Session K72: Mn-Te Magnetic Topology II

Magnetic defects in MnBi2Te4: friend or foe?

MnBi₂Te₄ and related compounds were intensively investigated in the last three years as the first family of intrinsic magnetic topological insulators, where an ideal marriage of non-trivial electronic topology and magnetism makes them a promising materials platform for exotic quantum phenomena. Like other crystalline solids, however, MnBi₂Te₄ has defects in its crystal lattice that can affect both magnetic and topological properties. Identifying these defects and understanding their behavior are critical in fine tuning of the magnetic and topological properties of MnBi₂Te₄. In this presentation, I will talk about the type, concentration, and distribution of lattice defects with special emphasis on magnetic defects and their effects on the physical properties.  I will discuss how these magnetic defects can be employed to engineer the magnetic and topological properties of MnBi₂Te₄ and related compounds.  I will also present our recent progress on the control of lattice defects during materials synthesis.   

Identifying Axion Insulator by Quantized Magnetoelectric Effect in antiferromagnetic MnBi2Te4 Tunnel Junction

Intrinsic magnetic topological insulator MnBi₂Te₄ is believed to be an axion insulator in its antiferromagnetic ground state. However, direct identification of axion insulators remains experimentally elusive because the observed vanishing Hall resistance, while indicating the onset of the axion field, is inadequate to distinguish the system from a trivial normal insulator. Using numerical Green's function method, we theoretically demonstrate the quantized topological magnetoelectric current in a tunnel junction with atomically thin MnBi₂Te₄ sandwiched between two metalic contacts, which is a smoking-gun signal that unambiguously confirms antiferromagnetic MnBi₂Te₄ to be an axion insulator. Our predictions can be verified directly by experiments.   

Anomalous Hall Effect Due to Magnetic Canting Phase in MnBi2Te4

The intrinsic magnetic topological insulator, MnBi₂Te₄, is an exciting material predicted to host a variety of quantum states, such as the quantum anomalous Hall insulator, the Weyl semimetal, and the axion insulator. Despite the rapid progress, the study of this material has generated many unanswered questions that have inspired experimental developments. Here, we experimentally observed an intrinsic anomalous Hall effect (AHE) in MnBi₂Te₄ sensitive to the perpendicular magnetic moments and to its canting angle. MnBi₂Te₄ was grown by molecular beam epitaxy on GaAs(111)B, which is allowed us to obtain a large area of 24-layer MnBi₂Te₄. AHE measurements yield the phase diagram that includes an antiferromagnetic response followed by the surface spin-flop transition, a canted phase, and a ferromagnetic state. Throughout this evolution, the AHE is scaling super-linear with magnetization rather than linearly. We demonstrate that this super-linear scaling is related to the canted angle and consistent with the symmetry of the crystal. Our findings suggest that novel topological responses may be found in non-collinear ferromagnetic and antiferromagnetic phases.   

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MnBi₂Te₄, made by intercalating a Mn-Te layer into a topological insulator Bi₂Te₃, is an antiferromagnetic topological insulator of abundant research potential due to its exotic physical properties—quantum anomalous Hall effect, Axion insulator, and Dirac semimetal depending on the magnetic state and its directions. Especially, the layered structure of MnBi₂Te₄ allows for the exhibition of many turntable capabilities, making it a valuable platform in device applications. In this article, we use Raman spectroscopy, which provides us with a structural fingerprint of the material, under various polarization and temperature dependent configurations to study the phonon modes of (100) direction of bulk MnBi₂Te₄. We observed intensity differences at 46, 113, and 146 cm^-1 peaks between the right and left cross circular polarization configurations while they had the same intensity in co-circular polarization. In addition, A-axis MnBi₂Te₄ had rotation anisotropic polarization Raman spectra, with a 6-degree rotation when comparing above and below the transition temperature of MnBi₂Te₄. Our results contribute to understanding the phonon modes of (100) direction MnBi₂Te₄, and paves the way to tailoring its properties for use in promising emerging applications.   

Evolution of the electronic structures of magnetic topological insulator MnBi2Te4 film and bulk

Intrinsic magnetic topological insulator MnBi₂Te₄, after been exfoliated to atomically thin film and electrostatically gated, exhibits rich and fascinating quantum properties such as quantum anomalous Hall effect and axion insulator state. However, despite these great breakthroughs, there are still many mysteries in the electronic structure of MnBi₂Te₄ to be understood. First of all, the topological surface states (TSSs) were observed by angle-resolved photoemission spectroscopy (ARPES) only at low photon-energies and exhibit a diminishing energy gap that is immune to the magnetic transition, in drastic contrast to the theoretical prediction and transport measurements. Secondly, the dispersion of TSSs shows a kink-like structure and is strongly broadened near the Fermi level (EF), which is out of the expectation of Fermi liquid theory. Thirdly and also importantly, although the intriguing transport properties are realized in MnBi₂Te₄ thin films, their electronic structure is not sufficiently investigated by experiments and seems to be in drastic contrast to that of bulk material. In this talk, we will show our ARPES studies on the electronic structure of MnBi₂Te₄ film as well as its evolution with temperature, film thickness, and surface doping of alkali metals. A trivial surface band is induced by surface doping which may hold the key to understand the peculiar dispersion of the topological surface states in MnBi2Te4. Our results will shed light on the understanding of the electronic structure and intriguing transport properties of the system.   

Thermal signatures of strong magnon-phonon interactions in the canted anti-ferromagnetic phase of MnBi2Te4

Stacked van de Waals septuple layers in MnBi₂Te₄ displays an intriguing combination of spin-orbit coupling induced band inversion in Bi₂Te₃ layers and magnetic moments provided by MnTe layers. Below the Neel temperature T_N=25K, MnBi₂Te₄ becomes an antiferromagnetic (AFM) topological insulator. In an increasing magnetic field applied perpendicular to the layers, the magnetic ordering undergoes a spin-flop transition followed by a canted-antiferromagnetic (CAFM) ordering and finally into a ferromagnetic (FM) ordering where the interlayer magnetic exchange coupling induces a magnetic Weyl semimetal phase [1]. Here, we report field dependence of thermal conductivity k in the in-plane direction under an applied magnetic field in the cross-plane direction in MnBi₂Te₄ from 2K to 30K. k shows a drop in the CAFM phase which we attributed to strong magnon-phonon scattering. We also report thermal Hall data measured in the same configuration and compare it with the electrical Hall data.

[1] Lei & al. PNAS 117 (44) 27224-27230 (2020)   

New insights into the electronic bands of antiferromagnetic topological insulator MnBi2Te4

As the first intrinsic magnetic topological insulator, MnBi₂Te₄ has been attracting a wide range of interest in the condensed matter community as it potentially provides a platform to realize high-temperature topological phases, such as axion insulators and quantum anomalous Hall insulators. However, despite an enormous amount of work using angle-resolved photoemission spectroscopy (ARPES), the electronic band structure of MnBi₂Te₄ remains a subject under intense debate. In this talk, we will present an in-depth investigation of the electronic structure of MnBi₂Te₄ utilizing a multi-resolution photoemission spectroscopy (MRPES) setup developed in our lab.^{1, 2} We first estimate an upper bound of 3 meV for the gap size of the topological surface state (TSS) using 6 eV laser-based ARPES. Measurements on the same sample using 21.2 eV photons yield a gap of 150 meV, which suggests a photon-energy-dependent matrix element effect. Furthermore, our circular dichroism measurement reveals an “anti-crossing” behavior between the TSS and other quasi-2D states, leading to a new understanding of band hybridization. A numerical calculation elucidates that the quasi-2D states potentially originate from the surface quantum confinement. Our study represents a solid step forward in reconciling the existing controversies in the electronic structure of MnBi₂Te₄, and provides a conceptual framework to understand the electronic structures of other relevant topological materials MnBi_2nTe_3n+1.³   

Second Harmonic Generation in van der Waals Magnetic Topological Insulator MnBi2Te4

MnBi₂Te₄, a newly discovered van der Waals topological magnet, has attracted great interest due to the rich interplay of topology and intrinsic magnetism present in both bulk and atomically thin flakes. Its A-type antiferromagnetic order should also lead to layer- and magnetic-state-dependent c-type optical second harmonic generation (SHG), which can arise in centrosymmetric crystals when spin ordering breaks inversion symmetry. In this talk, I will present our measurements and progress toward understanding the SHG response of MnBi₂Te₄ from pristine bulk crystal to the atomically thin limit. I will discuss steady-state and time-resolved spectroscopic results that show how the SHG depends on excitation polarization, applied magnetic field, crystal layer thickness, and sample temperature. Our results provide useful symmetry-related information for gaining further insights into this new class of two dimensional magnetic topological insulators.   

Non-reciprocal circuits based on topological magnet

The chiral edge states (CES) which exist at the edges of quantum Hall insulators and Chern insulators are manifestations of extreme non-reciprocity – they move strictly in one direction, with backscattering impossible. The direction of propagation is dictated by a topological invariant of the system. One attractive system for CES engineering is the intrinsic topological magnet MnBi₂Te₄. In this talk, I will present our progress on using CES in MnBi₂Te₄ to redirect signals and construct non-reciprocal circuits. The atomically thin nature of this material and abundance of non-trivial topological states allows control of such circuits with gate voltage and magnetic field. Our results constitute the proof-of-concept non-reciprocal circuitry based on gate-tunable atomically thin Chern insulators.    

Native point defects and their implications for the Dirac point gap at MnBi2Te4(0001)

The Dirac point gap at the surface of the antiferromagnetic topological insulator MnBi₂Te₄ (MBT) is a highly debated issue [1-3]. While the early photoemission measurements reported on large gaps in agreement with theoretical predictions, other experiments found vanishingly small splitting of the MBT Dirac cone. Here, we study the effect of the native point defects on the MBT surface electronic structure using the density functional theory calculations [4]. The results of our scanning tunneling microscopy images simulations are consistent with the presence of the Bi_Te antisites (Bi atoms at the Te sites) and Mn_Bi substitutions (Mn atoms at the Bi sites). Our surface electronic structure calculations show that, due to the predominant localization of the topological surface state near the Bi layers, Mn_Bi defects can cause a strong reduction of the MBT Dirac point gap [4], given the recently proved [5] antiparallel alignment of the Mn_Bi moments with respect to those of the Mn layer. Our results provide important insights into the MBT Dirac point gap mystery.

1 M Otrokov et al, Nature 576, 412 (2019)

2 Y Hao et al, Phys. Rev. X 9, 041038 (2019)

3 A Shikin et al, Phys. Rev. B 104, 115168 (2021)

4 M Garnica, M. Otrokov et al, arXiv:2109.01615

5 Y Lai et al, Phys. Rev. B 103, 184429 (2021)   

Direct visualization of edge state in even-layer MnBi2Te4 at zero magnetic field

Being the first intrinsic antiferromagnetic (AFM) topological insulator, MnBi₂Te₄ is argued to be a topological axion state in its even-layer form due to the antiparallel magnetization between the top and bottom layers. Here we combine both transport and scanning microwave impedance microscopy (sMIM) to investigate such axion state in atomically thin MnBi₂Te₄ with even-layer thickness at zero magnetic field. While transport measurements show a zero Hall plateau signaturing the axion state, sMIM uncovers an unexpected edge state raising questions regarding the nature of the “axion state”. Based on our model calculation, we propose that the even-layer MnBi₂Te₄ at zero field is in an AFM quantum spin Hall (QSH) state hosting a pair of helical edge states. Such novel AFM QSH is originated from the combination of half translation symmetry and time-reversal symmetry in MnBi₂Te₄. Our finding thus signifies the richness of topological phases in MnB₂Te₄ that has yet to be fully explored.