Topological magnons offer unique opportunities for low-dissipation spin transport, but achieving nonvolatile control over their topological states remains a significant challenge. Here, using a Heisenberg-Dzyaloshinskii-Moriya model and symmetry analysis, we propose a ferroelectrically tunable magnonic platform that enables reversible switching among three distinct topological phases: a second-order topological magnon insulator, a topological magnon insulator, and a normal magnon insulator. This transition is characterized by the simultaneous emergence and reversal of spontaneous magnon valley polarization. We further identify the Ti_{3}I_{8} monolayer with a breathing kagome lattice as a promising material platform that supports electric-field-driven topological switching and reversal of spontaneous magnon valley polarization, as confirmed by first-principles calculations. Notably, this material platform also hosts electrically controllable valley-dependent magnonic transport, including valley Hall and valley Nernst effects. This Letter establishes topological magnons as a functional bridge linking ferroelectricity with magnon cornertronic and magnon valleytronic responses.

We theoretically show the switching between topologically nontrivial phases characterized by different bulk-boundary correspondences and, in particular, establish the intrinsic coupling between band topology and orbital Hall effect in two-dimensional (2D) ferromagnets. We further emphasize that the variation of orbital angular momentum induced by topological phase transitions accompanied by band inversion plays a crucial role in modulating the magnitude of the orbital Hall conductivity. Taking the hexagonal Janus SVSiN2 monolayer as a representative material candidate, we validate the feasibility of our proposal. The material hosts the 2D second-order topological insulator with intrinsic ferromagnetism, and strain-induced valley modulation drives successive topological phase transitions from second-order topological insulators to Chern insulator and, finally, to normal insulator. The topologically nontrivial natures in SVSiN2 are confirmed through the analysis of corner states, edge states, and Chern numbers. Notably, the topological phase transitions give rise to a switchable orbital Hall effect, highlighting their critical role in modulating orbital transport. These findings underscore the profound connection between nontrivial topology and the orbital Hall effect, opening promising avenues for future advances in topological spintronics and orbitronics.

Detecting insulator defects accurately and efficiently is vital for maintaining the reliability of power transmission systems, particularly during Unmanned Aerial Vehicle (UAV)-based inspections. In order to improve local and global feature extraction and detect minor, low-visibility flaws in complex settings, this paper suggests C2F-YOLOv11n, a lightweight detection framework that incorporates the Conv2Former attention mechanism. Experiments on a self-built insulator dataset show that C2F-YOLOv11n achieves 91.7% precision, 83.1% recall, 89.4% mAP50, and 58.9% mAP50-95, with inference speed at 194 FPS and a compact 2.70 MB model size, outperforming YOLOv8n and YOLOv10n. Additionally, a novel regression loss function, PW-IoU, combining PIoUv2's boundary-aware localization and WIoUv3's adaptive gradient reweighting, is introduced to address bounding box regression challenges. By integrating PW-IoU, the model achieves higher performance with precision reaching 92.6%, recall at 87.5%, mAP50 at 90.8%, and mAP50-95 at 59.3%, outperforming conventional Complete IoU (CIoU) and other IoU-related loss functions. PW-IoU enhances localization accuracy and convergence stability, especially for small targets in complex backgrounds. Furthermore, comparative experiments on the publicly available Chinese Power Line Insulator Dataset (CPLID) confirm the model's strong generalization, achieving competitive detection performance on both normal and defective insulators.

Under the DC field, live contamination is more likely to deposit on the surface of insulators due to the action of the external electric field. The deposition of dirt on the surface of Ultra High Voltage (UHV) insulators can lead to the occurrence of flashover phenomena, causing significant economic losses. Due to the particularity of UHV insulators, many traditional surface anti-pollution technologies designed for normal voltage insulators are not applicable to them. In order to prevent the harm of contamination accumulation affecting the safe operation of transmission lines, in this study, tetragonal BaTiO3 was mixed into room-temperature vulcanized silicone rubber for the first time to prepare a composite coating with piezoelectric properties. This coating can use the piezoelectric effect to remove the contamination adhering to the surface of UHV insulators under a DC field. In this study, the piezoelectric properties of the prepared tetragonal BaTiO3 were verified through material characterization. The results show that the introduction of piezoelectric fillers can significantly accelerate the dissipation of charges on the insulator surface under slight disturbances, which helps to reduce the accumulation of charged pollutants on the insulator surface. The anti-pollution performance under electric field conditions was verified through a simulation experimental device. Finally, through experiments in a real converter station environment, the anti-pollution effect of the insulator under actual working conditions was verified.

Weyl semimetals exhibit extraordinary electronic properties derived from their topologically protected Weyl nodes. However, their practical applications are constrained by the scarcity of natural materials and the inherent lack of tunability in their intrinsic properties. In this work, we investigate how the topological states of Weyl semimetals in artificial heterostructures composed of alternating layers of topological insulators and normal insulators can be dynamically regulated by an external alternating-current light field. To this end, we construct a lattice regularization of the continuum multilayer model, which enables a systematic analysis of light-induced Weyl semimetal phases. Based on the model, the Floquet–Weyl semimetal states can be investigated for both longitudinal and transverse light field. It is found that the field provides flexible controllability on the topological phase transition and exhibits the precise manipulation on Weyl nodes, including adjustments to their energy positions, chirality reversal, node annihilation or creation, and anisotropic deformation. The field-driven modification overcomes the rigidity of natural Weyl materials and paves the way for their applications in tunable optoelectronics and quantum computing.

This study proposes a novel autonomous inspection strategy for insulator strings using a drone. The proposed method not only optimizes the viewpoint of an optical camera for acquiring high-quality images of insulator strings but also detect anomalies of insulator strings from the acquired images. The proposed method features three key characteristics. First, an adaptive flight strategy is proposed based on the spatial configuration of transmission facilities. Specifically, the type of transmission tower is classified as either suspension or strain by analyzing the orientation of the insulator strings detected from optical images. Key structural features of transmission facilities are then extracted from point cloud data by addressing effective signal processing methods including random sample consensus, Euclidean distance clustering, and probabilistic downsampling. This feature enables the drone dynamically adjust the heading, altitude, and camera tilt to acquire optimal images of insulator strings. Second, a novel architecture of a deep neural network is proposed to detect defects in insulator strings based on the acquired images of insulator strings. Specifically, the architecture of the proposed network combines a multi-scale variational autoencoder and a lightweight classifier for anomaly detection. The variational autoencoder reconstructs normal insulator images at multiple scales to acquire hierarchical features, and the classifier distinguishes between normal and defective patterns by utilizing the extracted multi-scale features. Third, synthetic images of insulator strings are generated to mitigate a concern on the data imbalance between normal and abnormal images of insulator strings. Specifically, 3D models of insulator strings are constructed by using computer-aided design tools, and fault patterns are embedded to generate abnormal samples. 2D synthetic images are then rendered under varying viewpoints, lighting conditions, and backgrounds. Additionally, a generative adversarial network is addressed to produce realistic defect images to enhance the diversity of abnormal samples. These synthetic images contribute to improving the robustness of the proposed anomaly detection network. Systematic analyses conducted in both virtual and real-world environments show the effectiveness of the proposed method. The adaptive flight mission was successfully completed to acquire high-quality images of insulator strings without visual overlap between adjacent insulator strings. The proposed network achieves classification accuracy of 95.0% in distinguishing between normal and abnormal insulator strings for anomaly detection. The proposed strategy not only improves the performance of autonomous inspection but also enhances operational safety by reducing reliance on manual inspection in hazardous environments.

We show that a semi-Dirac (SD) system with an inversion symmetry breaking mass exhibits a topological phase transition when irradiated with off-resonant light. Using Floquet theory, we derive the band structure, Chern numbers, phase diagram, and we show that as the light intensity is swept at fixed mass, the SD system undergoes a normal-Chern-normal insulator transition. Along the phase boundaries, we observe single semi-Dirac-cone (SSDC) semimetal states in which one SD cone is gapless and the other is gapped. The nontrivial Berry curvature distribution Ω(k)≠−Ω(−k) generates an orbital magnetization M and anomalous Nernst (αxy) and thermal Hall (κxy) conductivities. We show that M remains constant as the Fermi level EF scans the insulating gap, but it changes linearly with it in the Chern insulator (CI) phase, as expected. In the normal insulator phase, we find that αxy exhibits a dip-peak profile that is reversed in the CI phase. We also find that switching the light's circular polarization from left to right induces a sign change in M, αxy, and κxy, regardless of the topological phase, thereby allowing us to reverse the direction of flow of the transverse charge and heat currents. Further, we evaluate the components of the charge (σaa), thermoelectric (αaa), and thermal (κaa) conductivity tensors (a=x,y) and examine the effect of light on them. With a linear dispersion along the y-direction, we find that αyy and κyy are significantly larger than αxx and κxx, respectively, due to the much larger squared Dirac velocity vy2 compared to vx2.

Abstract In this study, we investigate the magnetic and topological properties of Mn2X2Te5 (X = Bi, Sb) using first-principles calculations. We find that both Mn2Bi2Te5 and Mn2Sb2Te5 bilayers exhibit the A-type antiferromagnetic order, which can be understood based on the Goodenough-Kanamori-Anderson rules. We further find that an appropriate hole doping can induce a transition from the A-type antiferromagnetic phase to the ferromagnetic phase in these systems, which also experience a transition from a normal insulator to a quantum anomalous Hall phase. Our study thus demonstrates that tunable magnetism and band topology can be achieved in Mn2X2Te5, which can be used to design new functional electronic devices.

Abstract Topological insulators (TIs) are materials with unique surface conductive properties that distinguish them from normal insulators and have attracted significant interest due to their potential applications in electronics and spintronics. However, their weak magnetic field response in traditional setups has limited their practical applications. Here, we show that integrating TIs with active metamaterial substrates can significantly enhance the induced magnetic field by more than 104 times. Our results demonstrate that selecting specific permittivity and permeability values for the active metamaterial substrate optimizes the magnetic field at the interface between the TI layer and the metamaterial, extending it into free space. This represents a substantial improvement over previous methods, where the magnetic field decayed rapidly. The findings reveal that the TI-metamaterial approach enhances the magnetic field response, unveiling new aspects of TI electromagnetic behavior and suggesting novel pathways for developing materials with tailored electromagnetic properties. The integration of metamaterials with TIs offers promising opportunities for advancements in materials science and various technological applications. Overall, our study provides a practical and effective approach to exploring the unique magnetic field responses of TIs, potentially benefiting other complex material systems.

We present a theoretical study demonstrating that in InN/InGaN quantum wells, the topological insulator phase depends largely not only on the quantum well width, but also on the width of the barriers. We show that for structures with a large width of the barriers equal to 200 nm, the topological insulator exists only when the quantum well width is less than 4.5 nm. For quantum wells with widths of 4.5 nm, we obtain a unique topological phase transition from the normal insulator phase to the nonlocal topological semimetal via the Weyl semimetal phase. Decreasing the width of the barriers from 200 to 20 nm results in a large increase in the bulk energy gap in the topological insulator phase, which can greatly facilitate experimental verification of the topological insulator in InN/InGaN quantum wells. We reveal that this effect originates from increasing the built-in electric field in the barriers, which remarkably decreases the penetration of the conduction band wavefunction in the barrier. We also demonstrate that the bulk energy gap in the topological insulator phase is larger in the free-standing structures than in the structures grown on the substrates with the same In content as the barriers.

Abstract Topological superconductivity, generated in an engineered system with the proximity effect from an s-wave superconductor, usually requires the original sample to be a topological insulator. In this study, we propose a novel form of topological superconductivity in a honeycomb lattice arising from both antiferromagnetism and s-wave superconductivity. Eventhough the honeycomb lattice with antiferromagnetism is a normal insulator, the inherent topology of such a system is nontrivial. The topology of the system is determined by the relative values of the s-wave pairing potential and antiferromagnetic order. Notably, there are no chiral edge states at the open boundary if the engineered system is uniform everywhere, whether topologically trivial or not. However, when two parts with different topologies are brought together, two chiral edge states emerge at the topological phase boundary in the middle of the material. This challenges the bulk-edge correspondence observed in conventional topological materials. These chiral edge states are protected by valley symmetry and, owing to their Majorana fermion nature, can contribute to a half-integer quantized conductance.

We theoretically study the quantum spin Hall insulator (QSHI) in a perpendicular magnetic field. In the noninteracting case, the QSHI with space inversion and/or uniaxial spin rotation symmetry undergoes a topological transition into a normal insulator phase at a critical magnetic field B_{c}. The exciton condensation in the lowest Landau levels is triggered by Coulomb interactions in the vicinity of B_{c} at low temperature and spontaneously breaks the inversion and the spin rotation symmetries. We propose that the electron spin resonance spectroscopy with the ac magnetic field also aligned in the perpendicular direction can directly probe the exciton condensation order. Our results should apply to QSHIs such as the InAs/GaSb quantum wells and monolayer transition-metal dichalcogenides.

In this study, we present a comprehensive theoretical investigation of the strain-dependent elastic, electronic, and optical properties of a novel two-dimensional (2D) magnesium carbide (Mg2C) monolayer using density functional theory. Our calculations confirm the high energetic, dynamic, and mechanical stability of the monolayer, highlighting its robustness and suitability for flexible electronic and nanomechanical applications. Strain engineering significantly modulates the bandgap, with compressive strain reducing it and tensile strain increasing it, making the material highly adaptable for strain-controlled semiconductor devices, photodetectors, and nano-electronic applications. Furthermore, we find that compressive strain induces a topological phase transition, transforming the Mg2C monolayer from a normal insulator to a topological insulator, as evidenced by the band inversion and the emergence of a non-zero invariant. This opens possibilities for utilizing this material in quantum spintronics and dissipation less electronic devices. The optical properties exhibit substantial strain-induced shifts, with variations in the dielectric function, absorption coefficient, and optical conductivity. Enhanced absorption in the visible to ultraviolet range and tunable optical conductivity suggest potential applications in optoelectronic devices, including photovoltaics, optical modulators, and sensors. The ability to fine-tune the electronic and optical properties through external strain makes this material highly promising for next-generation flexible and tunable optoelectronic technologies. Future experimental studies are encouraged to validate these theoretical predictions and explore real-time mechanical deformation effects, further expanding the potential applications of this intriguing 2D Mg2C monolayer.

It is generally believed that topological insulators are highly immune to non-magnetic defects, but there is still a lack of verification on a mesoscopic scale of device applications. We take SiSnF<sub>2</sub> monolayer ribbons as an illustration to study the effects of defects and sizes on the electron transport in topological insulators. First-principles calculations show that SiSnF<sub>2</sub> is transformed into a topological insulator under a tensile strain greater than 2%. The data of an effective tight-binding model are obtained by using a genetic algorithm to calculate the transport properties of the topological insulator SiSnF<sub>2</sub> ribbons, and it is found that edge states can also be disrupted by random vacancy defects. For a ribbon with a length of 18.8 nm and a width of 8.2 nm, when it has no defects, the current is concentrated at its edge, and its conductance is an ideal value of the topological edge state, 2<i>e</i><sup>2</sup>/<i>h</i>. When the defect concentration is 1%, the edge current is appreciably disturbed, but the backscattering is still effectively suppressed, and the current bypasses the defect and still goes forward. When the concentration is 5%, the edge electrons are scattered deep into the ribbon and scattered with the opposite edge, destroying the topological edge state and reducing the conductance to 0.6<i>e</i><sup>2</sup>/<i>h</i>. Therefore, the transformation from topological to normal insulator caused by defects happens gradually rather than suddenly. Found in this study is an obvious transport quantum size effect, i.e. increasing the ribbon width can reduce electron scattering between edges and enhance the stability of topological edge states; while increasing the length will increase electron localization and electron scattering between edges, reducing the stability of topological edge states.

The Hall response can be dramatically different from its quantized value in materials with broken inversion symmetry. This stems from the leading Hall contribution beyond the linear order, known as the Berry curvature dipole (BCD). While the BCD is in principle always present, it is typically very small outside of a narrow window close to a topological transition and is thus experimentally elusive without careful tuning of external fields, temperature, or impurities. We transcend this challenge by devising optical driving and quench protocols that enable practical and direct access to large BCD. Varying the amplitude of an incident circularly polarized laser drives a topological transition between normal and Chern insulator phases, and importantly allows the precise unlocking of nonlinear Hall currents comparable to or larger than the linear Hall contributions. This strong BCD engineering is even more versatile with our two-parameter quench protocol, as demonstrated in our experimental proposal.

The formation of the topological surface state originates from bulk band inversion at the top of the valence band and the bottom of the conduction band. The transition between normal and topological insulators is known as a topological phase transition. Here we show spin-polarized electronic states of Pb-based ternary topological insulators Pb(Bi1-xSbx)2Te4 (x = 0.55, 0.70, 0.79) investigated by spin- and angle-resolved photoemission spectroscopy and first-principles calculations. We visualize a pair of spin-polarized surface resonances dispersing along the upper edge of projected bulk bands in occupied states. Interestingly, a branch of the spin-polarized surface resonances continuously connects to the topological surface state. The coexistence of the topological surface state and the spin-polarized surface resonances can be explained by considering the topological phase transition.

Abstract Recently, Chern insulators in an antiferromagnetic (AFM) phase have been suggested theoretically and predicted in a few materials. However, the experimental observation of two-dimensional (2D) AFM quantum anomalous Hall effect is still a challenge to date. In this work, we propose that an AFM Chern insulator can be realized in a 2D monolayer of NiOsCl6 modulated by a compressive strain. Strain modulation is accessible experimentally and used widely in predicting and tuning topological nontrivial phases. With first-principles calculations, we have investigated the structural, magnetic, and electronic properties of NiOsCl6. Its stability has been confirmed through molecular dynamical simulations, elasticity constant, and phonon spectrum. It has a collinear AFM order, with opposite magnetic moments of 1.3 μ B on each Ni/Os atom, respectively, and the Néel temperature is estimated to be 93 K. In the absence of strain, it functions as an AFM insulator with a direct gap with spin–orbital coupling included. Compressive strain will induce a transition from a normal insulator to a Chern insulator characterized by a Chern number C = 1, with a band gap of about 30 meV. This transition is accompanied by a structural distortion. Remarkably, the Chern insulator phase persists within the 3%–10% compressive strain range, offering an alternative platform for the utilization of AFM materials in spintronic devices.

The interaction between topology and magnetism can lead to novel topological materials including Chern insulators, axion insulators, and Dirac and Weyl semimetals. In this work, a family of van der Waals layered materials using MnTe and Sb2Te3 or Bi2Te3 superlattices as building blocks are systematically examined in a search for antiferromagnetic Weyl semimetals, preferably with a simple node structure. The approach is based on controlling the strength of the exchange interaction as a function of layer composition to induce the phase transition between the topological and the normal insulators. Our calculations, utilizing a combination of first-principles density functional theory and tight-binding analyses based on maximally localized Wannier functions, clearly indicate a promising candidate for a type-I magnetic Weyl semimetal. This centrosymmetric material, Mn10Sb8Te22 (or (MnTe) m (Sb2Te3) n with m = 10 and n = 4), shows ferromagnetic intralayer and antiferromagnetic interlayer interactions in the antiferromagnetic ground state. The obtained electronic bandstructure also exhibits a single pair of Weyl points in the spin-split bands consistent with a Weyl semimetal. The presence of Weyl nodes is further verified with Berry curvature, Wannier charge center, and surface state (i.e. Fermi arc) calculations. Other combinations of the MnSbTe-family materials are found to be antiferromagnetic topological or normal insulators on either side of the Mn:Sb ratio, respectively, illustrating the topological phase transition as anticipated. A similar investigation in the homologous (MnTe) m (Bi2Te3) n system produces mostly nontrivial antiferromagnetic insulators due to the strong spin–orbit coupling. When realized, the antiferromagnetic Weyl semimetals in the simplest form (i.e. a single pair of Weyl nodes) are expected to provide a promising candidate for low-power spintronic applications.

We propose a new electrical breakdown mechanism for exciton insulators in the BCS limit, which differs fundamentally from the Zener breakdown mechanism observed in traditional band insulators. Our new mechanism results from the instability of the many-body ground state for exciton condensation, caused by the strong competition between the polarization and condensation energies in the presence of an electric field. We refer to this mechanism as “many-body breakdown.” To investigate this new mechanism, we propose a BCS-type trial wave function under finite electric fields and use it to study the many-body breakdown numerically. Our results reveal two different types of electric breakdown behavior. If the system size is larger than a critical value, the Zener tunneling process is first turned on when an electrical field is applied, but the excitonic gap remains until the field strength reaches the critical value of the many-body breakdown, after which the excitonic gap disappears and the system becomes a highly conductive metallic state. However, if the system size is much smaller than the critical value, the intermediate tunneling phase disappears since the many-body breakdown happens before the onset of Zener tunneling. The sudden disappearance of the local gap leads to an “off-on” feature in the current-voltage (I−V) curve, providing a straightforward way to distinguish excitonic insulators from normal insulators. Published by the American Physical Society 2024

The correlated spinful Haldane model exhibits rich topological phases consisting of chiral topological superfluids (TSFs) and topological spin density waves. However, most of previous studies mainly focus on the case with the fixed hopping phase or at zero temperature. In this paper, we study the attractive spinful Haldane model with arbitrary phase at finite temperature. The chiral TSFs with Chern number C = 2 and 4 emerge driven by the phase and temperature. In particular, the temperature can drive a C = 2 topological superfluid from a trivial normal insulator phase at an appropriate interaction. The bulk topology of all TSFs is uncovered by the Wilson loop method, and confirmed by the responses of edge dislocations.