Quantum Anomalous Hall Research Articles

Unveiling the chirality of the quantum anomalous Hall effect

Unveiling the chirality of the quantum anomalous Hall effect

Two-dimensional ferromagnetic V2Cl3Br3 with tunable topological phases

In recent years, there has been significant research focus on topological states in two-dimensional materials, particularly those exhibiting intrinsic magnetic orderings. In this study, we employ theoretical calculations to uncover the remarkable band topology of monolayer V2Cl3Br3. This two-dimensional compound not only possesses a half-metallic ferromagnetic ground state but also demonstrates exceptional thermodynamic and mechanical stabilities. Notably, we observe clean band crossings with complete spin polarization, which manifest as phase transitions between Weyl semimetal states and quantum anomalous Hall states under different magnetization directions. Both of these topological phases exhibit pronounced edge states. Furthermore, utilizing Monte Carlo simulations with the Heisenberg model, we estimate a high Curie temperature of up to 79.2 K, indicating its potential for spintronic development. Additionally, we analyze the mechanical properties of the material, emphasizing its isotropic mechanical behavior, which offers advantageous features for practical applications. These findings establish the foundation for further exploration of practical applications involving two-dimensional ferromagnetic topological states. Importantly, the identified material candidate can be readily incorporated into experimental preparation, thus accelerating progress in topological quantum applications and the development of half-metallic spintronic devices.

Intertwined fractional quantum anomalous Hall states and charge density waves

Intertwined fractional quantum anomalous Hall states and charge density waves

On the zero-field quantization of the anomalous quantum Hall effect in two-dimensional moiré layers

On the zero-field quantization of the anomalous quantum Hall effect in two-dimensional moiré layers

Monolayer V_{2}MX_{4}: A New Family of Quantum Anomalous Hall Insulators.

We theoretically propose that the van der Waals layered ternary transition metal chalcogenide V_{2}MX_{4} (M=W, Mo; X=S, Se) is a new family of quantum anomalous Hall insulators with sizable bulk gap and Chern number C=-1. The large topological gap originates from the deep band inversion between spin-up bands contributed by d_{xz}, d_{yz} orbitals of V and spin-down band from d_{z^{2}} orbital of M at the Fermi level. Remarkably, the Curie temperature of monolayer V_{2}MX_{4} is predicted to be much higher than that of monolayer MnBi_{2}Te_{4}. Furthermore, the thickness dependence of the Chern number for few multilayers shows interesting oscillating behavior. The general physics from the d orbitals here applies to a large class of ternary transition metal chalcogenide such as Ti_{2}WX_{4} with the space group P-42m. These interesting predictions, if realized experimentally, could greatly promote the research and application of topological quantum physics.

A new 2D Janus family with multiple properties: auxetic behavior, straintunable photocatalyst, high Curie temperature ferromagnets, and piezoelectric quantum anomalous Hall insulator

A new 2D Janus family with multiple properties: auxetic behavior, straintunable photocatalyst, high Curie temperature ferromagnets, and piezoelectric quantum anomalous Hall insulator

Evolution from quantum anomalous Hall insulator to heavy-fermion semimetal in magic-angle twisted bilayer graphene

Evolution from quantum anomalous Hall insulator to heavy-fermion semimetal in magic-angle twisted bilayer graphene

Magnetization direction-controlled topological band structure in TlTiX (X = Si, Ge) monolayers

The quantum anomalous Hall (QAH) insulator is a vital material for the investigation of emerging topological quantum effects, but its extremely low working temperature limits experiments. Apart from the temperature challenge, effective regulation of the topological state of QAH insulators is another crucial concern. Here, by first-principles calculations, we find a family of stable two-dimensional materials TlTiX (X = Si, Ge) are large-gap QAH insulators. Their extremely robust ferromagnetic (FM) ground states are determined by both the direct- and super-exchange FM coupling. In the absence of spin–orbit coupling (SOC), there exist a spin-polarized crossing point located at each K and Kʹ points, respectively. The SOC effect results in the spontaneous breaking of C 2 symmetry and introduces a mass term, giving rise to a QAH state with sizable band gap. The tiny magnetocrystalline anisotropic energy (MAE) implies that an external magnetic field can be easily used to align magnetization deviating from z direction to the x–y plane, thereby leading to a transformation of the electronic state from the QAH state to the Weyl half semimetals state, which indicate monolayers TlTiX (X = Si, Ge) exhibit a giant magneto topological band effect. Finally, we examined the impact of stress on the band gap and MAE, which underlies the reasons for the giant magneto topological band effect attributed to the crystal field. These findings present novel prospects for the realization of large-gap QAH states with the characteristic of easily modifiable topological states.

Photoinduced high-Chern-number quantum anomalous Hall effect from higher-order topological insulators

Photoinduced high-Chern-number quantum anomalous Hall effect from higher-order topological insulators

Strain-driven valley-dependent Berry phase effects and topological transitions in Janus SVGeN2 monolayer

The manipulation of valley-dependent properties in two-dimensional (2D) materials is intriguing for developing valleytronics. Using first-principles calculations, we explore valley-dependent properties of Janus SVGeN2 monolayer and reveal large and tunable valley polarization by tensile strain. The SVGeN2 monolayer possesses excellent stability. Furthermore, strain-driven topological magneto-valley phase transitions are predicted for this monolayer, leading to the valley quantum anomalous Hall (VQAH) phenomenon. The VQAH state, which is featured by the coexistence of complete valley polarization and topological phase, is confirmed by sign reversal of Berry curvature and the nontrivial band topology. The calculated magnetic anisotropy energy indicates that the Janus SVGeN2 monolayer possesses a ferromagnetic ground state and in-plane magnetization. Our investigation provides some physical insights into the strain-driven topological phase transition and manipulation of valley-dependent properties to realize giant valley polarization in the Janus 2D magnet.

Fractional Quantum Anomalous Hall Phase for Raman Superarray of Rydberg Atoms

AbstractRydberg atom arrays offer promising platforms for quantum simulation of correlated quantum matter and raise great interests. This work proposes a novel stripe‐lattice model with “Raman superarray of Rydberg atoms” to realize bosonic fractional quantum anomalous Hall (FQAH) phase. Two types of Rydberg states, arranged in a supperarray configuration and with Raman‐assisted dipole‐exchange couplings, are implemented to realize a minimal QAH model for hard‐core bosons populated into a topological flat band with large bulk gap under proper tunable experimental condition. With this the bosonic FQAH phase can be further achieved and probed feasibly. In particular, a novel quench protocol is proposed to probe the fractionalized excitations by measuring the correlated quench dynamics featured by fractional charge tunneling between bulk and chiral edge modes in the open boundary.

Discovery of controllable high Chern number quantum anomalous Hall state in tetragonal lattice FeSIn

Quantum anomalous Hall (QAH) insulators have excellent properties driven by fancy topological physics, but their practical application is greatly hindered by the observed temperature of liquid nitrogen, and the QAH insulator with high Chern number is conducive to spintronic devices with lower energy consumption. Here, we find that monolayer FeSIn is a good candidate for realizing the QAH phase; it exhibits a high magnetic transition temperature of 221 K and tunable C = ±2 with respect to magnetization orientation in the y–z plane. After the application of biaxial strain, the magnetic axis shifts from the x–y plane to the z direction, and the effect of the high C and ferromagnetic ground state on the stress is robust. Also, the effect of correlation U on C has been examined. These properties are rooted in the large size of the Fe atom that contributes to ferromagnetic kinetic exchange with neighboring Fe atoms. These findings demonstrate monolayer FeSIn to be a major template for probing novel QAH devices at higher temperatures.

Fractional quantum anomalous Hall effect in multilayer graphene.

The fractional quantum anomalous Hall effect (FQAHE), the analogue of the fractional quantum Hall effect1 at zero magnetic field, is predicted to exist in topological flat bands under spontaneous time-reversal-symmetry breaking2-6. The demonstration of FQAHE could lead to non-Abelian anyons that form the basis of topological quantum computation7-9. So far, FQAHE has been observed only in twisted MoTe2 at a moiré filling factor v > 1/2 (refs. 10-13). Graphene-based moiré superlattices are believed to host FQAHE with the potential advantage of superior material quality and higher electron mobility. Here we report the observation of integer and fractional QAH effects in a rhombohedral pentalayer graphene-hBN moiré superlattice. At zero magnetic field, we observed plateaus of quantized Hall resistance [Formula: see text] at v = 1, 2/3, 3/5, 4/7, 4/9, 3/7 and 2/5 of the moiré superlattice, respectively, accompanied by clear dips in the longitudinal resistance Rxx. Rxy equals [Formula: see text] at v = 1/2 and varies linearly with v, similar to the composite Fermi liquid in the half-filled lowest Landau level at high magnetic fields14-16. By tuning the gate-displacement field D and v, we observed phase transitions from composite Fermi liquid and FQAH states to other correlated electron states. Our system provides an ideal platform for exploring charge fractionalization and (non-Abelian) anyonic braiding at zero magnetic field7-9,17-19, especially considering a lateral junction between FQAHE and superconducting regions in the same device20-22.

Magnetic and electrical properties of Co2Te3 single crystal

The Co-Te system has become a prominent field of research in recent times. CoTe2 is confirmed to be a Dirac semimetal, while Co2Te is predicted to host the quantum anomalous Hall effect. Here, we have successfully grown high-quality Co2Te3 single crystals using the solid-state reaction method and have characterized them using the energy dispersive X-ray spectroscopy, the X-ray diffraction and Laue diffraction. The X-ray rocking curve with a full-width at half-maximum of 0.067° shows Co2Te3 single crystals have good crystalline quality and mosaicity. Co2Te3 single crystals possess a hexagonal structure with the space group of P63/mmc. Resistivity measurements reveal typical metallic behavior with a transition from Non-Fermi liquid to low-temperature Landau Fermi-liquid behavior around 50 K, which can be fitted by the parallel resistance model. Magnetic measurements show no obvious anisotropy between the in-plane and out-of-plane directions with the presence of a small magnetic hysteresis at 1.8 K. Furthermore, Co2Te3 single crystals undergo a magnetic phase transition around 50 K and an antiferromagnetic transition modulated by the magnetic field below 8 K.

Maximally localized Wannier functions, interaction models, and fractional quantum anomalous Hall effect in twisted bilayer MoTe2

We investigate the moiré band structures and the strong correlation effects in twisted bilayer MoTe[Formula: see text] for a wide range of twist angles, employing a combination of various techniques. Using large-scale first-principles calculations, we pinpoint realistic continuum modeling parameters, subsequently deriving the maximally localized Wannier functions for the top three moiré bands. Simplifying our model with reasonable assumptions, we obtain a minimal two-band model, encompassing Coulomb repulsion, correlated hopping, and spin exchange. Our minimal interaction models pave the way for further exploration of the rich many-body physics in twisted MoTe[Formula: see text]. Furthermore, we explore the phase diagrams of the system through Hartree-Fock approximation and exact diagonalization (ED). Our two-band ED analysis underscores significant band-mixing effects in this system, which enlarge the optimal twist angle for fractional quantum anomalous Hall states.

Phonon and defect mediated quantum anomalous Hall insulator to metal transition in magnetically doped topological insulators

Phonon and defect mediated quantum anomalous Hall insulator to metal transition in magnetically doped topological insulators

Spin resolved topological bulk state in acoustics

Extremely rare topologically protected acoustic energy sink is presented in this article. Acoustic topological phenomena are generally described using quantum anomalous hall effects (QAHE), quantum valley hall effects (QVHE), and quantum spin hall effects (QSHE) where spin orbit coupling is predominant. Topological edge states are demonstrated by bulk-boundary distinction when the bulk is insulated. In this article topological acoustic conductor and its phenomena are theoretically demonstrated where the boundaries are insulated. This is exactly opposite to the behavior of a topological acoustic insulator. Phenomena presented in this article could not be explained by any of the trio Quantum Hall effects. To explain the phenomenon phononic crystals or PnCs were designed to obtain accidental triple degeneracies, resulting a Dirac-like cone at the Γ point (k→=0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\overrightarrow{k}=0$$\\end{document}). The phenomenon is microarchitecture and microrotation field independent. Here time reversal symmetry or the space inversion symmetry is not broken, and the degenerated ‘Deaf band’ dominates the local dispersion with a syncline top band. This scenario results in continuously changing ‘up spin’ and ‘down spin’ of the wave energy in the media and remain trapped without specific preferential direction of wave transport. The spin was found to generate the spin angular momentum, causing the switching in geometric phase from 0-2π\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0-2\\pi$$\\end{document} in cyclic pattern, keeping the energy trapped inside the bulk media.

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.

Scanning tunneling microscopy study of epitaxial Fe3GeTe2 monolayers on Bi2Te3

Introducing magnetism to the surface state of topological insulators, such as Bi2Te3, can lead to a variety of interesting phenomena. We use scanning tunneling microscopy (STM) to study a single quintuple layer (QL) of the van der Waals magnet Fe3GeTe2 (FGT) that is grown on Bi2Te3 via molecular beam epitaxy. STM topographic images show that the FGT grows as free-standing islands on Bi2Te3 and outwards from Bi2Te3 steps. Atomic resolution imaging shows triangular lattices of 390 ± 10 pm for FGT and 430 ± 10 pm for Bi2Te3, consistent with the respective bulk crystals. A moiré pattern is observed on FGT regions with a periodicity of 4.3 ± 0.4 nm that can be attributed solely to this lattice mismatch and thus indicates zero rotational misalignment. While most of the surface is covered by a single QL of the FGT, there are small double QL regions, as well as regions with distinct chemical terminations due to an incomplete QL. The most common partial QL surface termination is the FeGe layer, in which the top two atomic layers are missing. This termination has a distinctive electronic structure and a 3 x 3R30∘ reconstruction overlaid on the moiré pattern in STM images. Magnetic circular dichroism measurements confirm these thin FGT films are ferromagnetic with T C ∼190 K.

Orbital-Selectivity-Induced Robust Quantum Anomalous Hall Effect in Hund's Metals MgFeP.

Ferromagnetic (FM) states with high Curie temperatures (Tc) and strong spin-orbit coupling (SOC) are indispensable for the long-sought room-temperature quantum anomalous Hall (QAH) effects. Here, we propose a two-dimensional (2D) iron-based monolayer MgFeP that exhibits a notably high FM Tc (about 1525 K) along with exceptional structural stabilities. The unique multiorbital nature in MgFeP, where localized and dxz/yz orbitals coexist with itinerant dxy and dz2 orbitals, renders the monolayer a Hund's metal and in an orbital-selective Mott phase (OSMP). This OSMP triggers an FM double exchange mechanism, rationalizing the high Tc in the Hund's metal. This material transitions to a QAH insulator upon consideration of the SOC effect. By leveraging orbital selectivity, the QAH band gap can be enlarged by more than two times (to 137 meV). Our findings showcase Hund's metals as a promising material platform for realizing high-performance quantum topological electronic devices.