Intrinsic Quantum Anomalous Hall Research Articles

Layer-Dependent Quantum Anomalous Hall and Quantum Spin Hall Effects in Two-Dimensional LiFeTe.

The emergence of an intrinsic quantum anomalous Hall (QAH) insulator with long-range magnetic order triggers unprecedented prosperity for combining topology and magnetism in low dimensions. Here, based on stacked two-dimensional LiFeTe, we confirm that magnetic coupling and topological electronic states can be simultaneously manipulated by just changing the layer numbers. Monolayer LiFeTe shows intralayer ferrimagnetic coupling, behaving as a QAH insulator with Chern number C = 2. Beyond the monolayer, the odd and even layers of LiFeTe correspond to uncompensated and compensated interlayer antiferromagnets, resulting in unexpected QAH and quantum spin Hall (QSH) states, respectively. Moreover, the spin Chern number is proportional to the stacking layer numbers in even-layer LiFeTe, proving that the spin Hall conductivity can be continuously enhanced by increasing layer numbers. Therefore, the odd-even-layer-dependent QAH and QSH effects found in LiFeTe topological insulators offer new insight into regulating quantum states in two-dimensional topological materials.

Strain-induced high-Chern-number spin-unlocked edge states in monolayer MnAsO3 with intrinsic quantum anomalous Hall effect

Two-dimensional intrinsic magnetic topological materials that can realize device miniaturization have attracted significant attention recently based on their chiral dissipationless edge states. However, since the experimental observation of quantum anomalous Hall effect (QAHE) is still limited by low temperature, high operating temperature and large nontrivial gap are urgently needed. Here, monolayer MnAsO3 is predicted to be a room-temperature intrinsic magnetic topological material with high Chern number C = 3 based on first-principles calculations, which offers the possibility of achieving high-speed and low-energy-consumption electron transport in the future. Furthermore, the large and experimental feasible nontrivial gap up to 79.09 meV is obtained under compressive strain modulation. Moreover, the high-Chern-number topological phase transition and strain-induced spin-unlocked edge states are observed, indicating the possibility of tuning the electron transport of QAHE. All these findings suggest that monolayer MnAsO3 is a suitable and promising material for fabricating low-energy-consumption spintronics devices.

Chern number transition of quantum anomalous hall phases in kagome TM3Te4 (TM = Ti, Cr) monolayers by manipulating magnetization orientation

The development of quantum anomalous hall (QAH) insulator with high transition temperature is the key to realize practical applications in future quantum technology and spintronics. Here, we predicted two stable two-dimensional kagome structures, Ti3Te4 and Cr3Te4, and found that both of them are intrinsic QAH insulators, using density functional theory calculations. In the absence of spin–orbit coupling (SOC), both systems display ferromagnetism (FM) Weyl semimetal states. Remarkably, Ti3Te4 monolayer is revealed to be a robust ferromagnetic half metal with high Curie temperature (TC) of 403 K. When the SOC effect occurs, it spontaneously creates QAH states with large nontrivial bandgap and chiral edge states. As a result, the Ti(Cr)3Te4 monolayer is changed to be QAH insulators with Chern number C = ±1 by rotation of magnetization orientation. In addition, the phase change from FM QAH insulator to antiferromagnetic insulator can be manipulated by applying external strains. Moreover, a high-Chern number phase (C = 2) arises by building Ti3Te4/MoS2/Ti3Te4 heterostructure. Meanwhile, the topological phase transition can be well recurred by using a spinless three-band tight-binding (TB) model. The findings present ways to realize potential QAH insulators with high transition temperatures.

Tunable Topological States in Stacked Chern Insulator Bilayers

The emergence of intrinsic quantum anomalous Hall (QAH) insulators with a long-range ferromagnetic (FM) order triggers unprecedented prosperity for combining topology and magnetism in low dimensions. Built upon atom-thin Chern insulator monolayer MnBr3, we propose that the topologically nontrivial electronic states can be systematically tuned by inherent magnetic orders and external electric/optical fields in stacked Chern insulator bilayers. The FM bilayer illustrates a high-Chern-number QAH state characterized by both quantized Hall plateaus and specific magneto-optical Kerr angles. In antiferromagnetic bilayers, Berry curvature singularity induced by electrostatic fields or lasers emerges, which further leads to a novel implementation of the layer Hall effect depending on the chirality of irradiated circularly polarized light. These results demonstrate that abundant tunable topological properties can be achieved in stacked Chern insulator bilayers, thereby suggesting a universal routine to modulate d-orbital-dominated topological Dirac fermions.

Manipulation of intrinsic quantum anomalous Hall effect in two-dimensional MoYN2CSCl MXene

Quantum anomalous Hall effect (QAHE) is an innovative topological spintronic phenomenon with dissipationless chiral edge states and attracts rapidly increasing attention. However, it has only been observed in few materials in experiments. Here, according to the first-principles calculations, we report that the MXene MoYN2CSCl shows a topologically nontrivial band gap of 37.3 meV, possessing QAHE with a Chern number of C = 1, which is induced by band inversion between d xz and d yz orbitals. Also, the topological phase transition for the MoYN2CSCl can be realized via strain or by turning the magnetization direction. Remarkably, MoYN2CSCl shows the nodal-line semimetal state dependent on the electron correlation U. Our findings add an experimentally accessible and tunable member to the QAHE family, which stands a chance of enriching the applications in spintronics.

Topological Magnetic Materials of the (MnSb2Te4)·(Sb2Te3)n van der Waals Compounds Family.

Using density functional theory, we propose the (MnSb2Te4)·(Sb2Te3)n family of stoichiometric van der Waals compounds that harbor multiple topologically nontrivial magnetic phases. In the ground state, the first three members of the family (n = 0, 1, 2) are 3D antiferromagnetic topological insulators, while for n ≥ 3 a special phase is formed, in which a nontrivial topological order coexists with a partial magnetic disorder in the system of the decoupled 2D ferromagnets, whose magnetizations point randomly along the third direction. Furthermore, due to a weak interlayer exchange coupling, these materials can be field-driven into the FM Weyl semimetal (n = 0) or FM axion insulator states (n ≥ 1). Finally, in two dimensions, we reveal these systems to show intrinsic quantum anomalous Hall and AFM axion insulator states, as well as quantum Hall state, achieved under external magnetic field. Our results demonstrate that MnSb2Te4 is not topologically trivial as was previously believed that opens possibilities of realization of a wealth of topologically nontrivial states in the (MnSb2Te4)·(Sb2Te3)n family.

Robust quantum anomalous Hall effect in a pentagonal MoS2 monolayer grown on CuI(001) substrates

Recently, the square planar ${\mathrm{MoS}}_{2}$ monolayer, which exhibits the Cairo pentagonal tiling (termed as $1\mathrm{P}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$), was identified as an intrinsic quantum anomalous Hall (QAH) insulator. However, there is a paucity of theoretical work concerning a suitable substrate to support its nontrivial electron transport properties, which is the prerequisite for practical applications. Here. we demonstrate that CuI(001) serves an excellent substrate candidate for epitaxial growth of the $1\mathrm{P}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ sheet by showing that the intrinsic ferromagnetism and the QAH state of $1\mathrm{P}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ remain unaltered in the $1\mathrm{P}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$/CuI(001) system. Further analyses of the strain effect on $1\mathrm{P}\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ reveal that the QAH is robust within a strain range from $\ensuremath{-}$ 2% to 2%. Our findings will inspire the experimental realization of QAH effects in two-dimensional (2D) pentagon-based materials.

Electronic structure and magnetism of MnSb2Te4

MnSb2Te4 has the same crystal structure as MnBi2Te4. Whether it is an intrinsic antiferromagnetic TI, quantum anomalous Hall insulator or axion insulator like MnBi2Te4 [CHIN. PHYS. LETT. 36, (2019) 076801] has not been reported yet. The electronic structure and magnetism of MnSb2Te4 have been studied using first-principles calculations. The results show that the MnSb2Te4 is an antiferromagnetic semiconductor with a trivial energy gap (~ 0.132 eV). The band gap decreases to 0.057 eV under the tensile strain (1.03a0, 1.03c0). The feature of Weyl semimetal could be presented in MnSb2Te4 with ferromagnetic phase under strain 3%. Thin films (011) are metals with antiferromagnetic order and also metals with ferromagnetic order. Thin film (111) with alternate of thick (1 septuple layer–7 septuple layers) is an intrinsic magnetic semiconductor with a trivial energy gap (0.002–0.344 eV) rather than an intrinsic quantum anomalous Hall insulator or axion insulator.

Tuning topological states in a Ni-hexaaminobenzene framework by NO2 adsorption

Exploring a quantum topological phase, such as the quantum spin Hall (QSH) effect and quantum anomalous Hall (QAH) effect in two-dimensional metal-organic frameworks (MOFs), has been attracting significant attention. In this study, we propose to realize a topological phase transition via NO2 adsorption in a recently synthesized two-dimensional Ni-hexaaminobenzene [Ni3(HAB)2]. First-principles calculations show a QSH state of the pristine Ni3(HAB)2 nanosheet. However, when NO2 molecules are adsorbed on the Ni3(HAB)2 surface, the system changes to a ferromagnetic state and the time-reversal symmetry is broken. We find that the NO2-adsorbed Ni3(HAB)2 exhibits an intrinsic QAH state with the Fermi level exactly located within a topologically nontrivial bandgap. The calculated gapless edge states and quantized Hall conductance further confirm the QSH-to-QAH phase transition. Our finding offers an alternative approach of tuning the topological phase transition in a real MOF material.

Exchange bias and quantum anomalous nomalous Hall effect in the MnBi2Te4/CrI3 heterostructure.

The layered antiferromagnetic MnBi2Te4 films have been proposed to be an intrinsic quantum anomalous Hall (QAH) insulator with a large gap. It is crucial to open a magnetic gap of surface states. However, recent experiments have observed gapless surface states, indicating the absence of out-of-plane surface magnetism, and thus, the quantized Hall resistance can only be achieved at the magnetic field above 6 T. We propose to induce out-of-plane surface magnetism of MnBi2Te4 films via the magnetic proximity with magnetic insulator CrI3. A strong exchange bias of ∼40 meV originates from the long Cr-eg orbital tails that hybridize strongly with Te p orbitals. By stabilizing surface magnetism, the QAH effect can be realized in the MnBi2Te4/CrI3 heterostructure. Moreover, the high-Chern number QAH state can be achieved by controlling external electric gates. Thus, the MnBi2Te4/CrI3 heterostructure provides a promising platform to realize the electrically tunable zero-field QAH effect.

The intrinsic magnetism, quantum anomalous Hall effect and Curie temperature in 2D transition metal trihalides.

Searching for experimentally feasible intrinsic quantum anomalous Hall (QAH) insulators is of great significance for dissipationless electronics applications. Here we predict, based on density functional theory (DFT), that four monolayer transition metal tri-bromides (VBr3, FeBr3, NiBr3, and PdBr3) are endowed with intrinsic half-metallicity and possess quantum anomalous Hall insulating phases. DFT+U calculations reveal that the VBr3, NiBr3, and PdBr3 monolayers undergo nontrivial to Mott insulator transitions with increasing on-site Hubbard Coulomb interaction U at 0.5, 2 and 3 eV. The gap opening induced by the spin-orbit coupling drives the systems into the QAH state. The Curie temperatures of the VBr3, NiBr3, and PdBr3 monolayers are ∼190, 100 and 110 K. Additionally, the calculated cleavage energies suggest that the freestanding VBr3, FeBr3, NiBr3, and PdBr3 monolayers can be easily produced by exfoliation using adhesive tape, which may stimulate experimental research interest to achieve QAH phases.

Prediction of intrinsic two-dimensional non-Dirac topological insulators in triangular metal-organic frameworks

Two-dimensional (2D) metal-organic frameworks (MOFs) with topologically nontrivial states have attracted considerable attention recently. Based on ab initio calculations, we demonstrate that Cu(C21N3H15) with a triangular lattice is an intrinsic quantum anomalous Hall insulator. The multiple p (px and py)-like molecular orbitals of the C21N3H15 molecule with C3 symmetry give rise to a degenerate energy point at the Γ point with quadratic non-Dirac band dispersions. When the spin-orbit coupling is included, the bandgap is opened around the non-Dirac Γ point and a topologically nontrivial state is achieved. Quantum spin Hall effects can also be realized in this kind of 2D triangular lattice MOF if the time-reversal symmetry is preserved, as proposed in the Ni(C21N3H15) lattice. The physical mechanism is analyzed through a k · p model built. Our results present that the 2D triangular lattice MOFs are excellent candidates for producing 2D topological insulators and might have promising applications in future microelectronics and spintronics.

Large-gap quantum anomalous Hall phase in hexagonal organometallic frameworks

The nontrivial band gap plays a critical role in quantum anomalous Hall (QAH) insulators. In this work, we propose that the intrinsic QAH phase with sizable band gaps up to 367 meV is achieved in two-dimensional hexagonal organometallic frameworks (HOMFs). Based on first-principles calculations and effective model analysis, we uncover that these large band gaps in transition metal based HOMFs are opened by strong spin-orbital coupling of the localized 4d or 5d electrons. Importantly, we reveal that Coulomb correlations can further significantly enhance the nontrivial band gaps. In addition, we suggest a possible avenue to grow these organometallic QAH insulators on a semiconducting SiC(111) substrate, and the topological features are perfectly preserved due to the van der Waals interaction. Our work shows that the correlation remarkably enhances the nontrivial band gaps, providing exotic candidates to realize the QAH states at high temperatures.

Intrinsic Quantum Anomalous Hall Effect with In-Plane Magnetization: Searching Rule and Material Prediction.

So far, most theoretically predicted and experimentally confirmed quantum anomalous Hall effects (QAHEs) are limited in two-dimensional (2D) materials with out-of-plane magnetization. In this Letter, starting from 2D nodal-line semimetal, a general rule for searching QAHE with in-plane magnetization is mapped out. Because of spin-orbital coupling, we found that the magnetization will prefer an in-plane orientation if the orbital of degenerate nodal-line states at the Fermi level have the same absolute value of magnetic quantum number. Moreover, depending on the broken or conserved mirror symmetry, either a QAHE or 2D semimetal can be realized. Based on first principles calculations, we further predict a real material of monolayer LaCl to be an intrinsic QAHE with in-plane magnetization. By tuning the directions of in-plane magnetization, the QAHE in LaCl demonstrates a threefold rotational symmetry with a Chern number of either +1 or -1, and the transition point is characterized by a 2D semimetal phase. All these features are quantitatively reproduced by tight-binding model calculations, revealing the underlying physics clearly. Our results greatly extend the scope for material classes of QAHE and hence stimulate immediate experimental interest.

Electrically tunable valleytronics in quantum anomalous Hall insulating transition metal trihalides

We propose a different scheme for achieving valley splitting using an electric field in the materials with inversion symmetry but without time-reversal symmetry, and apply this scheme to two-dimensional transition metal trihalides $\text{V}{X}_{3}$ ($X=\text{Cl}$, Br). Based on ab initio calculations and the tight-binding model, we find few layer $\text{V}{X}_{3}$ can be readily tuned from intrinsic quantum anomalous Hall insulators to quantum valley Hall insulators by external electric fields. Especially, the electric-field-induced valley splitting of the bilayer $\text{V}{X}_{3}$ is extremely large, about two orders of magnitude higher than that induced by a magnetic field in the state-of-the-art valleytronic materials (e.g., ${\mathrm{MoS}}_{2}$ and ${\mathrm{WSe}}_{2}$). We further reveal rich topological phases of few layer $\text{V}{X}_{3}$ and valley-polarized states at the phase boundary. These findings may motivate further topology and valleytronics related researches in low-dimensional transition metal compounds.

Intrinsic quantum anomalous hall effect in a two-dimensional anilato-based lattice.

Using first-principles calculations, we predict an intrinsic quantum anomalous Hall (QAH) state in a monolayer anilato-based metal-organic framework M2(C6O4X2)3 (M = Mn and Tc, X = F, Cl, Br and I). The spin-orbit coupling of M d orbitals opens a nontrivial band gap up to 18 meV at the Dirac point. The electron counting rule is used to explain the intrinsic nature of the QAH state. The calculated nonzero Chern number, gapless edge states and quantized Hall conductance all confirm the nontrivial topological properties in the anilato-based lattice. Our findings provide an organic materials platform for the realization of the QAH effect without the need for magnetic and charge doping, which are highly desirable for the development of low-energy-consumption spintronic devices.

Intrinsic Dirac half-metal and quantum anomalous Hall phase in a hexagonal metal-oxide lattice

The quantum anomalous Hall (QAH) effect has attracted extensive attention due to time-reversal symmetry broken by a staggered magnetic flux emerging from ferromagnetic ordering and spin-orbit coupling. However, the experimental observations of the QAH effect are still challenging due to its small nontrivial bulk gap. Here, based on density functional theory and Berry curvature calculations, we propose the realization of intrinsic QAH effect in two-dimensional hexagonal metal-oxide lattice, $\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{3}$, which is characterized by the nonzero Chern number $(C=1)$ and chiral edge states. Spin-polarized calculations indicate that it exhibits a Dirac half-metal feature with temperature as large as ${T}_{C}=392\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ using spin-wave theory. When the spin-orbit coupling is switched on, $\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{3}$ becomes a QAH insulator. Notably, the nontrivial topology is robust against biaxial strain with its band gap reaching up to ${E}_{g}=75\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$, which is far beyond room temperature. A tight-binding model is further constructed to understand the origin of nontrivially electronic properties. Our findings on the Dirac half-metal and room-temperature QAH effect in the $\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{3}$ lattice can serve as an ideal platform for developing future topotronics devices.

Discovery of intrinsic quantum anomalous Hall effect in organic Mn-DCA lattice

The quantum anomalous Hall (QAH) phase is a topological state of matter characterized by a nonzero quantized Hall conductivity without an external magnetic field. The realizations of the QAH effect, however, are experimentally challengeable. Based on ab initio calculations, here, we propose an intrinsic QAH phase in the Mn–dicyanoanthracene (DCA) Kagome lattice. The nontrivial topology in Kagome bands is confirmed by the nonzero Chern number, quantized Hall conductivity, and gapless chiral edge states of the Mn-DCA lattice. A tight-binding model is further constructed to clarify the origin of the QAH effect. Furthermore, its Curie temperature, estimated to be ∼253 K using the Monte-Carlo simulation, is comparable with room temperature and higher than that of most of the two-dimensional ferromagnetic thin films. Our findings present a reliable material platform for the observation of the QAH effect in covalent-organic frameworks.

Quantum anomalous Hall effect in ferromagnetic transition metal halides

The quantum anomalous Hall (QAH) effect is a novel topological spintronic phenomenon arising from inherent magnetization and spin-orbit coupling. Various theoretical and experimental efforts have been devoted in search of robust intrinsic QAH insulators. However, up to now, it has only been observed in Cr or V doped (Bi,Sb)2Te3 film in experiments with very low working temperature. Based on the successful synthesis of transition metal halides, we use first-principles calculations to predict that RuI3 monolayer is an intrinsic ferromagnetic QAH insulator with a topologically nontrivial global band gap of 11 meV. This topologically nontrivial band gap at the Fermi level is due to its crystal symmetry, thus the QAH effect is robust. Its Curie temperature, estimated to be ~360 K using Monte-Carlo simulation, is above room temperature and higher than most of two-dimensional ferromagnetic thin films. We also discuss the manipulation of its exchange energy and nontrivial band gap by applying in-plane strain. Our work adds a new experimentally feasible member to the QAH insulator family, which is expected to have broad applications in nanoelectronics and spintronics.

Intrinsic large gap quantum anomalous Hall insulators inLaX(X=Br,Cl,I)

We report a theoretical prediction of a new class of bulk and intrinsic quantum Anomalous Hall (QAH) insulators La$X$ ($X$=Br, Cl, and I) via relativistic first-principle calculations. We find that these systems are innate long-ranged ferromagnets which, with the help of intrinsic spin-orbit coupling, become QAH insulators. A low-energy multiband tight binding model is developed to understand the origin of the QAH effect. Finally integer Chern number is obtained via Berry phase computation for each two-dimensional plane. These materials have the added benefit of a sizable band gap of as large as $\sim$ 25 meV, with the flexibility of enhancing it to above 75 meV via strain engineering. The synthesis of La$X$ materials will provide the impurity-free single crystals and thin-film QAH insulators for versatile experiments and functionalities.