Session N02: Magnetic Topological Insulators

Spin-resolved band structure in the antiferromagnetic topological insulators MnBi2Te4 and MnBi4Te7

Intrinsic magnetic topological insulators MnBi₂Te₄(Bi₂Te₃)_n (n=0,1,2…) have been extensively studied over the last few years, as they are predicted to host many exotic topological quantum states. However, despite what theory has predicted, angle-resolved photoemission spectroscopy (ARPES) has yet to see a clear gap opening associated with the antiferromagnetic transition, and discrepancies remain between DFT and the complete electronic structure as measured in ARPES. Therefore, the fundamental properties of the material family still need to be further explored.

 

In this talk, we will present our spin-resolved ARPES spectra on MnBi₂Te₄ (n=0) and MnBi₄Te₇ (n=1), showing both in-plane and out-of-plane spin polarization of the surface states. We analyze the spin-orbital texture and compare it to the behavior of typical topological insulators such as Bi₂Te₃ as described by effective theory and ab initio calculations [1]. We discuss the implications of our analysis and how it aids in the understanding of the electronic structures.   

Hydrogen-tuned charge transport in a van der Waals antiferromagnet MnSb2Te4 with antisite disorder

Van der Waals antiferromagnets have recently emerged as materials with unique electronic and magnetic properties, making them promising candidates for next-generation electronic devices. Among these materials, MnSb₂Te₄ (MST) stands out for its promising exotic electronic states and tunable transport properties. Due to easy formation of Mn and Sb interchange (antisite) defects MST can be grown as an antiferromagnet (AFM) or a ferromagnet (FM). Here we demonstrate that a more disordered FM MST can be converted into a robust AFM MST with the same Néel temperature T_N ≅ 18.5 K by injecting ionic hydrogen [1]. The change in magnetic anisotropy, seen in the magnetization measurements and confirmed by our density functional theory (DFT) calculations, reduces the two characteristic AFM fields in the converted MST  —  a spin-flop transition field H₁ at which the system is driven into a canted AFM state and field H₂ at which the system becomes aligned with the external field — but maintains the ratio H₁ / H₂ ≅ 2, allowing for easier tunability into a fully aligned FM state. We show that in both AFM systems, the in-plane longitudinal conductivity is field-linear and there is a large planar Hall effect, both characteristics of a Weyl semimetal. The results of the angularly resolved in-plane transport measurements and the effects of disorder in as-grown and converted AFM MST will be presented and the implications for potential applications in tunable spintronic devices will be discussed.   

Efficient Quantum Transduction Using Anti-Ferromagnetic Topological Insulators

Transduction of quantum information between distinct quantum systems is an essential step in various applications, including quantum networks and quantum computing. However, quantum transduction needs to mediate between photons with vastly different frequencies, making it challenging to design high-performance transducers, due to multifaceted and sometimes conflicting requirements. In this work, we first discuss some general principles for quantum transducer design, and then propose solid-state anti-ferromagnetic topological insulators to serve as highly effective transducers. First, topological insulators exhibit band-inversion, which can greatly enhance their optical responses. Coupled with their robust spin-orbit coupling and high spin density, this property leads to strong nonlinear interaction in topological insulators, thereby substantially improving transduction efficiency. Second, the anti-ferromagnetic order can minimize the detrimental influence on other neighboring quantum systems due to magnetic interactions. Using MnBi₂Te₄ as an example, we showcase that unit transduction fidelity can be achieved with modest experimental requirements, while the transduction bandwidth can reach the GHz range. The strong nonlinear interaction in magnetic topological insulators can find diverse applications, including the generation of entanglement between photons of distinct frequencies.   

Intralayer Non-Collinear Antiferromagnetism Induced Flat Bands in Two-Dimensional CoBi2Te4

The interplay of the topology of electronic wavefunctions with spin configurations in intrinsic magnetic topological insulators (TIs) gives rise to various exotic topological states. Here we predict existence of stable, two-dimensional CoBi₂Te₄ with an intralayer non-collinear antiferromagnetic state. Such an exotic magnetism is explained by the competing Heisenberg exchange interaction based on Co triangular lattice, where the weak exchange coupling between next-nearest neighbour Co plays a vital role in determining the non-collinear order. CoBi₂Te₄ presents trivial insulating state in one septuple-layer structure with a large band gap of 309 meV. In CoBi₂Te₄ two septuple layers, the band gap reduces to 52 meV, and the topological nature is determined by an obvious band inversion. Most interestingly, a flat band is observed in the edge state of nanoribbon with non-collinear antiferromagnetic termination. The appearance of the flat band can be attributed to the magnetic inhomogeneities caused by the intralayer non-collinear antiferromagnetic coupling.   

Evidence of additional Ferromagnetic Order in Antiferromagnetic Topological Insulator MnBi6Te10

MnBi₆Te₁₀ belongs to the antiferromagnetic topological insulator (TI) family MnBi₂Te₄(Bi₂Te₃)_n with n = 2. It can be constructed by intercalating two consecutive Bi₂Te₃ quintuple layers (QL) between adjacent septuple layers (SL) of MnBi₂Te₄. In previous studies, Bi₂Te₃ was considered as a nonmagnetic spacer, which reduces the antiferromagnetic exchange coupling between SLs [1,2]. Previous work demonstrated that the ground states of MnBi₂Te₄(Bi₂Te₃)_n become ferromagnetic for n > 2, while it remain as an A-type antiferromagnet with T_N ~ 12K in MnBi₆Te₁₀ (n=2) [1, 3-6]. There exists an extra step feature near zero field in the magnetic hysteresis loop of single crystal MnBi₆Te₁₀ below 6K, which cannot be explained by the conventional bulk spin flip transition [3]. In this talk, we will show evidence of an additional ferromagnetic order in MnBi₆Te₁₀ below 6K. Domains of this ferromagnetic order were visualized using our homemade cryogenic magnetic force microscope. Our results indicate that this “hidden” ferromagnetic order originates from the Bi₂Te₃ QLs, presumably due to the presence of Mn_Bi antisite defects.

1.Hu et al., Sci. Adv. 2020; 6 : eaba4275

2.Adv. Sci.2023,10, 2203239

3.PHYSICAL REVIEW MATERIALS4, 054202 (2020)

4.npj Quantum Mater. 5, 54 (2020).

5.PHYSICAL REVIEW B 100, 155144 (2019)

6.PHYSICAL REVIEW B 102, 035144 (2020)   

Defect-Driven Competing Magnetic Interactions in MnSb2Te4

The antiferromagnetic topological insulator MnBi₂Te₄ displays interesting properties related to the interaction between topological electronic bands and magnetic order. In isostructural MnSb₂Te₄, anti-site mixing between Mn and Sb atoms introduces magnetic vacancies and defects and determines whether the ground state is ferromagnetic or antiferromagnetic. Here, we present inelastic neutron scattering on single crystals of MnSb₂Te₄, where clear evidence of defect-induced magnetic excitations are observed. Qualitative analysis using linear spin wave theory and classical atomistic spin dynamic simulations are consistent with strong antiferromagnetic coupling to magnetic defects. We discuss how these defect-driven interactions control the global magnetic order in MnSb₂Te_4.   

Antiferromagnetic Quantum Spin Hall and Quantum Anomalous Hall Effect

The Quantum Spin Hall Effect (QSHE) and Quantum Anomalous Hall Effect (QAHE) are typically experimentally realized in topological insulators and magnetic topological insulators with ferromagnetic order. The ferromagnetic order breaks the time-reversal symmetry (TRS) and opens a nontrivial gap in the magnetic topological insulator for QAHE, characterized by the quantized Chern number. On the other hand, the TRS in topological insulators is preserved for the QSHE, characterized by the Z2 number. Here, we extend the Kane-Mele model to include an antiferromagnetic order that breaks the TRS. The system is found to exhibit the QAH phase with a robust edge state and, remarkably, a QSHE phase with a vanishing total Chern number. Our model realizes both the TRS-broken QSHE and QAHE within an antiferromagnetic honeycomb lattice, providing the insights for exploring the potential of antiferromagnetic topological materials.   

Evidence of a type-II to type-I Weyl semimetal transition controlled by magnetization rotation

Unlike their high-energy counterpart, the Weyl fermions in the Weyl semimetals (WSMs) do not need to obey the Lorentz symmetry, which allows the Weyl cones to tilt in the energy-momentum space. The tilting of the Weyl cones leads to two kinds of WSMs, type-I and type-II, each hosting distinct Fermi surface topology and magneto-transport properties. In this talk, I will present evidence of a transition from an ideal type-II to type-I WSM phase in MnBi_2-xSb_xTe₄, controlled by an easy knob of magnetic field angle. Evidence includes the anomalous angular dependence of Shubnikov de Haas oscillations and high-field Hall resistivity, both suggesting a Lifshitz transition associated with dramatic changes in Fermi surface topology. Furthermore, an unusual angle dependence of anomalous Hall conductivity further confirms the tilting of the Weyl cones that induces the Lifshitz transition. These findings establish MnBi_2-xSb_xTe₄ as an exceptional platform for exploring Weyl physics and topological phase transitions.   

Thermal Hall effect in yttrium iron garnet (YIG) potentially due to magnon-polarons

The theory of the magnon-polaron coupling interaction [1] in the ferrimagnet YIG has followed the experimental evidence substantiated by the spin Seebeck effect measurements conducted by Kikkawa. [2] It is also predicted that, at least in Kitaev antiferromagnets, a magnon-polaron driven thermal Hall effect can arise [3] due to the quantum-geometric Berry curvature of the magnon-polarons.  Motivated by this, we conducted thermal Hall measurement in a single-crystal YIG from 60K to 4K. We report new experimental data regarding the thermal Hall effect in YIG and show that the data demonstrate a field dependence in remarkable agreement with that of the magnon-polaron forming magnon and phonon dispersions.   

Investigation of Electric Field Induced Topological Magnons in MnPSe3

Topologically insulated surface magnons are of immense interest because of the prospective of their application in proposed highly efficient, versatile topological spin wave devices. We predict an electric field induced topological order transition for MnPSe₃ below its Neel temperature in which a magnon gap opens and enables topologically insulated surface magnon modes. Flip chip processing of few layered MnPSe₃ nanoflakes on nano-thickness platinum strips is used to create non-local spin transport devices. A red shift of the optical band edge due to the Franz-Keldysh effect is observed using spatially resolved photocurrent spectroscopy measurements in laterally biased devices at room temperature, which allows quantification of the lateral electric field. We will discuss the results of such measurements as a function of temperature and applied lateral electric field in relation to the topological transition and the associated nonlocal spin transport signals emerging with bias.   

Weyl points on non-orientable Brillouin zones: Nielsen-Ninomiya, Fermi arcs and Z2 topological charge

Weyl fermions are chiral particles in high-energy physics that can also emerge as low-energy quasiparticles near three-dimensional band crossing points. They are subject to the Nielsen--Ninomiya "no-go" theorem when placed on a lattice, requiring their total chirality to vanish. This constraint results from the topology of the (orientable) manifold on which they exist. In this talk, we discuss to what extent the concepts of topology and chirality of Weyl points remain well-defined when the underlying manifold is non-orientable. We show that this setting renders chirality a more subtle concept, as well as allowing for systems with a non-zero net chirality. Furthermore, we discover that Weyl points on non-orientable manifolds carry an additional Z2 invariant, satisfying a similar no-go theorem. We implement such Weyl points by constructing tight-binding models with a momentum-space non-symmorphic symmetry. Finally, we experimentally realize their phenomenology in a photonic platform with synthetic momenta.    

Antiferromagnetic Topological Insulators as Magnon-Based Light Dark Matter Detectors

In recent years, the search for dark matter (DM) has stretched to lower masses, with several well-motivated theories for sub-MeV DM. However, the detection of these light DM candidates presents a substantial challenge, requiring target materials that exhibit measurable responses with just a few meV of energy deposition from DM scattering or absorption. DM-magnon interaction in magnetic materials has recently been proposed as a possible detection scheme that would also be sensitive to spin-dependent DM couplings. However, the feasibility of these detection schemes is limited by the ability to read out single magnons. In this work, we instead explore the possibility of magnons in topological insulators as a potential route to sensing. Specifically, we study the spin-wave excitation spectra of antiferromagnetic topological insulators using first-principles calculations. We explore the influence of magnons on the bulk electronic structure and symmetry-protected topological surface states of these systems and examine spin-dependent interactions with several dark matter models. Finally, we establish criteria for selecting optimal targets for magnon-based DM detectors with potential read-out schemes.