Progress and prospects in magnetic topological materials

Quasi-Majorana modes in thep-wave Kitaev chains on a square lattice

The topological characteristics of thep-wave Kitaev chains on a square lattice with nearest-neighbor and next-nearest-neighbor inter-chains hopping and pairing are investigated. Besides gapless exact zero-energy modes, this model exhibits topological gapless phase hosting edge modes, which do not reside strictly at zero energy. However, these modes can be distinguished from the bulk states. These states are known as pseudo- or quasi-Majorana Modes (qMMs). The exploration of this system's bulk spectrum and Berry curvature reveals singularities and flux-carrying vortices within its Brillouin zone. These vortices indicate the presence of four-fold Dirac points arising from two-fold degenerate bands. Examining the Hamiltonian under a cylindrical geometry uncovers the edge properties, demonstrating the existence of topological edge modes. These modes are a direct topological consequence of the Dirac semimetal characteristics of the system. The system is analyzed under open boundary conditions to distinguish the multiple Majorana zero modes and qMMs. This analysis includes a study of the normalized site-dependent local density of states, which pinpoints the presence of localized edge states. Additionally, numerical evidence confirms the topological protection of the edge states due to the finite-size effect and their robustness against disorder perturbations. The emergence of topological edge states and Dirac points with net zero topological charge indicates that this model is a weak topological superconductor.

Photonic axion insulator with non-coplanar chiral hinge transport

Axion insulators represent a unique class of magnetic topological phases, linking the two-dimensional quantum anomalous Hall effect to the magnetic higher-order phase of three-dimensional topological insulators. Within axion insulators, axion electrodynamics exhibits novel topological magneto-electric phenomena such as quantized Faraday and Kerr rotation and half-integer surface Hall response. However, among them, the chiral hinge state with non-reciprocal hinge transport as their essential hallmark has yet to be experimentally observed since it was predicted theoretically. Here we report the first photonic axion insulator based on a three-dimensional antiferromagnetic-like structure in microwave bands. Such an artificial magnetic lattice consists of bilayer square-lattice arrays of ferrites imposed with equal but opposite embedded magnets, simultaneously with inversion-symmetric interlayer couplings. By probing all twelve hinges and detecting all eight vertices of the photonic axion insulator, we directly map out the non-coplanar chiral hinge states and observe the non-reciprocal robust hinge transport. The different performances between odd- and even-layer axion insulators are also investigated. These results enrich the family of topological photonics and the controllable dimension of electromagnetic waves, opening up a photonic way to study rich magnetic topological phases that have already been proposed but are challenging to implement in solid-state materials.

Epitaxial growth of antiferromagnetic MnBi2Te4/CdTe heterostructures on GaAs(001) using molecular beam epitaxy: structure and electronic properties

MnBi2Te4 is one of the most recent materials that integrates the class of topological quantum materials exhibiting topological insulating properties and magnetic ordering, thus providing the opportunity to investigate particular topological quantum states and design novel spintronic devices. The samples were grown on GaAs(100) substrates using molecular beam epitaxy with Bi2Te3, Te and Mn as sources. The growth was characterized through X-ray diffraction, atomic force microscopy and transmission electron microscopy. Topological insulator properties were probed through scanning tunneling microscopy and scanning tunneling spectroscopy, while antiferromagnetic order was investigated via magnetotransport measurements. Furthermore, we propose a high-quality CdTe thin film as a cap layer to protect topological surface states, thus opening the possibility of integrating topological quantum materials with group II-VI semiconductors via van der Waals epitaxy.

Nonzero Berry curvature dipole, magnetic gapped edge states and persistent spin texture in a rotational symmetry preserved van der Waals magnetic topological insulator

Berry curvature dipole and persistent spin textures are unconventional quantum marvels with paramount relevance towards nonlinear transport phenomena and futuristic spintronic applications. Here, we demonstrate that a magnetic van der Waals (vdW) heterobilayer, comprised of semiconducting monolayers 1H-MoSe2 (nonmagnetic) and ferromagnet 1H-VSe2, exhibits a finite Berry curvature dipole (DBcd) in the absence of strain or twist perturbation, where each constituent layer displays no DBcd owing to three-fold (C3v) rotational symmetry. A change in sign of DBcd is observed over an energy range, suggesting its oscillatory nature. Further, persistent spin texture (PST) with small spin canting and a prominent magnetic gap of 190 meV have emerged in the edge state spectrum of the vdW system. We find the emerged topological properties are crystal facet dependent. Moreover, an exotic quantum state of intrinsic spin-valley locking at two inequivalent K valleys with distinct spin identity is observed in the dispersion relation. We explore the subtle role of spin-valley locking in the generation of a finite DBcd, which has not been previously discussed in this context. Such quantum states promote right and left handed circular polarization and are perceived as a binary catalogue for information encoding and energy. Further, the role of symmetry breaking in the observed phenomena of the AB-stacked vdW heterobilayer is discussed. We propose that both time reversal symmetry breaking and spin-valley locking are essential for inducing nonzero DBcd in an AB stacked vdW heterostructure, where the three-fold (C3) rotational symmetry is preserved along the z-axis only.

Bilayer Kagome Ferrimagnet Exhibiting Exceptional Spontaneous Exchange Bias in TbMn6(Ge,Ga)6

Manipulating interlayer interactions in two-dimensional (2D) materials has led to intriguing behaviors. Borrowing these 2D signatures to bulk materials is likely to unlock exceptional properties. Here, we report an emergent 2D-like bilayer Kagome ferrimagnet through reducing the interbilayer magnetic interaction to nearly zero. This concept is realized within bulk TbMn6(Ge,Ga)6 compounds, characterized by an isolated and pure Mn Kagome lattice, simply by the chemical substitution of Ge with Ga. Specifically, the targeted compound TbMn6Ge5Ga1 exhibits a giant spontaneous exchange bias (SEB) of approximately 1.6 T, which is more than twice that observed in known materials. Field-dependent neutron diffraction reveals the robust nature of the compensated ferrimagnetic (FiM), characterized by almost two-thirds of the moments being pinned and irreversible under fields up to 9 T. Through magnetic and structural analysis, alongside theoretical calculations, we demonstrate that the substantial SEB is related to the intense competition between local robust and weak FiM states within the bilayer Kagome configuration, which are stabilized by an incommensurate spin arrangement. The concept of a bilayer Kagome magnet offers new opportunities for discovering attractive properties in 2D-like materials.

Controllable topological phase transition via ferroelectric-paraelectric switching in a ferromagnetic single-layer MIMIIGe2X6 family

Recently, the emergence of two-dimensional (2D) multiferroic materials has opened a new perspective for exploring topological states. However, instances of tuning topological phase transitions through ferroelectric (FE) polarization in 2D ferromagnetic (FM) materials are relatively rare. Here, we found that 11 single layer (SL) materials, named the MIMIIGe2X6 family, possess both FE and FM properties. Among them, 5 SL materials exhibit controllable topological phase transition via ferroelectric-paraelectric switching. Taking the SL ReIrGe2S6 as an example, we find that the paraelectric (PE) and FE phases exhibit half-metal and magnetic semiconductor behavior, respectively. In the spin-down channel of the PE phase, a critical-type Weyl point (WP) is observed with a robust Fermi arc edge state. In contrast, the FE phase of the SL ReIrGe2S6 clearly identifies 0D corner states in both spin channels. Therefore, under the control of external fields, the SL ReIrGe2S6 undergoes a transformation from PE to FE by overcoming the energy barrier of 0.62 eV, subsequently realizing the phase transition from the Weyl semimetal to the high order topological insulator. These findings not only combine topological states with multiferroics but also pave the way for the experimental discovery of 2D tunable topological phase transition.

Journey of the 2D Intrinsic Antiferromagnetic Topological Insulators in the (MnBi2Te4)(Bi2Te3)n Homologous Series

In recent years, the study of two-dimensional (2D) intrinsic antiferromagnetic (AFM) topological insulators (TIs) has attracted considerable attention due to their unique electronic and magnetic properties, which are promising for the advancement of quantum computing and spintronic applications. MnBi2Te4, recognized as the first intrinsic AFM TI, provides a unique platform for examining theoretical predictions in the field of quantum materials. This discovery has sparked extensive research and led to numerous new insights that have improved the understanding of the interplay between magnetism and topology in two-dimensional systems. The homologous series (MnBi2Te4)(Bi2Te3)n, with its alternating layers of MnBi2Te4 and Bi2Te3, exhibits tunable magnetic and topological properties, making it a subject of intense investigation. This review comprehensively examines advances in the (MnBi2Te4)(Bi2Te3)n homologous series, including their synthesis, structural characterization, and study of magnetic and electronic properties. Key experimental observations are highlighted, which have been instrumental in elucidating the fundamental physics of these materials. Additionally, several unresolved questions and potential future research directions are discussed, providing valuable insights for researchers seeking to advance this integrated field. This review serves as a reference for understanding the potential and future advancements of 2D AFM TIs, fostering further exploration of their complex and promising properties.

Unusually High Occupation of Co 3d State in Magnetic Weyl Semimetal Co3Sn2S2

The physical properties of magnetic topological materials are strongly influenced by their nontrivial band topology coupled with the magnetic structure. Co3Sn2S2 is a ferromagnetic kagome Weyl semimetal displaying giant intrinsic anomalous Hall effect which can be further tuned via elemental doping, such as Ni substitution for Co. Despite significant interest, the exact valency of Co and the magnetic order of the Ni dopants remained unclear. Here, we report a study of Ni-doped Co3Sn2S2 single crystals using synchrotron-based X-ray magnetic circular dichroism (XMCD), X-ray photoelectron emission microscopy (XPEEM), and hard/soft X-ray photoemission spectroscopy (XPS) techniques. We confirm the presence of spin-dominated magnetism from Co in the host material, and also the establishment of ferromagnetic order from the Ni dopant. The oxygen-free photoemission spectrum of the Co 2p core levels in the crystal well resembles that of a metallic Co film, indicating a Co0+ valency. Surprisingly, we find the electron filling in the Co 3d state can reach 8.7-9.0 electrons in these single crystals. Our results highlight the importance of element-specific X-ray spectroscopy in understanding the electronic and magnetic properties that are fundamental to a heavily studied Weyl semimetal, which could aid in developing future spintronic applications based on magnetic topological materials.

Continuous Evolution of Eu2+/Eu3+ Mixed Valency Driven by Pressure and Temperature

Continuous mixed valency involving Eu2+ and Eu3+ in Eu4Bi6Se13 can be induced under applied pressure or at reduced temperatures. The monoclinic structure of Eu4Bi6Se13, crystallizing in the P21/m space group (No. 11), features linear chains of Eu atoms aligned along the b-axis. Magnetic susceptibility measurements, conducted both parallel and perpendicular to the b-axis and analyzed using Curie-Weiss theory, alongside high-pressure partial fluorescence yield (PFY) data from X-ray absorption spectroscopy (XAS), indicate the material's propensity to adopt a mixed-valent state. Within this state, the trivalent Eu3+ configuration becomes increasingly favored as the pressure rises or the temperature decreases.

Chiral Phonons Enhance Ferromagnetism

Recent experiments suggest that the conditions for ferromagnetic order in magnetite can be modified by adsorption of chiral molecules. Especially, the coercivity of a ferromagnetic metal was increased by nearly 100% or 20 times the earth magnetic flux density at room temperature. The coercivity was, moreover, demonstrated to increase linearly with the temperature in a finite range around room temperature. On the basis of these results, a mechanism is proposed for providing the necessary enhancement of magnetic anisotropy. It is shown that nuclear vibrations (phonons) coupled to ferromagnetic spin excitations (magnons) absorb the thermal energy in the system, thereby diverting the excess energy that otherwise would excite magnons in the ferromagnet. This energy diversion not only restores the ferromagnetic order but also enhances its stability by increasing the anisotropy energy for magnon excitations. The coupling between phonons with magnons is enabled by chirality due to the lack of inversion symmetry.

Emergent Magnetic Skyrmions in a Topological Weyl Nodal Ring Semimetal

Topological magnetic materials are expected to show multiple transport responses because of their unusual bulk electronic topology in momentum space and their topological spin texture in real space. However, such multiple topological properties-hosting materials are rare in nature. In this work, we unambiguously reveal the emergence of magnetic skyrmions in Mn5Ge3 single crystal through detailed electrical transport and Lorentz transmission electron microscopy (L-TEM) combined with ab initio calculations. We demonstrate that Mn5Ge3 is a topological ferromagnet with multiple nodal rings in its electronic structure. Importantly, L-TEM experiments further reveal that the magnetic skyrmions appear in the (001) plane when an appropriate magnetic field is applied along the [001] direction. Skyrmion-induced topological Hall resistivity as large as ∼972 nΩ cm is also observed over a wide temperature-magnetic field region. These prove Mn5Ge3 as a rare magnetic topological nodal-ring semimetal with great significance to explore novel topological multifunctionality, which facilitate the development of spintronics.

Towards the quantized anomalous Hall effect in AlOx-capped MnBi2Te4

The quantum anomalous Hall effect in layered antiferromagnet MnBi2Te4 harbors a rich interplay between magnetism and topology, holding a significant promise for low-power electronic devices and topological antiferromagnetic spintronics. In recent years, MnBi2Te4 has garnered considerable attention as the only known material to exhibit the antiferromagnetic quantum anomalous Hall effect. However, this field faces significant challenges as the quantization at zero magnetic field depending critically on fabricating high-quality devices. In this article, we introduce a straightforward yet effective method to mitigate the detrimental effect of the standard fabrication on MnBi2Te4 by depositing an AlOx layer on the surface before fabrication. Optical contrast and magnetotransport measurements on over 50 MnBi2Te4 demonstrate that AlOx can effectively preserve the pristine states of the devices. Surprisingly, we find this simple method can significantly enhance the anomalous Hall effect towards quantization, which resolves a longstanding challenge in the field of MnBi2Te4. Scaling relation analysis further reveals the intrinsic mechanism of anomalous Hall effect dominated by Berry curvature at various magnetic configuration. By tuning the gate voltage, we uncover a gate independent magnetism in odd-layer MnBi2Te4 devices. Our experiments not only pave the way for the fabrication of high-quality dissipationless transport devices, but also advance the investigation of exotic topological quantum phenomena in 2D materials.

Designing Chiral Organometallic Nanosheets with Room-Temperature Multiferroicity and Topological Nodes

Two-dimensional (2D) room-temperature chiral multiferroic and magnetic topological materials are essential for constructing functional spintronic devices, yet their number is extremely limited. Here, by using the chiral and polar HPP (HPP = 4-(3-hydroxypyridin-4-yl)pyridin-3-ol) as an organic linker and transition metals (TM = Cr, Mo, W) as nodes, we predict a class of 2D TM(HPP)2 organometallic nanosheets that incorporate homochirality, room-temperature magnetism, ferroelectricity, and topological nodes. The homochirality is introduced by chiral HPP linkers, and the change in structural chirality induces a topological phase transition of Weyl phonons. The room-temperature magnetism arises from the strong d-p spin coupling between TM cations and HPP doublet anions. The ferroelectricity is attributed to the breaking of spatial inversion symmetry in the lattice structure. Additionally, by adjusting the type of TMs, these nanosheets show rich and tunable band structures. Notably, all predicted materials are topologically nontrivial, featuring a quadratic nodal point around the Fermi level.

The quantum anomalous Hall effect in two-dimensional hexagonal monolayers studied by first-principles calculations

The quantum anomalous Hall effect (QAHE) demonstrates the potential for achieving quantized Hall resistance without the need for an external magnetic field, making it highly promising for reducing energy loss in electronic devices. Its realization and research rely heavily on precise first-principles calculations, which are essential for analyzing the electronic structures and topological properties of novel two-dimensional (2D) materials. This review article explores the theoretical progress of QAHE in 2D hexagonal monolayers with strong spin-orbit coupling and internal magnetic ordering. We summarize current strategies and methods for realizing QAHE in these monolayers, focusing on material selection and fine-tuning to achieve stable QAHE at room temperature. We hope that this review will provide new perspectives for theoretical studies and enable researchers to more accurately predict materials with superior QAHE properties. Meanwhile, we anticipate that these theoretical advancements will further drive breakthroughs in experimental studies and promote its broader application in low-power electronic devices and quantum information technology.

Synthesis of a semimetallic Weyl ferromagnet with point Fermi surface

Quantum materials governed by emergent topological fermions have become a cornerstone of physics. Dirac fermions in graphene form the basis for moiré quantum matter and Dirac fermions in magnetic topological insulators enabled the discovery of the quantum anomalous Hall (QAH) effect1-3. By contrast, there are few materials whose electromagnetic response is dominated by emergent Weyl fermions4-6. Nearly all known Weyl materials are overwhelmingly metallic and are largely governed by irrelevant, conventional electrons. Here we theoretically predict and experimentally observe a semimetallic Weyl ferromagnet in van der Waals (Cr,Bi)2Te3. In transport, we find a record bulk anomalous Hall angle of greater than 0.5 along with non-metallic conductivity, a regime that is strongly distinct from conventional ferromagnets. Together with symmetry analysis, our data suggest a semimetallic Fermi surface composed of two Weyl points, with a giant separation of more than 75% of the linear dimension of the bulk Brillouin zone, and no other electronic states. Using state-of-the-art crystal-synthesis techniques, we widely tune the electronic structure, allowing us to annihilate the Weyl state and visualize a unique topological phase diagram exhibiting broad Chern insulating, Weyl semimetallic and magnetic semiconducting regions. Our observation of a semimetallic Weyl ferromagnet offers an avenue towards new correlated states and nonlinear phenomena, as well as zero-magnetic-field Weyl spintronic and optical devices.

A magneto-thermoelectric with a high figure of merit in topological insulator Bi88Sb12

High thermoelectric performance is generally achieved by synergistically optimizing two or even three of the contradictorily coupled thermoelectric parameters. Here we demonstrate magneto-thermoelectric correlation as a strategy to achieve simultaneous gain in an enhanced Seebeck coefficient and reduced thermal conductivity in topological materials. We report a large magneto-Seebeck effect and high magneto-thermoelectric figure of merit of 1.7 ± 0.2 at 180 K and 0.7 T in a single-crystalline Bi88Sb12 topological insulator. This result fills a gap of a high performance below 300 K and is promising for low-temperature thermoelectric applications. The large magneto-Seebeck response is attributed to the ultrahigh mobility and the Dirac band dispersion. The application of a low magnetic field to achieve a high thermoelectric performance can be extended to topological materials with similar features that are rapidly emerging because it synergistically optimizes the thermoelectric parameters.

Quantum spin Hall states in MX2 (M = Ru, Os; X = As, Sb) monolayers

The quantum spin Hall (QSH) effect has attracted extensive research interest due to its great promise in topological quantum computing and novel low-energy electronic devices. Here, using first-principles calculations, we find that MX2 (M = Ru and Os; X = As and Sb) monolayers are 2D topological insulators (TIs). The spin-orbit coupling (SOC) band gaps for RuAs2, RuSb2, OsAs2, and OsSb2 monolayers are predicted to be 80, 131, 118, and 221 meV, respectively. Additionally, the nontrivial topological states are further confirmed by calculating the topological invariant and the appearance of gapless edge states. More interestingly, for RuSb2 and OsSb2 monolayers, the position of node points in energy can be effectively tuned by applying in-plane strain. Our results consistently indicate that all MX2 monolayers can serve as an effective platform for achieving the room-temperature QSH effect.

Symmetry invariants and classes of quasiparticles in magnetically ordered systems having weak spin-orbit coupling

Symmetry invariants of a group specify the classes of quasiparticles, namely the classes of projective irreducible co-representations in systems having that symmetry. More symmetry invariants exist in discrete point groups than the full rotation group O(3), leading to new quasiparticles restricted to lattices that do not have any counterpart in a vacuum. We focus on the fermionic quasiparticle excitations under "spin-space group" symmetries, applicable to materials where long-range magnetic order and itinerant electrons coexist. We provide a list of 218 classes of new quasiparticles that can only be realized in the spin-space groups. These quasiparticles have at least one of the following properties that are qualitatively distinct from those discovered in magnetic space group(MSG)s, and distinct from each other:(i) degree of degeneracy,(ii) dispersion as function of momentum, and(iii) rules of coupling to external probe fields. We rigorously prove this result as a theorem that directly relates these properties to the symmetry invariants, and then illustrate this theorem with a concrete example, by comparing three 12-fold fermions having different sets of symmetry invariants including one discovered in MSG. Our approach can be generalized to realize more quasiparticles whose little co-groups are beyond those considered in our work.

Optical skyrmions from metafibers with subwavelength features

Optical skyrmions are an emerging class of structured light with sophisticated particle-like topologies with great potential for revolutionizing modern informatics. However, the current generation of optical skyrmions involves complex or bulky systems, hindering the development of practical applications. Here, exploiting the emergent "lab-on-fiber" technology, we demonstrate the design of a metafiber-integrated photonic skyrmion generator. We not only successfully generate high-quality optical skyrmions from metafibers, but also verify their remarkable properties, such as topology switchability and topology stability with subwavelength polarization features beyond the diffraction limits. Our flexible fiber-integrated optical skyrmions platform paves the avenue for future applications of topologically-enhanced remote super-resolution microscopy and robust information transfer.

The quantum anomalous Hall effect and strong robustness in two-dimensional p-state Dirac half-metals Y3X2 (Y = Li, Na; X = Se, Te)

Based on first-principles calculations, we have predicted a novel group of 2D p-state Dirac half-metal (DHM) materials, Y3X2 (Y = Li, Na; X = Se, Te) monolayers. All the monolayers exhibit intrinsic ferromagnetism. Among them, Li3Te2 and Na3Se2 open topologically nontrivial band gaps of 4.0 meV and 5.0 meV considering spin-orbit coupling (SOC), respectively. The Curie temperature of Li3Te2 is 355 K. The non-zero Chern number and the presence of edge states further confirm that the Li3Te2 monolayer is a room-temperature ferromagnetic material and a quantum anomalous Hall (QAH) insulator. Additionally, it is found that Y3X2 (Y = Li, Na; X = Se, Te) monolayers exhibit strong robustness against strain and electric fields. Finally, we have proposed the growth of Y3X2 (Y = Li, Na; X = Se, Te) monolayers on h-BN substrates, which shows promise for experimental synthesis. Our research indicates that Y3X2 (Y = Li, Na; X = Se, Te) monolayers exhibit strong robustness as DHMs, showcasing significant potential for realizing the intrinsic quantum anomalous Hall effect (QAHE).

High Chern number quantum anomalous Hall effect in monolayer Co3X3SSe (X = Sn, Pb) kagomes

The high Chern number quantum anomalous Hall effect can offer an ideal platform to develop exotic quantum materials with a dissipationless chiral edge. The investigation of kagome monolayer Co3X3SSe (X = Sn, Pb) materials enables a comprehensive exploration of their structural, magnetoelectric, and topological characteristics through first-principles calculations. The monolayers Co3Sn3SSe and Co3Pb3SSe are classified as kagome ferromagnets, and they exhibit stable perpendicular magnetic anisotropy energy. These materials can achieve the intrinsic high Chern number quantum anomalous Hall effect with C = -3. The band gaps of Co3Sn3SSe and Co3Pb3SSe are 46 and 59 meV, respectively, which are larger than the thermal energy at room-temperature scale. Additionally, our findings demonstrate that both the band gap and magnetic anisotropy energy of the monolayers Co3Sn3SSe and Co3Pb3SSe are sensitive to applied strain. This research presents intriguing and alternative possibilities for advancing intrinsic high Chern number quantum anomalous Hall devices.

Kitaev interactions through extended superexchange pathways in the j eff = 1 / 2 Ru3+ honeycomb magnet RuP3SiO11

Magnetic materials are composed of the simple building blocks of magnetic moments on a crystal lattice that interact via magnetic exchange. Yet from this simplicity emerges a remarkable diversity of magnetic states. Some reveal the deep quantum mechanical origins of magnetism, for example, quantum spin liquid (QSL) states in which magnetic moments remain disordered at low temperatures despite being strongly correlated through quantum entanglement. A promising theoretical model of a QSL is the Kitaev model, composed of unusual bond-dependent exchange interactions, but experimentally, this model is challenging to realise. Here we show that the material requirements for the Kitaev QSL survive an extended pseudo-edge-sharing superexchange pathway of Ru3+ octahedra within the honeycomb layers of the inorganic framework solid, RuP3SiO11. We confirm the requisitej eff = 1 2 state of Ru3+ in RuP3SiO11 and resolve the hierarchy of exchange interactions that provide experimental access to an unexplored region of the Kitaev model.

Real Topological Phonons in 3D Carbon Allotropes

There has been a significant focus on real topological systems that enjoy space-time inversion symmetry and lack spin-orbit coupling. While the theoretical classification of the real topology has been established, more progress has yet to be made in the materials realization of real topological phononic states in 3D. To address this crucial issue, high-throughput computing is performed to inspect the real topology in the phonon spectrums of the 3D carbon allotropes. Among 1661 carbon allotropes listed in the Samara Carbon Allotrope Database (SACADA), 79 candidates host a phononic real Chern insulating (PRCI) state, 2 candidates host a phononic real nodal line (PRNL) state, 12 candidates host a phononic real Dirac point (PRDP) state, and 10 candidates host a phononic real triple-point pair (PRTPP) state. The PRCI, PRNL, PRTPP, and PRDP states of 27-SG. 166-pcu-h, 1081-SG. 194-42T13-CA, 52-SG. 141-gis, and 132-SG. 191-3,4T157 are exhibited as illustrative examples, and the second-order phononic hinge modes are explored. This study broadens the understanding of 3D topological phonons and expands the material candidates with phononic hinge modes and phononic real topology.

Giant Optical Anisotropy Induced by Magnetic Order in FePS3/WSe2 Heterostructures

Magnetic 2D materials offer a promising platform for manipulating quantum states at the nanoscale. Recent studies have underscored the significant influence of 2D magnetic materials on the optical behaviors of transition-metal dichalcogenides (TMDs), revealing phenomena such as interlayer exciton-magnon interactions, magnetization-dependent valley polarization, and an enhanced Zeeman effect. However, the controlled manipulation of anisotropic optical properties in TMDs via magnetism remains challenging. Here, the magnetic ordering in FePS3 profoundly impacts the optical characteristics of WSe2, achieving a giant linear polarization degree of 5.1 in exciton emission is demonstrated. This is supported by a detailed analysis of low-temperature photoluminescence (PL) and Raman spectra from nL-FePS3/WSe2 heterostructures. These findings indicate that a phase transition in FePS3 from paramagnetic to antiferromagnetic enhances interlayer Coulomb interactions, inducing a transition from non-polar to polar behavior in the heterostructures. Additionally, valley-polarized PL spectra under magnetic fields from -9 to 9 T reveal the influence of FePS3 on valley polarization and Zeeman splitting of excitons in monolayer WSe2. These results present a novel strategy for tailoring the optoelectronic properties of 2D magnetic van der Waals heterostructures, paving the way for advancements in nanoscale device design.

Experimental progress in Eu(Al,Ga)4topological antiferromagnets

The non-trivial magnetic and electronic phases occurring in topological magnets are often entangled, thus leading to a variety of exotic physical properties. Recently, the BaAl4-type compounds have been extensively investigated to elucidate the topological features appearing in their real- and momentum spaces. In particular, the topological Hall effect and the spin textures, typical of the centrosymmetric Eu(Al,Ga)4family, have stimulated extensive experimental and theoretical research. In this topical review, we discuss the latest findings on the Eu(Al,Ga)4topological antiferromagnets and related materials, arising from a wide range of experimental techniques. We show that Eu(Al,Ga)4represents a suitable platform to explore the interplay between lattice-, charge-, and spin degrees of freedom, and associated emergent phenomena. Finally, we address some key questions open to future investigation.

Quasi-One-Dimensional Spin Transport in Altermagnetic Z^{3} Nodal Net Metals

In three dimensions, quasi-one-dimensional (Q1D) transport has traditionally been associated with systems featuring a Q1D chain structure. Here, based on first-principle calculations, we go beyond this understanding to show that the Q1D transport can also be realized in certain three-dimensional (3D) altermagnetic (AM) metals with a topological nodal net in momentum space but lacking Q1D chain structure in real space, including the existing compounds β-Fe_{2}(PO_{4})O, Co_{2}(PO_{4})O, and LiTi_{2}O_{4}. These materials exhibit an AM ground state and feature an ideal crossed Z^{3} Weyl nodal line in each spin channel around Fermi level, formed by three straight and flat nodal lines traversing the entire Brillouin zone. These nodal lines eventually lead to an AM Z^{3} nodal net. Surprisingly, the electronic conductivity σ_{xx} in these topological nodal net metals is dozens of times larger than σ_{yy} and σ_{zz} in the up-spin channel, while σ_{yy} dominates transport in the down-spin channel. This suggests a distinctive Q1D transport signature in each spin channel, and the principal moving directions for the two spin channels are orthogonal, resulting in Q1D direction-dependent spin transport. This novel phenomenon cannot be found in both conventional 3D bulk materials and Q1D chain materials. In particular, the Q1D spin transport gradually disappears as the Fermi energy moves away from the nodal net, further confirming its topological origin. Our Letter not only enhances the comprehension of topological physics in altermagnets but also opens a new direction for the exploration of topological spintronics.

Lieb-Schultz-Mattis Theorem for 1D Quantum Magnets with Antiunitary Translation and Inversion Symmetries

We study quantum many-body systems in the presence of an exotic antiunitary translation or inversion symmetry involving time reversal. Based on a symmetry-twisting method and spectrum robustness, we propose that a half-integer spin chain that respects any of these two antiunitary crystalline symmetries in addition to the discrete Z_{2}×Z_{2} global spin-rotation symmetry must either be gapless or possess degenerate ground states. This explains the gaplessness of a class of chiral spin models not indicated by the Lieb-Schultz-Mattis theorem and its known extensions. Moreover, we present symmetry classes with minimal sets of generators that give nontrivial Lieb-Schultz-Mattis-type constraints, argued by the bulk-boundary correspondence in 2D symmetry-protected topological phases as well as lattice homotopy. Our results for detecting the ingappability of 1D quantum magnets from the interplay between spin-rotation symmetries and magnetic space groups are applicable to systems with a broader class of spin interactions, including Dzyaloshinskii-Moriya and triple-product interactions.

Ubiquitous Order-Disorder Transition in the Mn Antisite Sublattice of the (MnBi2Te4)(Bi2Te3)n Magnetic Topological Insulators

Magnetic topological insulators (TIs) herald a wealth of applications in spin-based technologies, relying on the novel quantum phenomena provided by their topological properties. Particularly promising is the (MnBi2Te4)(Bi2Te3)n layered family of established intrinsic magnetic TIs that can flexibly realize various magnetic orders and topological states. High tunability of this material platform is enabled by manganese-pnictogen intermixing, whose amounts and distribution patterns are controlled by synthetic conditions. Here, nuclear magnetic resonance and muon spin spectroscopy, sensitive local probe techniques, are employed to scrutinize the impact of the intermixing on the magnetic properties of (MnBi2Te4)(Bi2Te3)n and MnSb2Te4. The measurements not only confirm the opposite alignment between the Mn magnetic moments on native sites and antisites in the ground state of MnSb2Te4, but for the first time directly show the same alignment in (MnBi2Te4)(Bi2Te3)n with n = 0, 1 and 2. Moreover, for all compounds, the static magnetic moment of the Mn antisite sublattice is found to disappear well below the intrinsic magnetic transition temperature, leaving a homogeneous magnetic structure undisturbed by the intermixing. The findings provide a microscopic understanding of the crucial role played by Mn-Bi intermixing in (MnBi2Te4)(Bi2Te3)n and offer pathways to optimizing the magnetic gap in its surface states.

Evidence for Two-Dimensional Weyl Fermions in Air-Stable Monolayer PtTe1.75

The Weyl semimetals represent a distinct category of topological materials wherein the low-energy excitations appear as the long-sought Weyl Fermions. Exotic transport and optical properties are expected because of the chiral anomaly and linear energy-momentum dispersion. While three-dimensional Weyl semimetals have been successfully realized, the quest for their two-dimensional (2D) counterparts is ongoing. Here, we report the realization of 2D Weyl Fermions in monolayer PtTe1.75, which has strong spin-orbit coupling and lacks inversion symmetry, by combined angle-resolved photoemission spectroscopy, scanning tunneling microscopy, second harmonic generation, X-ray photoelectron spectroscopy measurements, and first-principles calculations. The giant Rashba splitting and band inversion lead to the emergence of three pairs of critical Weyl cones. Moreover, monolayer PtTe1.75 exhibits excellent chemical stability in ambient conditions, which is critical for future device applications. The discovery of 2D Weyl Fermions in monolayer PtTe1.75 opens up new possibilities for designing and fabricating novel spintronic devices.

Robust Edge States of Quasi-1D Material Ta2NiSe7 and Applications in Saturable Absorbers

The helical edge states (ESs) protected by underlying Z2 topology in two-dimensional topological insulators (TIs) arouse upsurges in saturable absorptions thanks to the strong photon-electron coupling in ESs. However, limited TIs demonstrate clear signatures of topological ESs at liquid nitrogen temperatures, hindering the applications of such exotic quantum states. Here, we demonstrate the existence of one-dimensional (1D) ESs at the step edge of the quasi-1D material Ta2NiSe7 at 78 K by scanning tunneling microscopy. Such ESs are rather robust against the irregularity of the edges, suggesting a possible topological origin. The exfoliated Ta2NiSe7 flakes were used as saturable absorbers (SAs) in an Er-doped fiber laser, hosting a mode-locked pulse with a modulation depth of up to 52.6% and a short pulse duration of 225 fs, far outstripping existing TI-based SAs. This work demonstrates the existence of robust 1D ESs and the superior SA performance of Ta2NiSe7.

Spin Gapless Quantum Materials and Devices

Quantum materials, with nontrivial quantum phenomena and mechanisms, promise efficient quantum technologies with enhanced functionalities. Quantum technology is held back because a gap between fundamental science and its implementation is not fully understood yet. In order to capitalize the quantum advantage, a new perspective is required to figure out and close this gap. In this review, spin gapless quantum materials, featured by fully spin-polarized bands and the electron/hole transport, are discussed from the perspective of fundamental understanding and device applications. Spin gapless quantum materials can be simulated by minimal two-band models and could help to understand band structure engineering in various topological quantum materials discovered so far. It is explicitly highlighted that various types of spin gapless band dispersion are fundamental ingredients to understand quantum anomalous Hall effect. Based on conventional transport in the bulk and topological transport on the boundaries, various spintronic device aspects of spin gapless quantum materials as well as their advantages in different models for topological field effect transistors are reviewed.

Enhancement of the anomalous Hall effect by distorting the Kagome lattice in an antiferromagnetic material

In topological magnetic materials, the topology of the electronic wave function is strongly coupled to the structure of the magnetic order. In general, ferromagnetic Weyl semimetals generate a strong anomalous Hall conductivity (AHC) due to a large Berry curvature that scales with their magnetization. In contrast, a comparatively small AHC is observed in noncollinear antiferromagnets. We investigated HoAgGe, an antiferromagnetic (AFM) Kagome spin-ice compound, which crystallizes in a hexagonal ZrNiAl-type structure in which Ho atoms are arranged in a distorted Kagome lattice, forming an intermetallic Kagome spin-ice state in the ab-plane. It exhibits a large topological Hall resistivity of ~1.6 µΩ-cm at 2.0 K in a field of ~3 T owing to the noncoplanar structure. Interestingly, a total AHC of 2,800 Ω-1 cm-1 is observed at ~45 K, i.e., 4 TN, which is quite unusual and goes beyond the normal expectation considering HoAgGe as an AFM Kagome spin-ice compound with a TN of ~11 K. We demonstrate further that the AHC below TN results from the nonvanishing Berry curvature generated by the formation of Weyl points under the influence of the external magnetic field, while the skew scattering led by Kagome spins dominates above the TN. These results offer a unique opportunity to study frustration in AFM Kagome lattice compounds.

Bias-dependent photoresponse of Td-WTe2grown by chemical vapor deposition

The type-II Weyl semimetal Td-WTe2is one of the wonder materials for high-performance optoelectronic devices. We report the self-powered Td-WTe2photodetectors and their bias-dependent photoresponse in the visible region (405, 520, 638 nm) driven by the bulk photovoltaic effect. The device shows the responsivity of 15.8 mAW-1and detectivity of 5.2 × 109Jones at 520 nm. Besides, the response time of the WTe2photodetector shows the strong bias-voltage dependent property. This work offers a physical reference for understanding the photoresponse process of Td-WTe2photodetectors.

Even-Odd Layer-Dependent Exchange Bias Effect in MnBi2Te4 Chern Insulator Devices

Magnetic topological materials with coexisting magnetism and nontrivial band structures exhibit many novel quantum phenomena, including the quantum anomalous Hall effect, the axion insulator state, and the Weyl semimetal phase. As a stoichiometric layered antiferromagnetic topological insulator, thin films of MnBi2Te4 show fascinating even-odd layer-dependent physics. In this work, we fabricate a series of thin-flake MnBi2Te4 devices using stencil masks and observe the Chern insulator state at high magnetic fields. Upon magnetic field training, a large exchange bias effect is observed in odd but not in even septuple layer (SL) devices. Through theoretical calculations, we attribute the even-odd layer-dependent exchange bias effect to the contrasting surface and bulk magnetic properties of MnBi2Te4 devices. Our findings reveal the microscopic magnetic configuration of MnBi2Te4 thin flakes and highlight the challenges in replicating the zero magnetic field quantum anomalous Hall effect in odd SL MnBi2Te4 devices.

Design of Antiferromagnetic Second-Order Band Topology with Rotation Topological Invariants in Two Dimensions

The existence of fractionally quantized topological corner charge serves as a key indicator for two-dimensional (2D) second-order topological insulators (SOTIs), yet it has not been experimentally observed in realistic materials. Here, based on effective model analysis and symmetry arguments, we propose a strategy for achieving SOTI phases with in-gap corner states in 2D systems with antiferromagnetic (AFM) order. We discover that the band topology originates from the interplay between intrinsic spin-orbital coupling and interlayer AFM exchange interactions. Using first-principles calculations, we show that the 2D AFM SOTI phase can be realized in (MnBi2Te4)(Bi2Te3)m films. Moreover, we demonstrate that the SOTI states are linked to rotation topological invariants under 3-fold rotation symmetry C3, resulting in fractionally quantized corner charge, i.e., n 3 | e | (mod e). Due to the great achievements in (MnBi2Te4)(Bi2Te3)m systems, our results providing reliable material candidates for experimentally accessible AFM SOTIs should draw intense attention.

Spin stiffness and spin excitation gap of van der Waals ferromagneticFe3+δGeTe2

Fe3+δGeTe2(FGT) has proved to be an interesting van der Waals (vdW) ferromagnetic compound with a tunable Curie temperature (TC). However, the underlying mechanism for varyingTCremains elusive. Here, we systematically investigate and compare low-temperature magnetic properties of single crystalline FGT samples that exhibitTCs ranging from 160 K to 205 K. Spin stiffness (D) and spin excitation gap (Δ) are extracted using Bloch's theory for crystals with varying Fe content. Compared to Cr-based vdW ferromagnets, FGT compounds have higher spin stiffness values but lower spin wave excitation gaps. We discuss the implication of these relationships in Fe-Fe ion magnetic interactions in FGT unit cells. The itinerancy of magnetic electrons is measured and discussed under the Rhodes-Wohlfarth ratio (RWR) and the Takahashi theory.

Tuning of thermoelectric performance by modulating vibrational properties in Ni-doped Sb2Te3

Sb2Te3, a binary chalcogenide-based 3D topological insulator, attracts significant attention for its exceptional thermoelectric performance. We report the vibrational properties of magnetically doped Sb2Te3thermoelectric material. Ni doping induces defect/disorder in the system and plays a positive role in engineering the thermoelectric properties through tuning the vibrational phonon modes. Synchrotron powder x-ray diffraction study confirms good crystalline quality and single-phase nature of the synthesized samples. The change in structural parameters, includingBisoand strain, further corroborate with structural disorder. Detailed modification of phonon modes with doping and temperature variation is analysed from temperature-dependent Raman spectroscopic measurement. Compressive lattice strain is observed from the blue shift of Raman peaks owing to Ni incorporation in Sb site. An attempt is made to extract the lattice thermal conductivity from total thermal conductivity estimated through optothermal Raman studies. Hall concentration data support the change in temperature-dependent resistivity and thermopower. Remarkable increase in thermopower is observed after Ni doping. Simulation of the Pisarenko model, indicating the convergence of the valence band, explains the observed enhancement of thermopower in Sb2-xNixTe3. The energy gap between the light and heavy valence band at Γ point is found to be 30 meV (for Sb2Te3), which is reduced to 3 meV (in Sb1.98Ni0.02Te3). A significant increase in thermoelectric power factor is obtained from 715 μWm-1K-2for pristine Sb2Te3to 2415 μWm-1K-2for Ni-doped Sb2Te3sample. Finally, the thermoelectric figure of merit,ZTis found to increase by four times in Sb1.98Ni0.02Te3than that of its pristine counterpart.

Coupled Electronic and Magnonic Topological States in Two-Dimensional Ferromagnets

Magnetic materials offer a fertile playground for fundamental physics discovery, with not only electronic but also magnonic topological states intensively explored. However, one natural material with both electronic and magnonic nontrivial topologies is still unknown. Here, we demonstrate the coexistence of first-order topological magnon insulators (TMIs) and electronic second-order topological insulators (SOTIs) in 2D honeycomb ferromagnets, giving rise to the nontrivial corner states being connected by the charge-free magnonic edge states. We show that, with C 3 symmetry, the phase factor ± ϕ caused by the next nearest-neighbor Dzyaloshinskii-Moriya interaction breaks the pseudo-spin time-reversal symmetry T , which leads to the split of magnon bands, i.e., the emergence of TMIs with a nonzero Chern number of C = - 1 , in experimentally feasible candidates of MoI3, CrSiTe3, and CrGeTe3 monolayers. Moreover, protected by the C 3 symmetry, the electronic SOTIs characterized by nontrivial corner states are obtained, bridging the topological aspect of fermions and bosons with a high possibility of innovative applications in spintronics devices.

High-temperature concomitant metal-insulator and spin-reorientation transitions in a compressed nodal-line ferrimagnet Mn3Si2Te6

Symmetry-protected band degeneracy, coupled with a magnetic order, is the key to realizing novel magnetoelectric phenomena in topological magnets. While the spin-polarized nodal states have been identified to introduce extremely-sensitive electronic responses to the magnetic states, their possible role in determining magnetic ground states has remained elusive. Here, taking external pressure as a control knob, we show that a metal-insulator transition, a spin-reorientation transition, and a structural modification occur concomitantly when the nodal-line state crosses the Fermi level in a ferrimagnetic semiconductor Mn3Si2Te6. These unique pressure-driven magnetic and electronic transitions, associated with the dome-shaped Tc variation up to nearly room temperature, originate from the interplay between the spin-orbit coupling of the nodal-line state and magnetic frustration of localized spins. Our findings highlight that the nodal-line states, isolated from other trivial states, can facilitate strongly tunable magnetic properties in topological magnets.

Reversible non-volatile electronic switching in a near-room-temperature van der Waals ferromagnet

Non-volatile phase-change memory devices utilize local heating to toggle between crystalline and amorphous states with distinct electrical properties. Expanding on this kind of switching to two topologically distinct phases requires controlled non-volatile switching between two crystalline phases with distinct symmetries. Here, we report the observation of reversible and non-volatile switching between two stable and closely related crystal structures, with remarkably distinct electronic structures, in the near-room-temperature van der Waals ferromagnet Fe5-δGeTe2. We show that the switching is enabled by the ordering and disordering of Fe site vacancies that results in distinct crystalline symmetries of the two phases, which can be controlled by a thermal annealing and quenching method. The two phases are distinguished by the presence of topological nodal lines due to the preserved global inversion symmetry in the site-disordered phase, flat bands resulting from quantum destructive interference on a bipartite lattice, and broken inversion symmetry in the site-ordered phase.

Single-crystal growth, structure and thermal transport properties of the metallic antiferromagnet Zintl-phase β-EuIn2As2

Zintl-phase materials have attracted significant research interest owing to the interplay of magnetism and strong spin-orbit coupling, providing a prominent material platform for axion electrodynamics. Here, we report the single-crystal growth, structure, magnetic and electrical/thermal transport properties of the antiferromagnet layer Zintl-phase compound β-EuIn2As2. Importantly, the new layered structure of β-EuIn2As2, in rhombohedral (R3̄m) symmetry, contains triangular layers of Eu2+ ions. The in-plane resistivity ρ(H, T) measurements reveal metal behavior with an antiferromagnetic (AFM) transition (TN ∼ 23.5 K), which is consistent with the heat capacity Cp(H, T) and magnetic susceptibility χ(H, T) measurements. Negative MR was observed in the temperature range from 2 K to 20 K with a maximum MR ratio of 0.06. Unique 4f7J = S = 7/2 Eu2+ spins were supposed magnetically order along the c-axis. The Seebeck coefficient shows a maximum thermopower |Smax| of about 40 μV K-1. The kink around 23 K in the Seebeck coefficient originates from the effect of the antiferromagnetic phase on the electron band structure, while the pronounced thermal conductivity peak at around 10 K is attributed to the phonon-phonon Umklapp scattering. The results suggest that the Eu2+ spin arrangement plays an important role in the magnetic, electrical, and thermal transport properties in β-EuIn2As2, which might be helpful for future potential technical applications.

Emergent Three-Dimensional Electric Dipole Sinewave in Bulk Perovskite Oxides

The magnetic and electric dipoles of ferroics play a central role in their fascinating properties. In particular, topological configurations have shown promising potential for use in novel electromechanical and electronic devices. Magnetic configurations from simple collinear to complex topological are well-documented. In contrast, many complex topological features in the electric counterpart remain unexplored. Here, we report the first example of three-dimensional electric dipole sinewave topological structure in a PbZrO3-based bulk perovskite, which presents an interesting triple-hysteresis loop macroscopically. This polar configuration consists of two orthogonal sinewave electric dipole modulations decoded from a polar incommensurate phase by advanced diffraction and atomic-resolution imaging techniques. The resulting topology is unraveled to be the competition between the antiferroelectric and ferroelectric states, stabilized by the modulation of the Pb 6s2 lone pair and the antiferrodistortive effect. These findings further reinforce the similarity of the magnetic and electric topologies.

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.

Predictions of delafossite-hosted honeycomb and kagome phases

Delafossites, typically denoted by the formula ABO2, are a class of layered materials that exhibit a wide range of electronic and optical properties. Recently, the idea of modifying these delafossites into ordered kagome or honeycomb phases via strategic doping has emerged as a potential way to tailor these properties. In this study, we use high-throughput density functional theory calculations to explore many possible candidate kagome and honeycomb phases by considering dopants selected from the parent compounds of known ternary delafossite oxides from the inorganic crystal structure database. Our results indicate that while A-site in existing delafossites can host a limited range of elemental specifies, and display a low propensity for mixing or ordering, the oxide sub-units in the BO2 much more readily admit guest species. Our study identifies four candidate B-site kagome and fifteen candidate B-site honeycombs with a formation energy more than 50 meV f.u.-1 below other competing phases. The ability to predict and control the formation of these unique structures offers exciting opportunities in materials design, where innovative properties can be engineered through the selection of specific dopants. A number of these constitute novel correlated metals, which may be of interest for subsequent efforts in synthesis. These novel correlated metals may have significant implications for quantum computing, spintronics, and high-temperature superconductivity, thus inspiring future experimental synthesis and characterization of these proposed materials.

Spin-Reorientation-Driven Linear Magnetoelectric Effect in Topological Antiferromagnet Cu_{3}TeO_{6}

The search for new materials for energy-efficient electronic devices has gained unprecedented importance. Among the various classes of magnetic materials driving this search are antiferromagnets, magnetoelectrics, and systems with topological spin excitations. Cu_{3}TeO_{6} is a material that belongs to all three of these classes. Combining static electric polarization and magnetic torque measurements with phenomenological simulations we demonstrate that magnetic-field-induced spin reorientation needs to be taken into account to understand the linear magnetoelectric effect in Cu_{3}TeO_{6}. Our calculations reveal that the magnetic field pushes the system from the nonpolar ground state to the polar magnetic structures. However, nonpolar structures only weakly differing from the obtained polar ones exist due to the weak effect that the field-induced breaking of some symmetries has on the calculated structures. Among those symmetries is the PT (1[over ¯]^{'}) symmetry, preserved for Dirac points found in Cu_{3}TeO_{6}. Our findings establish Cu_{3}TeO_{6} as a promising playground to study the interplay of spintronics-related phenomena.

Tunable positions of Weyl nodes via magnetism and pressure in the ferromagnetic Weyl semimetal CeAlSi

The noncentrosymmetric ferromagnetic Weyl semimetal CeAlSi with simultaneous space-inversion and time-reversal symmetry breaking provides a unique platform for exploring novel topological states. Here, by employing multiple experimental techniques, we demonstrate that ferromagnetism and pressure can serve as efficient parameters to tune the positions of Weyl nodes in CeAlSi. At ambient pressure, a magnetism-facilitated anomalous Hall/Nernst effect (AHE/ANE) is uncovered. Angle-resolved photoemission spectroscopy (ARPES) measurements demonstrated that the Weyl nodes with opposite chirality are moving away from each other upon entering the ferromagnetic phase. Under pressure, by tracing the pressure evolution of AHE and band structure, we demonstrate that pressure could also serve as a pivotal knob to tune the positions of Weyl nodes. Moreover, multiple pressure-induced phase transitions are also revealed. These findings indicate that CeAlSi provides a unique and tunable platform for exploring exotic topological physics and electron correlations, as well as catering to potential applications, such as spintronics.

Effective electrical manipulation of a topological antiferromagnet by orbital torques

The electrical control of the non-trivial topology in Weyl antiferromagnets is of great interest for the development of next-generation spintronic devices. Recent studies suggest that the spin Hall effect can switch the topological antiferromagnetic order. However, the switching efficiency remains relatively low. Here, we demonstrate the effective manipulation of antiferromagnetic order in the Weyl semimetal Mn3Sn using orbital torques originating from either metal Mn or oxide CuOx. Although Mn3Sn can convert orbital current to spin current on its own, we find that inserting a heavy metal layer, such as Pt, of appropriate thickness can effectively reduce the critical switching current density by one order of magnitude. In addition, we show that the memristor-like switching behaviour of Mn3Sn can mimic the potentiation and depression processes of a synapse with high linearity-which may be beneficial for constructing accurate artificial neural networks. Our work paves a way for manipulating the topological antiferromagnetic order and may inspire more high-performance antiferromagnetic functional devices.

Antiferromagnetic topological insulator with selectively gapped Dirac cones

Antiferromagnetic (AF) topological materials offer a fertile ground to explore a variety of quantum phenomena such as axion magnetoelectric dynamics and chiral Majorana fermions. To realize such intriguing states, it is essential to establish a direct link between electronic states and topology in the AF phase, whereas this has been challenging because of the lack of a suitable materials platform. Here we report the experimental realization of the AF topological-insulator phase in NdBi. By using micro-focused angle-resolved photoemission spectroscopy, we discovered contrasting surface electronic states for two types of AF domains; the surface having the out-of-plane component in the AF-ordering vector displays Dirac-cone states with a gigantic energy gap, whereas the surface parallel to the AF-ordering vector hosts gapless Dirac states despite the time-reversal-symmetry breaking. The present results establish an essential role of combined symmetry to protect massless Dirac fermions under the presence of AF order and widen opportunities to realize exotic phenomena utilizing AF topological materials.

Magnetic properties and critical behaviors of the nodal-line semimetal candidate ErIn3

The AuCu3-type intermetallic compoundsReIn3(Re= a rare earth ion) with type-IV magnetic space groups are predicted to show topologically nontrivial electronic states. Here, we grow ErIn3single crystals, and study their magnetic properties and critical behaviors by means of the magnetic susceptibility, and magnetization isotherm measurements. Combining a detailed analysis of the magnetic susceptibility and isothermal magnetization, we find that this compound harbors a complicated magnetic phase diagram, and its magnetic moment arrangement appears not to simply follow the fashion as observed in the isostructural counterpart GdIn3(it adopts a conventional type-Cmagnetic structure that belongs to type-IV magnetic space groups). A careful study of the magnetic properties around the antiferromagnetic (AFM)-paramagnetic transition yields the critical exponentsβ= 0.309 (0.297),γ= 1.117 (1.038), andδ= 4.617 (4.454), indicating that the tricritical mean field model or the three-dimensional Ising model works for ErIn3's magnetic behaviors and the presence of a long-range AFM interaction therein. Besides, the exchange interaction distanceJ(r) ∼r-4.665as well confirms a long-range magnetic coupling in ErIn3. Our results offer the clues that the magnetic structure varies from one member ofReIn3family to another, and to confirm their electronic features in the AFM phases further experimental and theoretical studies are still desired.