Session D42: Two-dimensional Magnetic Topological Materials

Quantum spin Hall and quantum anomalous Hall effects in AB-stacked MoTe2/WSe2

Moiré materials provide a controllable platform to study the combined effects of strong electronic correlations and non-trivial band topology. Recently, robust quantum spin Hall and quantum anomalous Hall effects have been realized in a single material system, AB-stacked MoTe2/WSe2, simply by tuning the gate voltages in a field-effect device. In this talk, I will discuss the nature of the different topological states and topological phase transitions in AB-stacked MoTe2/WSe2 revealed by combining transport, optical and thermodynamic probes.   

Electrical manipulation and STM visualization of 1D chiral edge states in a moiré quantum anomalous Hall insulator

The quantum anomalous Hall (QAH) effect reflects an interplay between magnetism and non-trivial topology and is characterized by non-zero Chern number and chiral edge-states that carry dissipationless current along topologically defined interfaces. The discovery of the QAH effect in van der Waals moiré heterostructures provides a new opportunity for studying this exotic 2D state of matter. Here I will discuss our scanning tunneling microscopy and spectroscopy (STM/STS) measurements of twisted monolayer-bilayer graphene (tMBLG), a system that has recently been discovered to exhibit the QAH effect as well as gate-switchable orbital magnetism in transport measurements.¹  I will first discuss local spectroscopy measurements of the correlated insulating states at band-fillings of ν = 2 and ν = 3 electrons per moiré unit cell, where the ν = 3 state shows a total Chern number of |C| = 2. Under a small magnetic field, we are able to detect switching of the sign of the Chern number for the ν = 3 state induced by electrostatic gating, a result of the competition between bulk and edge orbital magnetization of the QAH state.²  Utilization of this gate-switchable Chern number allows us to directly visualize the chiral edge-state of a moiré QAH insulator for the first time through gate-dependent dI/dV mapping. We are additionally able to manipulate the spatial location and chirality of QAH edge-states by controlling local carrier concentration with the STM tip. I will discuss the implications of our experimental results for future electronic devices as well as the important role of local moiré strain in determining the correlated and topological properties of tMBLG.

¹ H. Polshyn et al., Nature, 588, 66-70 (2020)

² C. Zhang, T. Zhu et al., arXiv: 2210.06506 (2022)   

Intrinsic magnetic topological phases in a two-dimensional MnPSe3 monolayer

The recent observation of ferromagnetic (FM) topological insulators provides a promising platform for understanding the interplay between the band topology and magnetic order. Despite this, intrinsic magnetism of monolayer topological systems has not yet been experimentally realized so far. Here, we study the electronic properties of FM MnPSe3 monolayer to identify its topological nontrivial features with first-principles density-functional-theory (DFT) calculations and a designed tight-binding (TB) model. We demonstrate that magnetic-orientation-dependent different topological states including the quantum anomalous Hall effect (QAHE) and time-reversal symmetry broken quantum spin Hall effect (QSHE) can be obtained in MnPSe3 monolayer, manifesting a promising material realization of topological spintronics in two-dimensional ferromagnet.   

Magnetic and Topological Properties of Quasi-two-dimensional Ferromagnetic Cr2Te3

 Cr₂Te₃ is a ferromagnetic, quasi-two-dimensional layered material with perpendicular magnetic anisotropy, strong spin-orbit coupling, and non-trivial band topology. Applying density functional theory (DFT) and maximally localized Wannier functions (MLWFs), we extract values for the exchange coupling constants and magnetic anisotropy for a model magnetic Hamiltonian and we determine the band topological properties resulting in the anomalous Hall effect (AHE) as a function of strain. We find that Berry curvature switches sign under compressive strain, which results in a sign change in the anomalous Hall conductivity. The underlying mechanism for the strain-induced sign change is a large contribution to the Berry curvature resulting from two nearly degenerate, anti-crossing Cr-bands along high-symmetry paths in the Brillouin zone. These theoretical results are in agreement with the recent experimental results demonstrating that the intrinsic Berry phase mechanism is the primary origin of AHE in Cr₂Te₃ [1].   

Anomalous Hall effect in Fe3GeTe2: Microscopic mechanism and uniaxial strain effect

Among the several 2D magnetic van der Waals materials, Fe₃GeTe₂ (FGT) has garnered particular interest as a topological ferromagnetic semimetal. The presence of ferromagnetism with topological physics renders this material a platform for interesting phenomena, such as the enormous Berry phase effects. Here we discuss the uniaxial strain effect on the topological properties of FGT and the microscopic mechanism of the anomalous Hall effect (AHE). This study shows the stability of the AHE and the evidence of a nodal line of FGT under uniaxial strains using first-principle calculations and model analysis with symmetries. In addition, the topological origin of the intrinsic AHE is demonstrated that the orbital hybridization is responsible for the substantial Berry curvature near Γ point, which is large enough to compare with the previously reported K point contribution. It has important implications for understanding topological ferromagnetic materials and the discovery of magnetic materials exhibiting stronger topological features. Utilizing such topological stability, a quantum device that is resistant to external noise and information loss can be implemented, which is the subject of intensive global research.   

Tuning ferromagnetism in 2D magnet/topological insulator heterostructures across room temperature by epitaxial growth

2D van der Waals ferromagnet is Fe₃GeTe₂ (FGT) has a relatively high Curie temperature (T_C) with strong perpendicular magnetic anisotropy (PMA). Also, observation of novel phenomena such as spin-orbit torque switching and skyrmion spin textures were reported in this material, which makes it an excellent 2D magnetic candidate to be integrated with topological insulators (TIs) for spin-orbit torque studies. Here, we study the optimum growth conditions for the FGT/Bi₂Te₃ system using molecular beam epitaxy, through which we observed different magnetic properties for the heterostructures of different growth conditions. With different growth temperature and Fe:Ge atomic ratio, we are able to controllably achieve above room temperature ferromagnetism. Cross-sectional scanning transmission electron microscopy (STEM) was used to characterize our high-quality epitaxial van der Waals film. Optical magnetic circular dichroism (MCD) enables us to observe and compare the shape of the hysteresis loops of FGT/Bi₂Te₃ samples grown with different growth conditions and determine their Curie temperature. This in turn gives us more insights into the origin of the elevated Curie temperature in FGT/Bi₂Te₃ heterostructure, leading to a more systematic study of synthesis and magnetic properties of this all-vdW 2D magnet/TI heterostructure. In addition, we looked into possibilities of integration of other materials belonging to FGT family (Fe_mGe_nTe₂) with Bi₂Te₃.   

Scanning Tunneling Microscopy Study of Epitaxially Grown Fe3GeTe2 Layer on the Topological Insulator Bi2Te3

Introducing magnetism to the surface state of topological insulators, such as Bi₂Te₃, can lead to a variety of interesting phenomena such as magnetoelectric effects, Weyl semimetal phases, and the quantum anomalous Hall effect. We use spin-polarized scanning tunneling microscopy to study a single quintuple layer (QL) of Fe₃GeTe₂ (FGT) that is grown on Bi₂Te₃ via molecular beam epitaxy.  FGT is a 2D ferromagnetic van der Waals material with a Curie temperature (Tc) that can exceed room temperature.  Large topographic images show that the FGT grows as isolated islands on Bi₂Te₃ and from Bi₂Te₃ steps.  Atomic resolution imaging shows atomic lattices of 3.9(9)Å for FGT and 4.4(0)Å for Bi₂Te₃. The FGT has a Moire pattern with a periodicity of 4.38(6)nm attributed solely to the lattice mismatch showing that the FGT is rotationally aligned to the Bi₂Te₃. Much of the surface is covered by a single QL of the FGT, with some small double QL regions. There are also regions with partial terminations, where the second quintuple layer is incomplete. The most common partial termination is the FeGe layer, in which the top two atomic layers are missing. This termination has a distinctive electronic structure and a root 3 reconstruction resulting in a 6.8Å lattice constant on top of the Moire pattern associated with the FGT 3.9(9)Å atomic lattice.  Preliminary measurements have been done in a magnetic field with an anti-ferromagnetic Cr tip to probe the magnetic structure and search for magnetic textures.    

Spin transport in topological insulator nanoribbons sandwiched between two-dimensional magnetic materials

Nontrivial band topology along with magnetism leads to some novel quantum phases. When time-reversal-symmetry is broken in 3D topological insulators (TIs) by applying high enough magnetic field or proximity effect, different phases such as quantum Hall or quantum anomalous Hall (QAH)  emerge and display interesting transport properties for spintronic applications. The Quantum Anomalous Hall (QAH) phase displays sidewall chiral edge states  which leads to the QAH effect. In a finite slab, contribution of the surfaces states depends on both the cross-section and thickness of the system. Having a small cross-section and a thin thickness leads to direct coupling of the surfaces, on the other hand, a thicker slab results in a higher contribution of the sidewall states which connect top and bottom surfaces. In this regard, we have considered a heterostructure consisting of a TI, namely Bi₂Se₃ , which is sandwiched between two-dimensional magnetic monolayers of CrI₃ to study its topological and transport properties. Combining DFT and tight-binding calculations along with non-equilibrium Green’s function formalism, we show that a well-defined exchange gap appears in the band structure in which spin polarised edge states flow. We also study the width and finite-size effect on the transmission and topological properties of this magnetised TI nanoribbon.   

Evolution of magnetic exchange between transition-metal atoms in TiS2

In this study, we examine the magnetic and electrical properties of transition-metal atoms substituted into 2D titanium disulfide (TiS2). Using density functional theory and SCAN functional, we computationally substituted Ti with vanadium, chromium, and manganese and determined the magnetic ground state and Heisenberg exchange as a function of distance. Through our analysis, we find that, as the transition-metal atom increases in valance electrons and overall spin, the materials move from a standard FM exchange interaction to an RKKY-like magnetic exchange as the materials transition from semiconductors to metals.   

Topological switch in a near room temperature van der Waals ferromagnet

Two-dimensional (2D) van der Waals magnets have offered a large playground for exploring exotic correlated phenomena emerging from the interplay between dimensionality, magnetism, and topology. Their versality also allows a stronger tunability under various experimental tuning knobs. We will present the evidence for a switchable topological phase in a metallic near room temperature van der Waals ferromagnet from high resolution angle-resolved photoemission spectroscopy measurements supported by a combination of other experimental techniques and theoretical models. We will demonstrate the topological switching to hinge upon a symmetry change that results in a switching of the electronic state between a magnetic Weyl nodal line phase and a geometrical flat bands phase. Our work not only reveals a rich range of quantum phases emergent in 2D van der Waals ferromagnets, but also uncovers the potential of utilizing site occupancy as a novel degree of freedom for tuning symmetry and therefore topology in quantum materials for the realization of exotic emergent phases.