Magnetic proximity and nonreciprocal current switching in a monolayer WTe2 helical edge

Intrinsic Grain Boundary Structure and Enhanced Defect States in Air-Sensitive Polycrystalline 1T'-WTe2 Monolayer

Monolayer WTe2 has attracted significant attention for its unconventional superconductivity and topological edge states. However, its air sensitivity poses challenges for studying intrinsic defect structures. This study addresses this issue using a custom-built inert gas interconnected system, and investigate the intrinsic grain boundary (GB) structures of monolayer polycrystalline 1T' WTe2 grown by nucleation-controlled chemical vapor deposition (CVD) method. These findings reveal that GBs in this system are predominantly governed by W-Te rhombi with saturated coordination, resulting in three specific GB prototypes without dislocation cores. The GBs exhibit anisotropic orientations influenced by kinks formed from these fundamental units, which in turn affect the distribution of grains in various shapes within polycrystalline flakes. Scanning tunneling microscopy/spectroscopy (STM/S) analysis further reveals metallic states along the intrinsic 120° twin grain boundary (TGB), consistent with computed band structures. This systematic exploration of GBs in air-sensitive 1T' WTe2 monolayers provides valuable insights into emerging GB-related phenomena.

Observation of giant non-reciprocal charge transport from quantum Hall states in a topological insulator

Symmetry breaking in quantum materials is of great importance and can lead to non-reciprocal charge transport. Topological insulators provide a unique platform to study non-reciprocal charge transport due to their surface states, especially quantum Hall states under an external magnetic field. Here we report the observation of non-reciprocal charge transport mediated by quantum Hall states in devices composed of the intrinsic topological insulator Sn-Bi1.1Sb0.9Te2S, which is attributed to asymmetric scattering between quantum Hall states and Dirac surface states. A giant non-reciprocal coefficient of up to 2.26 × 105 A-1 is found. Our work not only reveals the properties of non-reciprocal charge transport of quantum Hall states in topological insulators but also paves the way for future electronic devices.

Unification of Nonlinear Anomalous Hall Effect and Nonreciprocal Magnetoresistance in Metals by the Quantum Geometry

The quantum geometry has significant consequences in determining transport and optical properties in quantum materials. Here, we use a semiclassical formalism coupled with perturbative corrections unifying the nonlinear anomalous Hall effect and nonreciprocal magnetoresistance (longitudinal resistance) from the quantum geometry. In the dc limit, both transverse and longitudinal nonlinear conductivities include a term due to the normalized quantum metric dipole. The quantum metric contribution is intrinsic and does not scale with the quasiparticle lifetime. We demonstrate the coexistence of a nonlinear anomalous Hall effect and nonreciprocal magnetoresistance in films of the doped antiferromagnetic topological insulator MnBi_{2}Te_{4}. Our work indicates that both longitudinal and transverse nonlinear transport provide a sensitive probe of the quantum geometry in solids.

Electronic Band Structure Changes across the Antiferromagnetic Phase Transition of Exfoliated MnPS3 Flakes Probed by μ-ARPES

Exfoliated magnetic 2D materials enable versatile tuning of magnetization, e.g., by gating or providing proximity-induced exchange interaction. However, their electronic band structure after exfoliation has not been probed, presumably due to their photochemical sensitivity. Here, we provide micrometer-scale angle-resolved photoelectron spectroscopy of the exfoliated intralayer antiferromagnet MnPS3 above and below the Néel temperature down to one monolayer. Favorable comparison with density functional theory calculations enables identifying the orbital character of the observed bands. Consistently, we find pronounced changes across the Néel temperature for bands consisting of Mn 3d and 3p levels of adjacent S atoms. The deduced orbital mixture indicates that the superexchange is relevant for the magnetic interaction. There are only minor changes between monolayer and thicker films, demonstrating the predominant 2D character of MnPS3. The novel access is transferable to other MPX3 materials (M: transition metal, P: phosphorus, X: chalcogenide), providing several antiferromagnetic arrangements.

Gate-Defined Josephson Weak-Links in Monolayer WTe2

Systems combining superconductors with topological insulators offer a platform for the study of Majorana bound states and a possible route to realize fault tolerant topological quantum computation. Among the systems being considered in this field, monolayers of tungsten ditelluride (WTe2 ) have a rare combination of properties. Notably, it has been demonstrated to be a quantum spin Hall insulator (QSHI) and can easily be gated into a superconducting state. Measurements on gate-defined Josephson weak-link devices fabricated using monolayer WTe2 are reported. It is found that consideration of the 2D superconducting leads are critical in the interpretation of magnetic interference in the resulting junctions. The reported fabrication procedures suggest a facile way to produce further devices from this technically challenging material and the results mark the first step toward realizing versatile all-in-one topological Josephson weak-links using monolayer WTe2 .

Electrical detection of spin pumping in van der Waals ferromagnetic Cr2Ge2Te6 with low magnetic damping

The discovery of magnetic order in atomically-thin van der Waals materials has strengthened the alliance between spintronics and two-dimensional materials. An important use of magnetic two-dimensional materials in spintronic devices, which has not yet been demonstrated, would be for coherent spin injection via the spin-pumping effect. Here, we report spin pumping from Cr₂Ge₂Te₆ into Pt or W and detection of the spin current by inverse spin Hall effect. The magnetization dynamics of the hybrid Cr₂Ge₂Te₆/Pt system are measured, and a magnetic damping constant of ~ 4-10 × 10^-4 is obtained for thick Cr₂Ge₂Te₆ flakes, a record low for ferromagnetic van der Waals materials. Moreover, a high interface spin transmission efficiency (a spin mixing conductance of 2.4 × 10¹⁹/m²) is directly extracted, which is instrumental in delivering spin-related quantities such as spin angular momentum and spin-orbit torque across an interface of the van der Waals system. The low magnetic damping that promotes efficient spin current generation together with high interfacial spin transmission efficiency suggests promising applications for integrating Cr₂Ge₂Te₆ into low-temperature two-dimensional spintronic devices as the source of coherent spin or magnon current.

Quantum metric nonlinear Hall effect in a topological antiferromagnetic heterostructure

Quantum geometry in condensed matter physics has two components: the real part quantum metric and the imaginary part Berry curvature. Whereas the effects of Berry curvature have been observed through phenomena such as the quantum Hall effect in 2D electron gases and the anomalous Hall effect (AHE) in ferromagnets, quantum metric has rarely been explored. Here, we report a nonlinear Hall effect induced by quantum metric dipole by interfacing even-layered MnBi₂Te₄ with black phosphorus. The quantum metric nonlinear Hall effect switches direction upon reversing the AFM spins and exhibits distinct scaling that is independent of the scattering time. Our results open the door to discovering quantum metric responses predicted theoretically and pave the way for applications that bridge nonlinear electronics with AFM spintronics.

Realization of quasi-1D topological magnetism at the V-alloyed MoS2 zigzag edge

Topological magnetism in quasi-1D systems can be interesting because of the significant quantum confinement. However, the realization is missing. In this letter, we propose the use of 3× periodicities related edge reconstructions of MoS₂ zigzag edges to construct a topological quasi-1D spin chain. Specifically, a trimer Su-Schrieffer-Heeger model can be applied to illustrate the topological and spin order when the inter-cell hopping integral is larger than the intra-cell ones. As a result, topological ferromagnetic order is achieved for S-oriented edge states magnetized by V atoms and confirmed by first-principles calculations and Wannier functions analysis. Finally, gap opening and spin-polarized end states are realized.

Physical Vapor Transport Growth of Antiferromagnetic CrCl3 Flakes Down to Monolayer Thickness

The van der Waals magnets CrX₃ (X = I, Br, and Cl) exhibit highly tunable magnetic properties and are promising candidates for developing novel two-dimensional (2D) spintronic devices such as magnetic tunnel junctions and spin tunneling transistors. Previous studies of the antiferromagnetic CrCl₃ have mainly focused on mechanically exfoliated samples. Controlled synthesis of high quality atomically thin flakes is critical for their technological implementation but has not been achieved to date. This work reports the growth of large CrCl₃ flakes down to monolayer thickness via the physical vapor transport technique. Both isolated flakes with well-defined facets and long stripe samples with the trilayer portion exceeding 60 µm have been obtained. High-resolution transmission electron microscopy studies show that the CrCl₃ flakes are single crystalline in the monoclinic structure, consistent with the Raman results. The room temperature stability of the CrCl₃ flakes decreases with decreasing thickness. The tunneling magnetoresistance of graphite/CrCl₃ /graphite tunnel junctions confirms that few-layer CrCl₃ possesses in-plane magnetic anisotropy and Néel temperature of 17 K. This study paves the path for developing CrCl₃ -based scalable 2D spintronic applications.

Two-Dimensional Van Der Waals Topological Materials: Preparation, Properties, and Device Applications

Over the past decade, 2D van der Waals (vdW) topological materials (TMs), including topological insulators and topological semimetals, which combine atomically flat 2D layers and topologically nontrivial band structures, have attracted increasing attention in condensed-matter physics and materials science. These easily cleavable and integrated TMs provide the ideal platform for exploring topological physics in the 2D limit, where new physical phenomena may emerge, and represent a potential to control and investigate exotic properties and device applications in nanoscale topological phases. However, multifaced efforts are still necessary, which is the prerequisite for the practical application of 2D vdW TMs. Herein, this review focuses on the preparation, properties, and device applications of 2D vdW TMs. First, three common preparation strategies for 2D vdW TMs are summarized, including single crystal exfoliation, chemical vapor deposition, and molecular beam epitaxy. Second, the origin and regulation of various properties of 2D vdW TMs are introduced, involving electronic properties, transport properties, optoelectronic properties, thermoelectricity, ferroelectricity, and magnetism. Third, some device applications of 2D vdW TMs are presented, including field-effect transistors, memories, spintronic devices, and photodetectors. Finally, some significant challenges and opportunities for the practical application of 2D vdW TMs in 2D topological electronics are briefly addressed.

Controlled large non-reciprocal charge transport in an intrinsic magnetic topological insulator MnBi2Te4

Symmetries, quantum geometries and electronic correlations are among the most important ingredients of condensed matters, and lead to nontrivial phenomena in experiments, for example, non-reciprocal charge transport. Of particular interest is whether the non-reciprocal transport can be manipulated. Here, we report the controllable large non-reciprocal charge transport in the intrinsic magnetic topological insulator MnBi₂Te₄. The current direction relevant resistance is observed at chiral edges, which is magnetically switchable, edge position sensitive and stacking sequence controllable. Applying gate voltage can also effectively manipulate the non-reciprocal response. The observation and manipulation of non-reciprocal charge transport reveals the fundamental role of chirality in charge transport of MnBi₂Te₄, and pave ways to develop van der Waals spintronic devices by chirality engineering.

Proximity-magnetized quantum spin Hall insulator: monolayer 1 T' WTe2/Cr2Ge2Te6

Van der Waals heterostructures offer great versatility to tailor unique interactions at the atomically flat interfaces between dissimilar layered materials and induce novel physical phenomena. By bringing monolayer 1 T’ WTe₂, a two-dimensional quantum spin Hall insulator, and few-layer Cr₂Ge₂Te₆, an insulating ferromagnet, into close proximity in an heterostructure, we introduce a ferromagnetic order in the former via the interfacial exchange interaction. The ferromagnetism in WTe₂ manifests in the anomalous Nernst effect, anomalous Hall effect as well as anisotropic magnetoresistance effect. Using local electrodes, we identify separate transport contributions from the metallic edge and insulating bulk. When driven by an AC current, the second harmonic voltage responses closely resemble the anomalous Nernst responses to AC temperature gradient generated by nonlocal heater, which appear as nonreciprocal signals with respect to the induced magnetization orientation. Our results from different electrodes reveal spin-polarized edge states in the magnetized quantum spin Hall insulator.

Van der Waals heterostructures allow for the integration of several materials with different properties in the one heterostructure. Here, Li et al combine a quantum spin hall insulator, WTe2, with an insulating ferromagnet, Cr2Ge2Te6, in a van der Waals heterostructure, with resulting proximity-induced magnetism in the WTe2 layer leading to an anomalous Hall and Nernst effect.

Low energy electrodynamics of CrI3 layered ferromagnet

We report on the optical properties from terahertz (THz) to Near-Infrared (NIR) of the layered magnetic compound CrI₃ at various temperatures, both in the paramagnetic and ferromagnetic phase. In the NIR spectral range, we observe an insulating electronic gap around 1.1 eV which strongly hardens with decreasing temperature. The blue shift observed represents a record in insulating materials and it is a fingerprint of a strong electron-phonon interaction. Moreover, a further gap hardening is observed below the Curie temperature, indicating the establishment of an effective interaction between electrons and magnetic degrees of freedom in the ferromagnetic phase. Similar interactions are confirmed by the disappearance of some phonon modes in the same phase, as expected from a spin-lattice interaction theory. Therefore, the optical properties of CrI₃ reveal a complex interaction among electronic, phononic and magnetic degrees of freedom, opening many possibilities for its use in 2-Dimensional heterostructures.

Terahertz response of monolayer and few-layer WTe2 at the nanoscale

Tungsten ditelluride (WTe2) is an atomically layered transition metal dichalcogenide whose physical properties change systematically from monolayer to bilayer and few-layer versions. In this report, we use apertureless scattering-type near-field optical microscopy operating at Terahertz (THz) frequencies and cryogenic temperatures to study the distinct THz range electromagnetic responses of mono-, bi- and trilayer WTe2 in the same multi-terraced micro-crystal. THz nano-images of monolayer terraces uncovered weakly insulating behavior that is consistent with transport measurements. The near-field signal on bilayer regions shows moderate metallicity with negligible temperature dependence. Subdiffractional THz imaging data together with theoretical calculations involving thermally activated carriers favor the semimetal scenario with [Formula: see text] over the semiconductor scenario for bilayer WTe2. Also, we observed clear metallic behavior of the near-field signal on trilayer regions. Our data are consistent with the existence of surface plasmon polaritons in the THz range confined to trilayer terraces in our specimens. Finally, data for microcrystals up to 12 layers thick reveal how the response of a few-layer WTe2 asymptotically approaches the bulk limit.

Recent progress on emergent two-dimensional magnets and heterostructures

Intrinsic two-dimensional (2D) magnetic materials own strong long-range magnetism while their characteristics of the ultrathin thickness and smooth surface provide an ideal platform for manipulating the magnetic properties at 2D limit. This makes them to be potential candidates in various spintronic applications compared to their corresponding bulk counterparts. The discovery of magnetic ordering in 2D CrI₃and Gr₂Ge₂Te₆nanostructures stimulated tremendous research interest in both experimental and theoretical studies on various intrinsic magnets at 2D limit. This review gives a comprehensive overview of the recent progress on the emergent 2D magnets and heterostructures. Firstly, several kinds of typical 2D magnetic materials discovered in the last few years and their fabrication methods are summarized in detail. Secondly, the current strategies for manipulating magnetic properties in 2D materials are further discussed. Then, the recent advances on the construction of representative van der Waals magnetic heterostructures and their respective performance are provided. With the hope of motivating the researchers in this area, we finally offered the challenges and outlook on 2D magnetism.

Van der Waals heterostructures for spintronics and opto-spintronics

The large variety of 2D materials and their co-integration in van der Waals heterostructures enable innovative device engineering. In addition, their atomically thin nature promotes the design of artificial materials by proximity effects that originate from short-range interactions. Such a designer approach is particularly compelling for spintronics, which typically harnesses functionalities from thin layers of magnetic and non-magnetic materials and the interfaces between them. Here we provide an overview of recent progress in 2D spintronics and opto-spintronics using van der Waals heterostructures. After an introduction to the forefront of spin transport research, we highlight the unique spin-related phenomena arising from spin-orbit and magnetic proximity effects. We further describe the ability to create multifunctional hybrid heterostructures based on van der Waals materials, combining spin, valley and excitonic degrees of freedom. We end with an outlook on perspectives and challenges for the design and production of ultracompact all-2D spin devices and their potential applications in conventional and quantum technologies.

Tuning Electronic Properties of 2D Materials Using Metal Adsorbates: Cu at WTe2 Edges

Two-dimensional materials exhibit properties promising for novel applications. Topologically protected states at their edges can be harnessed for use in quantum devices. We use ab initio simulations to examine properties of edges in 1T'-WTe₂ monolayers, known to exhibit topological order, and their interactions with Cu atoms. Comparison of (010)-oriented edges that have the same composition but different terminations shows that, as the number of Cu atoms increases, their thermodynamically preferred arrangement depends on the details of the edge structure. Cu atoms aggregate into a cluster at the most stable edge; while the cluster is nonmagnetic, it spin-polarizes the W atoms along the edge, which removes the topological protection. At the metastable edge, Cu atoms form a chain incorporated into the WTe₂ lattice; the topological state is preserved in spite of the dramatic edge restructuring. This suggests that exploiting interactions of metal species with metastable edge terminations can provide a path toward noninvasive interfaces.

Atomically Thin Quantum Spin Hall Insulators

Atomically thin topological materials are attracting growing attention for their potential to radically transform classical and quantum electronic device concepts. Among them is the quantum spin Hall (QSH) insulator-a 2D state of matter that arises from interplay of topological band inversion and strong spin-orbit coupling, with large tunable bulk bandgaps up to 800 meV and gapless, 1D edge states. Reviewing recent advances in materials science and engineering alongside theoretical description, the QSH materials library is surveyed with focus on the prospects for QSH-based device applications. In particular, theoretical predictions of nontrivial superconducting pairing in the QSH state toward Majorana-based topological quantum computing are discussed, which are the next frontier in QSH materials research.

Geometric Photon-Drag Effect and Nonlinear Shift Current in Centrosymmetric Crystals

The nonlinear shift current, also known as the bulk photovoltaic current generated by linearly polarized light, has long been known to be absent in crystals with inversion symmetry. Here we argue that a nonzero shift current in centrosymmetric crystals can be activated by a photon-drag effect. Photon-drag shift current proceeds from a "shift current dipole" (a geometric quantity characterizing interband transitions) and manifests a purely transverse response in centrosymmetric crystals. This transverse nature proceeds directly from the shift-vector's pseudovector nature under mirror operation and underscores its intrinsic geometric origin. Photon-drag shift current can be greatly enhanced by coupling to polaritons and provides a new and sensitive tool to interrogate the subtle interband coherences of materials with inversion symmetry previously thought to be inaccessible via photocurrent probes.

Chirality-Induced Giant Unidirectional Magnetoresistance in Twisted Bilayer Graphene

Twisted bilayer graphene (TBG) exhibits fascinating correlation-driven phenomena like the superconductivity and Mott insulating state, with flat bands and a chiral lattice structure. We find by quantum-transport calculations that the chirality leads to a giant unidirectional magnetoresistance (UMR) in TBG, where the unidirectionality refers to the resistance change under the reversal of the direction of current or magnetic field. We point out that flat bands significantly enhance this effect. The UMR increases quickly upon reducing the twist angle, and reaches about 20% for an angle of 1.5° in a 10 T in-plane magnetic field. We propose the band structure topology (asymmetry), which leads to a direction-sensitive mean free path, as a useful way to anticipate the UMR effect. The UMR provides a probe for chirality and band flatness in the twisted bilayers.

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Twisted bilayer graphene (TBG) was recently discovered to exhibit fascinating phenomena like the superconductivity and Mott insulating state

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TBG's chirality leads to a unidirectional magnetoresistance (UMR), where magnetoresistance is lower in a chirality-dependent direction. The flat bands of TBG enhance the UMR dramatically

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The giant UMR represents diode-like current rectification and can convert radiation (e.g., terahertz) into currents for energy-harvesting and photodetection

Controlling bimerons as skyrmion analogues by ferroelectric polarization in 2D van der Waals multiferroic heterostructures

Atom-thick van der Waals heterostructures with nontrivial physical properties tunable via the magnetoelectric coupling effect are highly desirable for the future advance of multiferroic devices. In this work on LaCl/In₂Se₃ heterostructure consisting of a 2D ferromagnetic layer and a 2D ferroelectric layer, reversible switch of the easy axis and the Curie temperature of the magnetic LaCl layer has been enabled by switching of ferroelectric polarization in In₂Se₃. More importantly, magnetic skyrmions in the bimerons form have been discovered in the LaCl/In₂Se₃ heterostructure and can be driven by an electric current. The creation and annihilation of bimerons in LaCl magnetic nanodisks were achieved by polarization switching. It thus proves to be a feasible approach to achieve purely electric control of skyrmions in 2D van der Waals heterostructures. Such nonvolatile and tunable magnetic skyrmions are promising candidates for information carriers in future data storage and logic devices operated under small electrical currents.

van der Waals Epitaxy of Soft Twisted Bilayers: Lattice Relaxation and Mass Density Waves

Interfaces comprising incommensurate or twisted hexagonal lattices are ubiquitous and of great interest, from adsorbed organic/inorganic interfaces in electronic devices, to superlubricants, and more recently to van der Waals bilayer heterostructures (vdWHs) of graphene and other 2D materials that demonstrate a range of properties such as superconductivity and ferromagnetism. Here we show how growth of 2D crystalline domains of soft block copolymers (BCPs) on patterned hard hexagonal lattices provide fundamental insights into van der Waals heteroepitaxy. At moderate registration forces, it is experimentally found that these BCP-hard lattice vdWHs do not adopt a simple moiré superstructure, but instead adopt local structural relaxations known as mass density waves (MDWs). Simulations reveal that MDWs are a primary mechanism of energy minimization and are the origin of the observed preferential twist angle between the lattices.

Antisymmetric Magnetoresistance in a van der Waals Antiferromagnetic/Ferromagnetic Layered MnPS3/Fe3GeTe2 Stacking Heterostructure

The presence of two-dimensional (2D) layer-stacking heterostructures that can efficiently tune the interface properties by stacking desirable materials provides a platform to investigate some physical phenomena, such as the proximity effect and magnetic exchange coupling. Here, we report the observation of antisymmetric magnetoresistance in a van der Waals (vdW) antiferromagnetic/ferromagnetic (AFM/FM) heterostructure of MnPS₃/Fe₃GeTe₂ when the temperature is below the Neel temperature of MnPS₃. Distinguished from two resistance states in conventional giant magnetoresistance, the magnetoresistance in the MnPS₃/Fe₃GeTe₂ heterostructure exhibits three states, of high, intermediate, and low resistance. This antisymmetric magnetoresistance spike is determined by an unsynchronized magnetic switching between the AFM/FM interface layer and the bulk of Fe₃GeTe₂ during magnetization reversal. Our work highlights that the artificial vdW stacking structure holds potential to explore some physical phenomena and spintronic device applications.

Two-Dimensional Materials for Energy-Efficient Spin-Orbit Torque Devices

Spin-orbit torques (SOTs), which rely on spin current generation from charge current in a nonmagnetic material, promise an energy-efficient scheme for manipulating magnetization in magnetic devices. A critical topic for spintronic devices using SOTs is to enhance the charge to spin conversion efficiency. Besides, the current-induced spin polarization is usually limited to in-plane, whereas out-of-plane spin polarization could be favored for efficient perpendicular magnetization switching. Recent advances in utilizing two important classes of two-dimensional materials-topological insulators and transition-metal dichalcogenides-as spin sources to generate SOT shed light on addressing these challenges. Topological insulators such as bismuth selenide have shown a giant SOT efficiency, which is larger than those from three-dimensional heavy metals by at least 1 order of magnitude. Transition-metal dichalcogenides such as tungsten telluride have shown a current-induced out-of-plane spin polarization, which is allowed by the reduced symmetry. In this review, we use symmetry arguments to predict and analyze SOTs in two-dimensional material-based heterostructures. We summarize the recent progress of SOT studies based on topological insulators and transition-metal dichalcogenides and show how these results are in line with the symmetry arguments. At last, we identify unsolved issues in the current studies and suggest three potential research directions in this field.

Probing the Ferromagnetism and Spin Wave Gap in VI3 by Helicity-Resolved Raman Spectroscopy

Circularly polarized light carries light spin angular momentum, which may lead helicity-resolved Raman scattering to be sensitive to the electronic spin configuration in magnetic materials. Here, we demonstrate that all Raman modes in the 2D ferromagnet VI₃ show different scattering intensities to left and right circularly polarized light at low temperatures, which gives direct evidence of the time-reversal symmetry breaking. By measuring the circular polarization of the dominant Raman mode with respect to the temperature and magnetic field, the ferromagnetic (FM) phase transition and hysteresis behavior can be clearly resolved. Besides the lattice excitations, quasielastic scattering is detected in the paramagnetic phase, and it gradually evolves into the acoustic magnon mode at 18.5 cm^-1 in the FM state, which gives the spin wave gap that results from large magnetic anisotropy. Our findings demonstrate that helicity-resolved Raman spectroscopy is an effective tool to directly probe the ferromagnetism in 2D magnets.