometric magnetic topological-crystalline axion insulator protected by the combination of a 180∘ rotation and time-reversal symmetries: $${C}_{2}\times {\mathcal{T}}={2}^{\prime}$$ C 2 × T = 2 ′ . Surfaces protected by $${2}^{\prime}$$ 2 ′ are expected to have an exotic gapless Dirac cone which is unpinned to specific crystal momenta. All other surfaces have gapped Dirac cones and exhibit half-integer quantum anomalous Hall conductivity. We predict that the direction of a modest applied magnetic field of μ0H ≈ 1 to 2 T can tune between gapless and gapped surface states.

Field-induced Topological Hall effect in antiferromagnetic axion insulator candidate EuIn$_2$As$_2$

“…We note that a possibility of a field-induced band structure changing, as observed in EuO [41], can be safely excluded as the origin of THE in this material. The band structure calculation of EuIn 2 As 2 predicts that the energy shift of the unoccupied band closes the inverted band gap near the Γ point when the magnetic order is changed from helical to FM, suggesting a possibility of the band-structure control by applying a magnetic field [20]. However, in contrast to these band calculations predicting a semiconducting state for EuIn 2 As 2 , this compound is a naturally hole-doped metal as shown by our Hall measurements.…”

“…It has been verified that noncollinear antiferromagnets with zero net magnetization can produce a large AHE when their electronic structure exhibits a nonvanishing Berry curvature that acts like a large fictitious magnetic field [10,29,30]. According to the recent neutron diffraction experiment, EuIn 2 As 2 has a helical magnetic structure [20], in which the magnetic moment ferromagnetically aligning in the ab plane rotates antiferromagnetically along the c axis with the magnetic space group of P 6 1 2'2'/C2'2'21 (Fig. 3(b)).…”

“…Angle-resolved photoemission spec-troscopy (ARPES) results show an inversion of the bulk band as entering to the AFM state, suggesting that there is a topological phase transition accomplished with the magnetic transition [18,19]. Although, recent neutron diffraction experiments show that EuIn 2 As 2 has a lowsymmetry helical AFM order, not an A-type AFM order, it has also been predicted that the direction of a modest applied magnetic field of B ≈ 1-2 T can tune between gapless and gapped surface states [20]. Thus, the fruitful topological nature and easily magnetic field tunability of this system will be a promising platform to realize "clean" topological quantum effect and many novel transport phenomena.…”

Canted Antiferromagnetic phases in the layered candidate Weyl material EuMnSb$_2$

“…Further, topological materials with a net magnetization M that can be readily tuned by temperature or external fields are especially sought after because they can offer direct control of Dirac surface states [4]. For example, the presence of a net M in a topological-crystalline insulator can result in the gapping of a surface-Dirac cone: gapless surface-Dirac cones are associated with chiral-locked dissipationless surface conductivity or edge states whereas gapped surface-Dirac cones lead to quantum-anomalous-Hall type conductivity [6].…”

A-type antiferromagnetic order in semiconducting EuMg$_2$Sb$_2$ single crystals

“…In recent years, studies of Eu-based intermetallic compounds carried out in the search for novel electronic states have been reported [9][10][11][12]. EuMg 2 Bi 2 is one such material which belongs to a class of rare-earth-based compounds that exhibit novel electronic states arising from a complex interplay of magnetism and electronband topology [13][14][15].…”