Session D72: Mn-Te Magnetic Topology I

Tunable Higher-Order Topology in MnBi2nTe3n+1

We propose MnBi_2nTe_3n+1, a recently discovered family of magnetic topological materials, as a highly tunable platform for realizing various symmetry-protected higher-order topological phases. Its canted antiferromagnetic phase can host exotic topological surface states with a Möbius twist that are protected by nonsymmorphic symmetry. Moreover, opposite surfaces hosting Möbius fermions are connected by one-dimensional chiral hinge modes, which offers the first material candidate of a higher-order topological Möbius insulator. We uncover a general mechanism to feasibly induce this exotic physics by applying a small in-plane magnetic field to the antiferromagnetic topological insulating phase of MnBi_2nTe_3n+1, as well as other proposed axion insulators. For other magnetic configurations, two classes of inversion-protected higher-order topological phases are ubiquitous in this system, which both manifest gapped surfaces and gapless chiral hinge modes. We systematically discuss their classification, microscopic mechanisms, and experimental signatures. Remarkably, the magnetic-field-induced transition between distinct chiral hinge mode configurations provides an effective “topological magnetic switch”.   

Thermoelectric and Thermal Conductivity Studies on Mn(Bi1-xSbx)2Te4

MnBi₂Te₄ is the first established intrinsic antiferromagnetic topological insulator [1], which can support various topological quantum states such as quantum anomalous Hall insulator and axion insulator in 2D thin layers. Theory predicts it can also host an ideal type-II Weyl state in its ferromagnetic (FM) phase [2,3]. While the FM phase of MBT is accessible through the spin flop/flip transition under magnetic fields, the predicted Weyl state is not observed in pristine MBT. Recently, Lee et al. [4] found the ideal Weyl state can be achieved by tuning the Sb concentration in Mn(Bi_1-xSb_x)₂Te₄. In this talk, we will report thermopower and thermal conductivity studies on the lightly hole-doped samples which display the Weyl state. We find the thermopower under the magnetic fields above the spin flip transition field (H_c2) is clearly suppressed when the material undergoes the paramagnetic-to-FM transition, consistent with the expected electronic structure transition driven by the magnetic fields. Furthermore, our experiments also showed the linear thermal conductivity enhancement above H_c2 is much more significant for H//c than for H//ab, which is in line with the prediction that the Weyl state is present for H//c but not for H//ab.   

Out-of-plane magnetotransport studies of the magnetic topological insulator Mn(Bi1-xSbx)2Te4

MnBi₂Te₄ is an intrinsic magnetic topological insulator^1-3, which can support various topological quantum states, including QAHI, axion insulator, and an ideal Weyl semimetal (WSM)^4-6. In this talk, we report systematic c-axis transport studies on Mn(Bi_1-xSb_x)₂Te₄ under fields up to 35T. We found that the electronic anisotropy and spin-valve effect of this system are sensitively dependent on chemical potential, which can be tuned by Sb-content. The electronic anisotropy is remarkably enhanced as the chemical potential is close to the band edge, resulting in nonmetallic transport in the c-axis. Moreover, the electronic anisotropy increase results in a giant spin-valve effect in lightly electron-doped samples whose negative MR due to the spin flop transition reaches ~-95%. This is sharply contrasted with lightly hole-doped sample whose MR is dominated by chiral anomaly effect due to the presence of an ideal WSM, indicating Weyl fermions experience very weak magnetic scattering.

1.Otrokov et al, Nature 576, 416 (2019)

2.Zhang et al., PRL 122, 206401 (2019)

3.Li et al, Sci. Adv. 5, eaaw5685 (2019)

4.Deng et al, Science 367, 895 (2020)

5.Liu et al, Nat. Mater. 19, 522 (2020)

6.Lee et al, PRX 11, 031032 (2021)   

Stability of type-II Weyl points in MnBi2–xSbxTe4 based on orbital interactions

One strategy of realizing ideal Weyl semimetals – with only one pair of Weyl points in the Brillouin zone – is to break only time-reversal symmetry while preserving inversion symmetry, as has been experimentally investigated in the ferromagnetic phase of the Weyl semimetal MnBi₂Te₄ [1]. Whether these Weyl points are of type-II character is yet debated, since this character is sensitive to MnBi₂Te₄ lattice parameter changes on the order of 1%. Such stringent lattice parameter requirements can in fact be relaxed by tuning MnBi_2–xSb_xTe₄ alloy composition x from 0 to 0.5. Employing density functional theory, we demonstrate the robustness of this procedure by articulating the conditions of realizing type-II Weyl points based on general principles of orbital interactions as applied to MnBi₂Te₄, including zone folding of p_z orbital dispersions and modifications of these band dispersions due to spin-orbit interaction [2]. The stability of type-II Weyl points in MnBi_1.5Sb_0.5Te₄ is thus intimately associated with orbital interactions, providing a generalizable strategy for future efforts in the rational design and engineering of topological electronic structures.   

Nanomechanical Measurements of an Antiferromagnetic Topological Insulator

The antiferromagnetic topological insulator Mn(Bi_1-xSb_x)₂Te₄ exhibits an ideal platform to study exotic topological phenomena and magnetic properties. The transport signatures of the magnetic phase transitions in the MnBi₂Te₄ family material have been well studied. However, their mechanical properties and magneto-mechanical coupling, in particular, have not been explored in the magnetic topological insulator system. In this work, we use nanoelectromechanical systems to study the intrinsic magnetism in Mn(Bi_1-xSb_x)₂Te₄ thin flakes via their magnetostrictive coupling. We investigate mechanical resonance signatures of the magnetic phase transitions from antiferromagnetic (AFM) to canted antiferromagnetic (cAFM) to ferromagnetic phases versus magnetic field and temperature. The spin-flip transitions in MBST are revealed by frequency shifts of mechanical resonance. With temperatures going above T_N (~25 K) the transitions disappear in the resonance frequency map, consistent with transport measurements. We develop a model to correlate the mechanical frequency shifts with the spin canting states. Our study shows a quadratic relationship between resonance frequency and magnetization of the material and reveals a technique to study the phase transitions and magnetization of the magnetic topological insulator.   

Coexistence of Chern Band and Landau Quantization in Intrinsic Magnetic Topological Insulator

The intrinsic magnetic topological insulators MnBi₂Te₄ family exhibits rich topological quantum states and magnetic phases. By introducing the Sb substitution, the fermi level of the bulk band can be effectively controlled and thus allows access to the exotic topological bulk and surface states. Here, we fabricated Mn(Bi,Sb)₂Te₄ devices with different flake thicknesses and studied their quantum transport properties at the optimal Sb concentrations. In higher electron mobility devices, we observe the development of quantum Hall (ν=-3, -5, etc.) states in addition to the Chern insulating state (C=-1) when the magnetic moment is aligned into the ferromagnetic phase by a magnetic field. The Landau level spectrum of the Mn(Bi,Sb)₂Te₄ behaves very differently from the bulk-insulating three-dimensional topological insulators. In the relatively thick Mn(Bi,Sb)₂Te₄ flake, the dual-gating feature for the quantum Hall plateaus remains a stripe-like pattern at odd integer Landau level degeneracy, indicating an intrinsic bulk-surface state coupling effect. The results suggest that the Mn(Bi,Sb)₂Te₄ is a promising candidate for the exploration of the unique quantization in a coupled topological surface and Weyl states in magnetic topological insulators.   

Transport and Magnetic Properties of MnSb2Te4 Ferromagnetic Layers Grown by MBE

The interaction between magnetic impurities and topological electronic states of 3D topological insulators (TIs) has attracted many studies of predicted exotic phenomena, including quantum anomalous hall effect. In the presence of magnetic dopants, time-reversal symmetry is broken and promotes a Dirac gap. Incorporation of Mn in Sb₂Te₃ results in the formation of a new crystal structure, MnSb₂Te₄, with septuple layer units rather than typical quintuple layers of the undoped TIs. However, high bulk conductivity, due largely to anti-site defects, is pervasive in these materials and limits the ability to detect the surface states and their possible applications. This led to developing new growth procedures of MnSb₂Te₄ that reduce the bulk conductivity. Here we will compare the transport properties, obtained from Hall Effect, of MnSb₂Te₄:Sb₂Te₃ grown by molecular beam epitaxy with varying Mn content, and compare them to first-principle calculations. We use first principle calculations to understand the scattering mechanisms that control the bulk conductivity in the materials, as well as the magnetic spin interactions and magnetic exchange mechanisms that promote topological phase transitions in samples with high Curie temperature values.   

Tuning Fermi level in ferromagnetic MnSb2Te4 by hydrogenation.

Recent realizations of quantum anomalous Hall (QAH) state made it apparent that when long-range ferromagnetism and band topology combine the problem of bulk conduction can become particularly acute. In a newly discovered van der Waals topological magnets of the MnBi₂Te₄ class consisting of antiferromagetically (AFM) coupled septuple layers (SL) of atoms with ferromagnetic (FM) coupling (with T_C ~ 13 K) within each SL the overall order is antiferromagnetic with Neél temperature T_N ~ 25 K.  Recently, a FM MnSb₂Te₄ (MST) with a much higher T_C was realized, where the overall ferrimagnetic coupling is thought to be induced by the abundance of Mn_Sb antisites. Here we investigate flux-grown MST single crystals with Tc ~ 30 K. In addition to high hole density (10²⁰ cm^-3 range) this system also features a non-equilibrium magnetic relaxation promoted by the randomly distributed antisites, with two distinct relaxation rates. We show that ionic hydrogen [1] can move the Fermi level into the bulk gap to access the surface/edge states. We will discuss how the magnetic relaxation is affected by hydrogen tuning and how it can be stabilized towards QAH.   

Growth, characterization and Chern insulator state in MnBi2Te4 via the chemical vapor transport method

As the first intrinsic magnetic topological insulator (MTI), Mn-Bi-Te family has been a fruitful platform to investigate the interplay of band topology and magnetism, which can lead to emergent phenomena such as the quantum anomalous Hall effect (QAHE). In this talk, I will present our study of MnBi₂Te₄ grown by the chemical-vapor-transport (CVT) method. Our result suggests that the CVT-grown MnBi₂Te₄ single crystals are marked with lower overall defects, smaller carrier concentration with a Fermi level closer to the Dirac point, and higher mobility. In particular, a 6-layer device made from the CVT-grown sample shows by far the highest mobility of 2500 cm²V/s in MnBi₂Te₄ devices with the quantized Hall conductance appearing at 1.8 K and 8 T. I then show that this method can be applied to obtain high-quality crystals of the Mn-Bi-Te natural heterostructural families. Our study provides a new route to obtain high-quality single MnBi₂Te₄ crystals that are promising to make superior devices, and to extend the existing natural heterostructural MTI family of Mn-Bi-Te.   

Complexity in Growth Mechanism of MnBi2Te4 using Molecular Beam Epitaxy.

The successful growth of bulk single crystalline intrinsic magnetic topological insulators (MTI) such as MnBi2Te4 has provided a platform to understand exotic quantum phenomena of Quantum Anomalous Hall effect and Axion insulator state. There is a strong driving force behind using molecular beam epitaxy (MBE) to control layer-by-layer growth of MnBi2Te4 as well as the MnBi2Te4/Bi2Te3 superlattices to exploit the rich topological quantum phase diagram. To achieve this goal, it is critical to gain an understanding of the growth kinetics which is a very non-trivial task due to the participation of many chemical species. By combining MBE growth with in-situ scanning tunneling microscopy and ex-situ atomic force microscopy, we report the observation of rich combinations of step heights which represent intermediate phases of MBT in the process of forming a septuple layer. STM study reveals ubiquitous Mn_Bi anti-site defects (Mn in the Bi-layer) and possibly Bi_Mn anti-site defects, suggesting an entropically driven intermixing of Mn and Bi in the septuple layer. We argue that the observed rich step structure on the surface is linked to this entropically driven intermixing in the septuple layer.    

Mechanism of Even-Odd Layer-Dependent Anomalous Hall Effect in MnBi2Te4

Magnetic topological insulator is an ideal platform to explore the interaction between magnetism and topological orders. As an intrinsic magnetic topological insulator, MnBi₂Te₄ has gained a lot of attention, but its properties are yet to be fully understood. Due to its antiferromagnetic nature below Néel temperature, MnBi₂Te₄ thin film is a quantum anomalous Hall (QAH) insulator at odd layers, while an axion insulator at even layers. Recent experiments have shown that the anomalous Hall measurements in MnBi₂Te₄ films exhibit unique even-odd layer-dependent patterns, which could be linked to its distinct topological states at even and odd layers. Here we theoretically studied the underlying mechanism of this intriguing behavior, and discussed the interplay between the axion electrodynamics and the antiferromagnetism for the even-layer systems. These results may provide a deeper insight of the newly-discovered material MnBi₂Te₄ and the interaction between topology and magnetic orders.   

Thickness-dependent electronic behavior of MBE grown MnBi2Te4

Quantum anomalous Hall insulator is seen as a promising candidate for the application of spintronics due to their symmetry-protected dissipationless edge states. Recently, van de Waals layered magnetic topological insulator, MnBi₂Te₄, was reported to show the anomalous hall effect at a temperature that is orders of magnitude higher than the previous magnetically doped topological insulators. However, its electronic structure, especially its surface state gaps, is still under debate. In this study, we report a controlled growth of MnBi₂Te₄ thin films using molecular beam epitaxy, and we revealed their thickness-dependent electronic behavior using scanning tunneling microscopy/spectroscopy. Our experimental observation is further supported by the theoretical calculation. This study provides insight into the intrinsic property of MnBi₂Te₄ and sheds light on the spintronics application based on the topological insulator platform.

    

Local manifestations of thickness dependent topology and edge states in topological magnet MnBi2Te4

The interplay of non-trivial band topology and magnetism gives rise to a series of exotic quantum phenomena, such as the emergent quantum anomalous Hall (QAH) effect and topological magnetoelectric effect. Many of these quantum phenomena have local manifestations when the global symmetry is broken. Here, we report local signatures of the thickness dependent topology in intrinsic magnetic topological insulator MnBi₂Te₄ (MBT), using scanning tunneling microscopy and spectroscopy on molecular beam epitaxy grown MBT thin films. A thickness-dependent band gap with an oscillatory feature is revealed, which we reproduce with theoretical calculations. Our theoretical results indicate a topological quantum phase transition beyond a film thickness of one monolayer, with alternating QAH and axion insulating states for odd and even layers, respectively. At step edges, we observe localized electronic states, in agreement with axion insulator and QAH edge states, respectively, indicating topological phase transitions across the steps. The demonstration of thickness-dependent topological properties highlights the role of nanoscale control over novel quantum states, reinforcing the necessity of thin film technology in quantum information science applications.