High-temperature Quantum Anomalous Hall Effect Research Articles

Experimental observation of gapped topological surface states in Sb-doped MnBi4Te7

The realization of intrinsic magnetic topological insulators offers an ideal platform to investigate high-temperature quantum anomalous Hall effect as well as quantum devices. The family of MnBi2Te4(Bi2Te3)n has been confirmed that it belongs to this system. However, whether there is a bandgap in the surface states remains controversial. In this work, the ferromagnetism in Sb-doped MnBi4Te7 is confirmed by magnetic transport. Utilizing angle-resolved photoemission spectroscopy, we demonstrate the nontrivial topology in Mn(Bi0.7Sb0.3)4Te7 with an energy gap on the MnBi2Te4 termination. As well, we detect distinct topological surface states on two different terminations. Our results provide spectral evidence of an energy gap in the topological surface states.

Magneto-optical Effects of an Artificially Layered Ferromagnetic Topological Insulator with a TC of 160 K.

Magnetic topological insulators are a fertile platform for studying the interplay between magnetism and topology. The unique electronic band structure can induce exotic transport and optical properties. However, a comprehensive optical study at both near-infrared and terahertz frequencies has been lacking. Here, we report magneto-optical effects from a heterostructure of a Cr-incorporated topological insulator, CBST. By measuring the magneto-optical Kerr effect, we observe a high temperature ferromagnetic transition (160 K) in the CBST film. We also use time-domain terahertz polarimetry to reveal a terahertz Faraday rotation of 1.5 mrad and a terahertz Kerr rotation of 3.6 mrad at 2 K. The calculated terahertz Hall conductance is 0.42 e2/h. Our work shows the optical responses of an artificially layered magnetic topological insulator, paving the way toward a high-temperature quantum anomalous Hall effect via heterostructure engineering.

Bi2Te3-intercalated MnBi2Te4: ideal candidate to explore intrinsic Chern insulator and high-temperature quantum anomalous Hall effect

The recently discovered magnetic topological insulator of MnBi2Te4 (MBT), has been demonstrated to realize the quantum anomalous Hall (QAH) effect, while the naturally antiferromagnetic (AFM) interlayer coupling in MBT results in that the QAH effect can only be realized in odd-layered systems and at low temperature. Using first-principles calculations, we find that intercalating Bi2Te3 (BT) layers into MBT by forming MBT/(BT) n /MBT (n = 1–6) heterostructures can induce magnetic phase transition from AFM to ferromagnetic (FM) interlayer coupling when n⩾ 3. Specifically, MBT/(BT)3/MBT and MBT/(BT)4/MBT respectively host Curie temperatures T c of 14 K and 11 K, which fits well the experimentally measured T c of 12 K. Detailed band structure calculations and topological identification show that the QAH phases are well preserved for all FM heterostructures. And the topological mechanism of MBT/(BT) n /MBT as a function of n is revealed by employing continuum model analysis. Most importantly, the FM MBT/(BT)4/MBT has already been experimentally fabricated. Thus, our work provides a practical guideline to explore high-temperature QAH effect in MBT family of materials.

A generic designing rule for realizing quantum anomalous Hall phase in a transition-metal trichalcogenide family

Different from introducing ferromagnetism in three-dimensional (3D) topological insulators, empowering nontrivial band topology with 2D ferromagnetic (2DFM) materials is another way to realize quantum anomalous Hall effect (QAHE). The recently discovered 2DFM insulators provide unprecedented opportunities for realizing such an intriguing quantum phenomenon. Here, by using first-principles approaches, we design a generic rule for realizing QAHE in the transition-metal trichalcogenide family materials of Cr2A2X6 (where A = Si, Ge, Sn; X = S, Se, Te), whose design principle is based on that a 2D topologically trivial magnetic insulator can be converted into a QAH system via proper surface modification. Taking silicene-passivated Cr2Sn2Se6 as an illustration, we show that the Cuire temperature of Cr2Sn2Se6 monolayer can be enhanced up to ∼92 K after silicene passivation. Most strikingly, such a heterostructure harbors QAHE with a band gap of ∼30 meV. Further low-effective model analyses reveal that the topologically nontrivial states are attributed to the honeycomb lattice of Cr atoms. All those results demonstrate that our proposal is general, which opens a new avenue to explore high-temperature QAHE.

Gate-Tunable Proximity Effects in Graphene on Layered Magnetic Insulators.

The extreme versatility of van der Waals materials originates from their ability to exhibit new electronic properties when assembled in close proximity to dissimilar crystals. For example, although graphene is inherently nonmagnetic, recent work has reported a magnetic proximity effect in graphene interfaced with magnetic substrates, potentially enabling a pathway toward achieving a high-temperature quantum anomalous Hall effect. Here, we investigate heterostructures of graphene and chromium trihalide magnetic insulators (CrI3, CrBr3, and CrCl3). Surprisingly, we are unable to detect a magnetic exchange field in the graphene but instead discover proximity effects featuring unprecedented gate tunability. The graphene becomes highly hole-doped due to charge transfer from the neighboring magnetic insulator and further exhibits a variety of atypical gate-dependent transport features. The charge transfer can additionally be altered upon switching the magnetic states of the nearest CrI3 layers. Our results provide a roadmap for exploiting proximity effects arising in graphene coupled to magnetic insulators.

Chern Number Tunable Quantum Anomalous Hall Effect in Monolayer Transitional Metal Oxides via Manipulating Magnetization Orientation.

Although much effort has been made to explore quantum anomalous Hall effect (QAHE) in both theory and experiment, the QAHE systems with tunable Chern numbers are yet limited. Here, we theoretically propose that NiAsO_{3} and PdSbO_{3}, monolayer transitional metal oxides, can realize QAHE with tunable Chern numbers via manipulating their magnetization orientations. When the magnetization lies in the x-y plane and all mirror symmetries are broken, the low-Chern-number (i.e., C=±1) phase emerges. When the magnetization exhibits nonzero z-direction component, the system enters the high-Chern-number (i.e., C=±3) phase, even in the presence of canted magnetization. The global band gap can approach the room-temperature energy scale in monolayer PdSbO_{3} (23.4meV), when the magnetization is aligned to z direction. By using Wannier-based tight-binding model, we establish the phase diagram of magnetization induced topological phase transition. Our work provides a high-temperature QAHE system with tunable Chern number for the practical electronic application.

Prediction of quantum anomalous Hall effect in CrI3/ScCl2 bilayer heterostructure

Based on first-principles calculations, a two-dimensional (2D) van der Waals (vdW) bilayer heterostructure consisting of two topologically trivial ferromagnetic (FM) monolayers CrI3 and ScCl2 is proposed to realize the quantum anomalous Hall effect (QAHE) with a sizable topologically nontrivial band gap of 4.5 meV. Its topological nature is attributed to an interlayer band inversion between the monolayers and critically depends on the symmetry of the stacking configuration. We further demonstrate that the topologically nontrivial band gap can be increased nearly linearly by the application of a perpendicular external pressure and reaches 8.1 meV at 2.7 GPa, and the application of an external out-of-plane electric field can also modulate the band gap and convert the system back to topologically trivial via eliminating the band inversion. An effective model is developed to describe the topological phase evolution in this bilayer heterostructure. This work provides a new candidate system based on 2D vdW materials for realization of potential high-temperature QAHE with considerable controllability.

A generic dual d-band model for interlayer ferromagnetic coupling in a transition-metal doped MnBi2Te4 family of materials

Realization of ferromagnetic (FM) interlayer coupling in magnetic topological insulators (TIs) of the MnBi2Te4 family of materials (MBTs) may pave the way for realizing the high-temperature quantum anomalous Hall effect (high-T QAHE). Here we propose a generic dual d-band (DDB) model to elucidate the energy difference (ΔE = EAFM - EFM) between the AFM and FM coupling in transition-metal (TM)-doped MBTs, where the valence of TMs splits into d-t2g and d-eg sub-bands. Remarkably, the DDB shows that ΔE is universally determined by the relative position of the dopant (X) and Mn d-eg/t2g bands, . If ΔEd > 0, then ΔE > 0 and the desired FM coupling is favored. This surprisingly simple rule is confirmed by first-principles calculations of hole-type 3d and 4d TM dopants. Significantly, by applying the DDB model, we predict the high-T QAHE in the V-doped Mn2Bi2Te5, where the Curie temperature is enhanced by doubling of the MnTe layer, while the topological order mitigated by doping can be restored by strain.

Intrinsic ferromagnetism and the quantum anomalous Hall effect in two-dimensional MnOCl2 monolayers.

Due to their potential application in spintronic devices, two-dimensional (2D) ferromagnetic materials are highly desired. We used first-principles calculations and Monte Carlo simulations to investigate the electronic structure and magnetic characteristics of the MnOCl2 monolayers. We discovered two stable monolayer structures, Pmna-MnOCl2 and Pmmn-MnOCl2. Our findings show that the Pmna-MnOCl2 monolayer is an intrinsic ferromagnetic semiconductor with an indirect band gap of 0.152 eV and a Curie temperature (TC) of 202 K, while the Pmmn-MnOCl2 monolayer is an intrinsic ferromagnetic Dirac semimetal with a high TC (910 K) and triaxial magnetic anisotropy. We also show that a Pmmn-MnOCl2 monolayer with a nontrivial band gap of 6.2 meV can achieve the quantum anomalous Hall effect (QAHE) with Chern number C = 1. Additionally, the existence of a gapless edge state can be flexibly regulated by choosing the terminal edges. Our studies reveal that the Pmmn-MnOCl2 monolayer can serve as a candidate material to achieve high-temperature QAHE.

Charge carrier mediation and ferromagnetism induced in MnBi6Te10 magnetic topological insulators by antimony doping

A new kind of intrinsic magnetic topological insulator (MTI) MnBi2Te4 family has shed light on the observation of novel topological quantum effects such as the quantum anomalous Hall effect (QAHE). However, strong anti-ferromagnetic (AFM) coupling and high carrier concentration in the bulk hinder practical applications. In closely related materials MnBi4Te7 and MnBi6Te10, the interlayer magnetic coupling is greatly suppressed by Bi2Te3 layer intercalation. However, AFM is still the ground state in these compounds. Here, by magnetic and transport measurements, we demonstrate that a Sb substitutional dopant plays a dual role in MnBi6Te10, which can not only adjust the charge carrier type and concentration, but also induces the solid into a ferromagnetic (FM) ground state. The AFM ground state region, which is also close to the charge neutral point, can be found in the phase diagram of Mn(Sb x Bi1−x )6Te10 when x ∼ 0.25. An intrinsic FM-MTI candidate is thus demonstrated, which may take us a step closer to realizing a high-quality and high-temperature QAHE and related topological quantum effects in the future.

Quantum anomalous Hall effect in MnBi2Te4 van der Waals heterostructures

Quantum anomalous Hall effect (QAHE) has been experimentally realized in the intrinsic antiferromagnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$. However, at ambient condition the magnetization of the $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ sample decreases significantly after it is exposed in air for a couple of days, which hinders its potential application. Here, we theoretically propose to cover 2D van der Waals materials such as hexagonal boron nitride (h-BN) monolayer onto the surface of $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$. This not only protects the stability of $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$, but also leads to interlayer ferromagnetic coupling and further realizes the QAHE at higher temperature. We find that interlayer ferromagnetic transition occurs generally when monolayer h-BN, $\mathrm{Mo}{\mathrm{S}}_{2}$, or $\mathrm{W}{\mathrm{Se}}_{2}$ is covered onto two or three septuple-layer $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ with interlayer antiferromagnetic coupling. Band-structure and topological properties calculations show that $\mathrm{h}\ensuremath{-}\mathrm{BN}/\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ heterostructure exhibits a topologically nontrivial band gap around 64--75 meV, hosting QAHE with a Chern number of $C=1$. Our proposed materials system should be considered as an ideal platform to explore high-temperature QAHE due to their relatively simple and stable structures.

Coexistence of ferromagnetism and topology by charge carrier engineering in the intrinsic magnetic topological insulator MnBi4Te7

Intrinsic magnetic topological insulators (MTIs) $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ and $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}/{({\mathrm{Bi}}_{2}{\mathrm{Te}}_{3})}_{n}$ are expected to realize the high-temperature quantum anomalous Hall effect and dissipationless electrical transport. However, there is still a lack of ideal MTI candidates with magnetic ordering of the ferromagnetic (FM) ground state. Here, we show a MTI sample of $\mathrm{Mn}{({\mathrm{Bi}}_{0.7}{\mathrm{Sb}}_{0.3})}_{4}{\mathrm{Te}}_{7}$ which holds the coexistence of a FM behavior state and topological nontriviality. The dramatic modulation of the magnetism is induced by a charge carrier engineering process via the Sb substitution in the $\mathrm{Mn}{\mathrm{Bi}}_{4}{\mathrm{Te}}_{7}$ matrix with antiferromagnetic ordering. The evolution of magnetism in $\mathrm{Mn}{({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{4}{\mathrm{Te}}_{7}$ is systematically investigated by our magnetic measurements and theoretical calculations. The clear topological surface states of the FM sample of $\mathrm{Mn}{({\mathrm{Bi}}_{0.7}{\mathrm{Sb}}_{0.3})}_{4}{\mathrm{Te}}_{7}$ are further verified by angle-resolved photoemission spectroscopy. The demonstration of the intrinsic FM-MTI of $\mathrm{Mn}{({\mathrm{Bi}}_{0.7}{\mathrm{Sb}}_{0.3})}_{4}{\mathrm{Te}}_{7}$ in this paper sheds light on further material optimization of intrinsic MTIs and paves the way for further studies to clarify the relationships between topology, magnetism, and charge carriers in topological materials.

Interlayer ferromagnetism and high-temperature quantum anomalous Hall effect in p -doped MnBi2Te4 multilayers

The interlayer antiferromagnetic coupling hinders the observation of quantum anomalous Hall effect in magnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$. We demonstrate that interlayer ferromagnetism can be established by utilizing the $p$-doping method in $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ multilayers. In two septuple layers system, the interlayer ferromagnetic coupling appears by doping nonmagnetic elements (e.g., N, P, As, Na, Mg, K, and Ca), due to the redistribution of orbital occupations of Mn. We further find that Mg and Ca elements are the most suitable candidates because of their low formation energy. Although, the $p$-doped two septuple layers exhibit topologically trivial band structure, the increase of layer thickness to three (four) septuple layers with Ca (Mg) dopants leads to the formation of the quantum anomalous Hall effect. Our proposed $p$-doping strategy without introducing additional magnetic disorder not only makes $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ become an ideal platform to realize the high-temperature quantum anomalous Hall effect without external magnetic field, but also can compensate the electrons from the intrinsic $n$-type defects in $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$.

Quarter-filled Kane-Mele Hubbard model: Dirac half metals

Recent experimental success in the realization of two-dimensional (2D) magnetism has stimulated the search for new magnetic 2D materials with strong magnetic anisotropy and high Curie temperature. One promising subgroup of 2D magnetic systems are Dirac half-metals (DHM) which have gained a lot of interest recently, as they host a high-temperature quantum anomalous Hall effect (QAHE). This article discusses predictions for intrinsic DHMs and identifies them as realizations of the Kane-Mele Hubbard model at quarter filling. This proposed unification contributes to a firmer understanding of these materials and suggests pathways for the discovery of new DHM systems.

Prediction of monolayered ferromagnetic CrMnI6 as an intrinsic high-temperature quantum anomalous Hall system

Quantized Hall conductance without an external magnetic field, known as the quantum anomalous Hall effect (QAHE), may have important applications in dissipationless spintronics, yet to date, it has only been realized in magnetically doped topological insulators and at very low temperatures. Here we design a physically realistic system for realizing QAHE by expanding the recently discovered two-dimensional ferromagnetic insulators as a new class of candidate materials. Based on first-principles calculations, we predict that a ${\mathrm{CrMnI}}_{6}$ monolayer is energetically stable and can be readily exfoliated. This system is further shown to be a ferromagnetic insulator, with a transition temperature of $\ensuremath{\sim}87\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, higher than that of ${\mathrm{CrI}}_{3}$. Most strikingly, such a monolayer is characterized as an intrinsic QAHE system with a high Chern number of $C$ = 2, and the underlying mechanism for the nontrivial topology is attributed to the two inequivalent subset sites of the Cr and Mn atoms. The present study thus provides an ideal platform for realizing high-temperature QAHE beyond the prevailing materials class of magnetically doped topological insulators.

Nonmagnetic doping induced quantum anomalous Hall effect in topological insulators

Quantum anomalous Hall effect (QAHE) has been experimentally observed in magnetically doped topological insulators. However, ultra-low temperature (usually below 300 mK), which is mainly attributed to inhomogeneous magnetic doping, becomes a daunting challenge for potential applications. Here, a \textit{nonmagnetic}-doping strategy is proposed to produce ferromagnetism and realize QAHE in topological insulators. We numerically demonstrated that magnetic moments can be induced by nitrogen or carbon substitution in Bi$_2$Se$_3$, Bi$_2$Te$_3$, and Sb$_2$Te$_3$, but only nitrogen-doped Sb$_2$Te$_3$ exhibits long-range ferromagnetism and preserve large bulk band gap. We further show that its corresponding thin-film can harbor QAHE at temperatures of 17-29 Kelvin, which is two orders of magnitude higher than the typical temperatures in similar systems. Our proposed \textit{nonmagnetic} doping scheme may shed new light in experimental realization of high-temperature QAHE in topological insulators.

The mechanism exploration for zero-field ferromagnetism in intrinsic topological insulator MnBi2Te4 by Bi2Te3 intercalations

Recent research on intrinsic magnetic topological insulators (MTIs), MnBi2Te4, sheds new light on the observation of a long-expected high-temperature quantum anomalous Hall effect (QAHE). However, the strong interlayered anti-ferromagnetic (AFM) coupling hinders the practical applications without applying a magnetic field. Thus, how to adjust the magnetism of this compound under zero field is essential. Here, we theoretically and experimentally study the magnetic properties of two new promising intrinsic MTI candidates MnBi4Te7 and MnBi6Te10, formed by intercalating the Bi2Te3 layer into MnBi2Te4. The first-principles calculations reveal that the relative energy between ferromagnetic (FM) and AFM states is greatly reduced by Bi2Te3 intercalations. The calculated energy barriers for the spin flipping process also point out that the metastable FM state is more easily retained by intercalation. Meanwhile, we also experimentally carry out magnetic and transport measurements on these materials. By increasing Bi2Te3 intercalations, the AFM coupling becomes weaker, and an almost fully polarized FM state can be preserved in MnBi6Te10 at low temperatures, which are consistent with our calculations. We believe that the demonstration of the intrinsic MTI preserving zero-field FM state and the in-depth investigation for the mechanism behind pave the way for investigating the high-temperature QAHE and the related physics.

Pursuing the high-temperature quantum anomalous Hall effect in MnBi2Te4/Sb2Te3 heterostructures

Quantum anomalous Hall effect (QAHE) has been experimentally realized in magnetically-doped topological insulators or intrinsic magnetic topological insulator MnBi$_2$Te$_4$ by applying an external magnetic field. However, either the low observation temperature or the unexpected external magnetic field (tuning all MnBi$_2$Te$_4$ layers to be ferromagnetic) still hinders further application of QAHE. Here, we theoretically demonstrate that proper stacking of MnBi$_2$Te$_4$ and Sb$_2$Te$_3$ layers is able to produce intrinsically ferromagnetic van der Waals heterostructures to realize the high-temperature QAHE. We find that interlayer ferromagnetic transition can happen at $T_{\rm C}=42~\rm K$ when a five-quintuple-layer Sb$_2$Te$_3$ topological insulator is inserted into two septuple-layer MnBi$_2$Te$_4$ with interlayer antiferromagnetic coupling. Band structure and topological property calculations show that MnBi$_2$Te$_4$/Sb$_2$Te$_3$/MnBi$_2$Te$_4$ heterostructure exhibits a topologically nontrivial band gap around 26 meV, that hosts a QAHE with a Chern number of $\mathcal{C}=1$. In addition, our proposed materials system should be considered as an ideal platform to explore high-temperature QAHE due to the fact of natural charge-compensation, originating from the intrinsic n-type defects in MnBi$_2$Te$_4$ and p-type defects in Sb$_2$Te$_3$.

High-temperature and multichannel quantum anomalous Hall effect in pristine and alkali-metal-doped CrBr3 monolayers.

The realization of the high-temperature and multichannel quantum anomalous Hall effect (QAHE) has been a central research area in the development of low-power-consumption electronics and quantum computing. Recently discovered two-dimensional (2D) ferromagnetic (FM) materials provide unprecedented opportunities for the exploration of the high-temperature QAHE. Based on first-principles approaches, we first reveal that a FM CrBr3 monolayer harbors topologically nontrivial conduction bands with a high Chern number of C = 2. Then, we reveal that the interesting conduction bands can be moved downwards to the Fermi levels by electron and alkali-metal-doping; meanwhile, the QAHE characteristics can be preserved. Most strikingly, the Na-doped CrBr3 system possesses a higher Chern number of C = -4 with a transition temperature of ∼54 K, which is attributed to the constructive coupling effect of the quadratic non-Dirac and linear Dirac band dispersions. The present study, together with recent achievements in the field of 2D FM materials, provides an experimentally achievable guide for realizing the high-temperature and multichannel QAHE based purely on 2D FM systems.

The Road to High-Temperature Quantum Anomalous Hall Effect in Magnetic Topological Insulators

Elevating the temperature needed to realize the quantum anomalous Hall effect (QAHE) to above 77 K or higher is the key to practical applications of the effect as well as many others based on it. Here, we briefly review the recent progresses, especially the experimental ones, on searching for magnetic topological insulators that can exhibit the QAHE at higher temperature. Based on our understanding of these results, we will give the principles and roadmap for designing and constructing high-temperature quantum anomalous Hall systems.