Experimental Observation of Anisotropic Adler-Bell-Jackiw Anomaly in Type-II Weyl Semimetal WTe_{1.98} Crystals at the Quasiclassical Regime

Antichirality Emergent in Type-II Weyl Phononic Crystals

Chiral anomaly as the hallmark feature lies in the heart of the researches for Weyl semimetal. It is rooted in the zeroth Landau level of the system with an applied magnetic field. Chirality or antichirality characterizes the propagation property of the one-way zeroth Landau level mode, and antichirality means an opposite group velocity compared to the case of chirality. Chirality is commonly observed for Weyl semimetals. Interestingly, the type-II Weyl point, with the overtilted dispersion, may flip the chirality to the antichirality, which, however, is yet to be evidenced despite numerous previous experimental efforts. Here, we implement the type-II Weyl point in sonic crystals, and by creating the pseudomagnetic fields with geometric deformation, the chirality flip of zeroth Landau levels is unambiguously demonstrated. Our Letter unveils the novel antichiral transport in the presence of time-reversal symmetry, and paves the way toward the state-of-the-art manipulation of sound waves.

Giant Magnetochiral Anisotropy in Weyl Semimetal WTe_{2} Induced by Diverging Berry Curvature

The concept of Berry curvature is essential for various transport phenomena. However, an effect of the Berry curvature on magnetochiral anisotropy, i.e., nonreciprocal magnetotransport, is still elusive. Here, we report that the Berry curvature induces the large magnetochiral anisotropy. In Weyl semimetal WTe_{2}, we observe the strong enhancement of the magnetochiral anisotropy when the Fermi level is located near the Weyl points. Notably, the maximal figure of merit γ[over ¯] reaches 1.2×10^{-6}  m^{2} T^{-1} A^{-1}, which is the largest ever reported in bulk materials. Our semiclassical calculation shows that the diverging Berry curvature at the Weyl points strongly enhances the magnetochiral anisotropy.

Room-temperature valley transistors for low-power neuromorphic computing

Valley pseudospin is an electronic degree of freedom that promises highly efficient information processing applications. However, valley-polarized excitons usually have short pico-second lifetimes, which limits the room-temperature applicability of valleytronic devices. Here, we demonstrate room-temperature valley transistors that operate by generating free carrier valley polarization with a long lifetime. This is achieved by electrostatic manipulation of the non-trivial band topology of the Weyl semiconductor tellurium (Te). We observe valley-polarized diffusion lengths of more than 7 μm and fabricate valley transistors with an ON/OFF ratio of 105 at room temperature. Moreover, we demonstrate an ion insertion/extraction device structure that enables 32 non-volatile memory states with high linearity and symmetry in the Te valley transistor. With ultralow power consumption (~fW valley contribution), we enable the inferring process of artificial neural networks, exhibiting potential for applications in low-power neuromorphic computing.

Controlling Spin-Orbit Coupling to Tailor Type-II Dirac Bands

NiTe₂, a type-II Dirac semimetal with a strongly tilted Dirac band, has been explored extensively to understand its intriguing topological properties. Here, using density functional theory calculations, we report that the strength of the spin-orbit coupling (SOC) in NiTe₂ can be tuned by Se substitution. This results in negative shifts of the bulk Dirac point (BDP) while preserving the type-II Dirac band. Indeed, combined studies using scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy confirm that the BDP in the NiTe_2-xSe_x alloy moves from +0.1 eV (NiTe₂) to -0.3 eV (NiTeSe) depending on the Se concentrations, indicating the effective tunability of type-II Dirac Fermions. Our results demonstrate an approach to tailor the type-II Dirac band in NiTe₂ by controlling the SOC strength via chalcogen substitution. This approach can be applicable to different types of topological materials.

Direct observation of vortices in an electron fluid

Vortices are the hallmarks of hydrodynamic flow. Strongly interacting electrons in ultrapure conductors can display signatures of hydrodynamic behaviour, including negative non-local resistance^1-4, higher-than-ballistic conduction^5-7, Poiseuille flow in narrow channels^8-10 and violation of the Wiedemann-Franz law¹¹. Here we provide a visualization of whirlpools in an electron fluid. By using a nanoscale scanning superconducting quantum interference device on a tip¹², we image the current distribution in a circular chamber connected through a small aperture to a current-carrying strip in the high-purity type II Weyl semimetal WTe₂. In this geometry, the Gurzhi momentum diffusion length and the size of the aperture determine the vortex stability phase diagram. We find that vortices are present for only small apertures, whereas the flow is laminar (non-vortical) for larger apertures. Near the vortical-to-laminar transition, we observe the single vortex in the chamber splitting into two vortices; this behaviour is expected only in the hydrodynamic regime and is not anticipated for ballistic transport. These findings suggest a new mechanism of hydrodynamic flow in thin pure crystals such that the spatial diffusion of electron momenta is enabled by small-angle scattering at the surfaces instead of the routinely invoked electron-electron scattering, which becomes extremely weak at low temperatures. This surface-induced para-hydrodynamics, which mimics many aspects of conventional hydrodynamics including vortices, opens new possibilities for exploring and using electron fluidics in high-mobility electron systems.

Enhancing Perpendicular Magnetic Anisotropy in Garnet Ferrimagnet by Interfacing with Few-Layer WTe2

Engineering magnetic anisotropy in a ferro- or ferrimagnetic (FM) thin film is crucial in a spintronic device. One way to modify the magnetic anisotropy is through the surface of the FM thin film. Here, we report the emergence of a perpendicular magnetic anisotropy (PMA) induced by interfacial interactions in a heterostructure comprised of a garnet ferrimagnet, Y₃Fe₅O₁₂ (YIG), and a low-symmetry, high spin-orbit coupling (SOC) transition metal dichalcogenide, WTe₂. At the same time, we also observed an enhancement in Gilbert damping in the WTe₂-covered YIG area. Both the magnitude of interface-induced PMA and the Gilbert damping enhancement have no observable WTe₂ thickness dependence down to a single quadruple layer, indicating that the interfacial interaction plays a critical role. The ability of WTe₂ to enhance the PMA in FM thin film, combined with its previously reported capability to generate out-of-plane damping like spin torque, makes it desirable for magnetic memory applications.

Positive Magnetoresistance and Chiral Anomaly in Exfoliated Type-II Weyl Semimetal Td-WTe2

Layered van der Waals semimetallic Td-WTe2, exhibiting intriguing properties which include non-saturating extreme positive magnetoresistance (MR) and tunable chiral anomaly, has emerged as a model topological type-II Weyl semimetal system. Here, ∼45 nm thick mechanically exfoliated flakes of Td-WTe2 are studied via atomic force microscopy, Raman spectroscopy, low-T/high-μ0H magnetotransport measurements and optical reflectivity. The contribution of anisotropy of the Fermi liquid state to the origin of the large positive transverse MR⊥ and the signature of chiral anomaly of the type-II Weyl Fermions are reported. The samples are found to be stable in air and no oxidation or degradation of the electronic properties is observed. A transverse MR⊥∼1200 % and an average carrier mobility of 5000 cm2V-1s-1 at T=5K for an applied perpendicular field μ0H⊥=7T are established. The system follows a Fermi liquid model for T≤50K and the anisotropy of the Fermi surface is concluded to be at the origin of the observed positive MR. Optical reflectivity measurements confirm the anisotropy of the electronic behaviour. The relative orientation of the crystal axes and of the applied electric and magnetic fields is proven to determine the observed chiral anomaly in the in-plane magnetotransport. The observed chiral anomaly in the WTe2 flakes is found to persist up to T=120K, a temperature at least four times higher than the ones reported to date.

The Axial Anomaly in Lorentz Violating Theories: Towards the Electromagnetic Response of Weakly Tilted Weyl Semimetals

Using the path integral formulation in Euclidean space, we extended the calculation of the abelian chiral anomalies in the case of Lorentz violating theories by considering a new fermionic correction term provided by the standard model extension, which arises in the continuous Hamiltonian of a weakly tilted Weyl semimetal, and whose cones have opposite tilting. We found that this anomaly is insensitive to the tilting parameter, retaining its well-known covariant form. This independence on the Lorentz violating parameters is consistent with other findings reported in the literature. The initially imposed gauge invariant regularization was consistently recovered at the end of the calculation by the appearance of highly non-trivial combinations of the covariant derivatives, which ultimately managed to give only terms containing the electromagnetic tensor. We emphasize that the value of the anomaly with an arbitrary parameter is not automatically related to the effective action describing the electromagnetic response of such materials.

Defect induced ferromagnetic ordering and room temperature negative magnetoresistance in MoTeP

The magneto-transport, magnetization and theoretical electronic-structure have been investigated on type-II Weyl semimetallic MoTeP. The ferromagnetic ordering is observed in the studied sample and it has been shown that the observed magnetic ordering is due to the defect states. It has also been demonstrated that the presence of ferromagnetic ordering in effect suppresses the magnetoresistance (MR) significantly. Interestingly, a change-over from positive to negative MR is observed at higher temperature which has been attributed to the dominance of spin scattering suppression.

Longitudinal magnetoconductivity of tilted type-I Weyl semimetals from semiclassical to ultra-quantum regime

Weyl semimetals (WSMs) display many unusual magnetotransport phenomena. Here, based on the Landau quantization and Boltzmann equation, we theoretically study the longitudinal magnetoconductivity for tilted type-I WSMs from weak to strong magnetic field within a unified framework. It is found that, in semiclassical (weak magnetic field) regime, the conductivity has an angular dependentB-linear term besides theB-quadratic term common to isotropic nodes. In ultra-quantum (strong magnetic field) regime, the magnetoconductivity shows a linear dependence onB, and this dependence is affected by the tilt. In the intermediate regime, magnetoconductivity shows a tilt-modified quantum oscillation behavior due to the oscillation in the density of state. These findings recover the results for isotropic nodes without tilt, and suggest a possible way to identify the tilt axis of tilted WSMs through magnetotransport experiment.

Anomalous Hall transport in tilted multi-Weyl semimetals

We study the effect of a perpendicular magnetic fieldBon a multinode Weyl semimetal (mWSM) of arbitrary integer monopole chargen, with the two Weyl multinodes separated ink-space. Besides type-I mWSMs, there exist type-II mWSMs which are characterized by the tilted minimal dispersion for low-energy excitations; the Weyl points in type-II mWSMs are still protected crossings but appear at the contact of the electron and hole pockets, after the Lifshitz transition. We find that the presence of a perpendicular magnetic field quantizes the occupation pockets due to the presence of Fermi tubes. In this theory, the Hilbert space is spanned by a set ofnchiral degenerate ground states, and a countably infinite number of particle-hole symmetric Landau levels (LLs). We calculate the Hall conductivity for the tilt-symmetric case of type-I mWSM using the Kubo formula, in the zero-frequency (DC) limit, and recover the well-known vacuum contribution. We compute the Fermi surface corrections and show that the expression generalizes from the formula for elementary (n= 1) type-I WSMs. We derive an expression for the type-II mWSM Hall conductivity, which is bounded by a LL cutoff introduced on physical grounds. Interestingly, we find that the anomalous vacuum Hall conductivity is vanishing in the type-II phase at all temperatures. The corresponding thermal Hall and Nernst conductivities are evaluated and characterized for both phases. The qualitative and quantitative observations presented here may serve in the characterization of generic mWSMs of both types.

A combined laser-based angle-resolved photoemission spectroscopy and two-photon photoemission spectroscopy study ofTd-WTe2

Laser-based angle-resolved photoemission spectroscopy and two-photon photoemission spectroscopy are employed to study the valence electronic structure of the Weyl semimetal candidateTd-WTe2along two high symmetry directions and for binding energies between ≈ -1 eV and 5 eV. The experimental data show a good agreement with band structure calculations. Polarization dependent measurements provide further information on initial and intermediate state symmetry properties with respect to the mirror plane of theTdstructure of WTe2.

Mode-resolved reciprocal space mapping of electron-phonon interaction in the Weyl semimetal candidate Td-WTe2

The excitation of coherent phonons provides unique capabilities to control fundamental properties of quantum materials on ultrafast time scales. Recently, it was predicted that a topologically protected Weyl semimetal phase in the transition metal dichalcogenide Td-WTe₂ can be controlled and, ultimately, be destroyed upon the coherent excitation of an interlayer shear mode. By monitoring electronic structure changes with femtosecond resolution, we provide here direct experimental evidence that the shear mode acts on the electronic states near the phase-defining Weyl points. Furthermore, we observe a periodic reduction in the spin splitting of bands, a distinct electronic signature of the Weyl phase-stabilizing non-centrosymmetric Td ground state of WTe₂. The comparison with higher-frequency coherent phonon modes finally proves the shear mode-selectivity of the observed changes in the electronic structure. Our real-time observations reveal direct experimental insights into electronic processes that are of vital importance for a coherent phonon-induced topological phase transition in Td-WTe₂.

It is predicted that topological phase transitions in quantum materials can be triggered by selective excitation of coherent phonons. Upon excitation of a shear mode, Hein et al. observe distinct perturbations of electronic Weyl semimetal fingerprints in Td-WTe₂.

Ideal Type-II Weyl Phase and Topological Transition in Phononic Crystals

Ideal Weyl points, which are related by symmetry and thus reside at the same frequency, could offer further insight into the Weyl physics. The ideal type-I Weyl points have been observed in photonic crystals, but the ideal type-II Weyl points with tilted conelike band dispersions are still not realized. Here we present the observation of the ideal type-II Weyl points of the minimal number in three-dimensional phononic crystals and, in the meantime, the topological phase transition from the Weyl semimetal to the valley insulators of two distinct types. The Fermi-arc surface states are shown to exist on the surfaces of the Weyl phase, and the Fermi-circle surface states are also observed, but on the interface of the two distinct valley phases. Intriguing wave partition of the Fermi-circle surface states is demonstrated.

Planar Hall effect induced by anisotropic orbital magnetoresistance in type-II Dirac semimetal PdTe2

We measure planar Hall effect (PHE) and longitudinal anisotropic magnetoresistance (AMR) with a magnetic field rotating in the a-b plane in the type-II Dirac semimetal PdTe₂. The measured PHE and AMR curves can be fitted by the theoretical equations; however, a detailed analysis of the extracted data demonstrates that the parameter related to PHE and AMR has no relationship with the chiral anomaly due to the absence of negative longitudinal magnetoresistance (MR) when the electric and magnetic fields are parallel to each other. Meanwhile, we prove that the origin of PHE in PdTe₂ is the anisotropic orbital MR. Our work suggests that negative longitudinal MR is necessary to identify chiral anomaly, and we cannot in general use PHE as a signal for the presence of the chiral anomaly in Dirac/Weyl semimetals.

Strong and Tunable Electrical Anisotropy in Type-II Weyl Semimetal Candidate WP2 with Broken Inversion Symmetry

A transition metal diphosphide, WP₂ , is a candidate for type-II Weyl semimetals (WSMs) in which spatial inversion symmetry is broken and Lorentz invariance is violated. As one of the prerequisites for the presence of the WSM state in WP₂ , spatial inversion symmetry breaking in this compound has rarely been investigated. Furthermore, the anisotropy of the WP₂ electrical properties and whether its electrical anisotropy can be tuned remain elusive. Angle-resolved polarized Raman spectroscopy, electrical transport, optical spectroscopy, and first-principle studies of WP₂ are reported. The energies of the observed Raman-active phonons and the angle dependences of the detected phonon intensities are consistent with results obtained by first-principle calculations and analysis of the proposed crystal symmetry without spatial inversion, showing that spatial inversion symmetry is broken in WP₂ . Moreover, the measured ratio (R_c /R_a ) between the crystalline c-axis and a-axis electrical resistivities exhibits a weak dependence on temperature (T) in the temperature range from 100 to 250 K, but increases abruptly at T ≤ 100 K, and then reaches the value of ≈8.0 at T = 10 K, which is by far the strongest in-plane electrical resistivity anisotropy among the reported type-II WSM candidates with comparable carrier concentrations. Optical spectroscopy study, together with the first-principle calculations on the electronic band structure, reveals that the abrupt enhancement of the electrical resistivity anisotropy at T ≤ 100 K mainly arises from a sharp increase in the scattering rate anisotropy at low temperatures. More interestingly, the R_c /R_a of WP₂ at T = 10 K can be tuned from 8.0 to 10.6 as the magnetic field increases from 0 to 9 T. The so-far-strongest and magnetic-field-tunable electrical resistivity anisotropy found in WP₂ can serve as a degree of freedom for tuning the electrical properties of type-II WSMs, which paves the way for the development of novel electronic applications based on type-II WSMs.

Direct Evidence for Charge Compensation-Induced Large Magnetoresistance in Thin WTe2

Since the discovery of extremely large nonsaturating magnetoresistance (MR) in WTe₂, much effort has been devoted to understanding the underlying mechanism, which is still under debate. Here, we explicitly identify the dominant physical origin of the large nonsaturating MR through in situ tuning of the magneto-transport properties in thin WTe₂ film. With an electrostatic doping approach, we observed a nonmonotonic gate dependence of the MR. The MR reaches a maximum (10600%) in thin WTe₂ film at certain gate voltage where electron and hole concentrations are balanced, indicating that the charge compensation is the dominant mechanism of the observed large MR. Besides, we show that the temperature-dependent magnetoresistance exhibits similar tendency with the carrier mobility when the charge compensation is retained, revealing that distinct scattering mechanisms may be at play for the temperature dependence of magneto-transport properties. Our work would be helpful for understanding mechanism of the large MR in other nonmagnetic materials and offers an avenue for achieving large MR in the nonmagnetic materials with electron-hole pockets.

Magnetotransport Properties of Layered Topological Material ZrTe2 Thin Film

ZrTe₂ is a candidate topological material from the layered two-dimensional transition-metal dichalcogenide family, and thus the material may show exotic electrical transport properties and may be promising for quantum device applications. In this work, we report the successful growth of layered ZrTe₂ thin film by pulsed-laser deposition and the experimental results of its magnetotransport properties. In the presence of a perpendicular magnetic field, the 60 nm thick ZrTe₂ film shows a large magnetoresistance of 3000% at 2 K and 9 T. A robust linear magnetoresistance is observed under an in-plane magnetic field, and negative magnetoresistance appears in the film when the magnetic field is parallel to the current direction. Furthermore, the Hall results reveal that the ZrTe₂ thin film has a high electron mobility of about 1.8 × 10⁴ cm² V^-1 s^-1 at 2 K. These findings provide insights into further investigations and potential applications of this layered topological material system.

Anomalous Photothermoelectric Transport Due to Anisotropic Energy Dispersion in WTe2

Band structures are vital in determining the electronic properties of materials. Recently, the two-dimensional (2D) semimetallic transition metal tellurides (WTe₂ and MoTe₂) have sparked broad research interest because of their elliptical or open Fermi surface, making distinct from the conventional 2D materials. In this study, we demonstrate a centrosymmetric photothermoelectric voltage distribution in WTe₂ nanoflakes, which has not been observed in common 2D materials such as graphene and MoS₂. Our theoretical model shows the anomalous photothermoelectric effect arises from an anisotropic energy dispersion and micrometer-scale hot carrier diffusion length of WTe₂. Further, our results are more consistent with the anisotropic tilt direction of energy dispersion being aligned to the b-axis rather than the a-axis of the WTe₂ crystal, which is consistent with the previous first-principle calculations as well as magneto-transport experiments. Our work shows the photothermoelectric current is strongly confined to the anisotropic direction of the energy dispersion in WTe₂, which opens an avenue for interesting electro-optic applications such as electron beam collimation and electron lenses.

Nonlinear magnetotransport shaped by Fermi surface topology and convexity

The nature of Fermi surface defines the physical properties of conductors and many physical phenomena can be traced to its shape. Although the recent discovery of a current-dependent nonlinear magnetoresistance in spin-polarized non-magnetic materials has attracted considerable attention in spintronics, correlations between this phenomenon and the underlying fermiology remain unexplored. Here, we report the observation of nonlinear magnetoresistance at room temperature in a semimetal WTe₂, with an interesting temperature-driven inversion. Theoretical calculations reproduce the nonlinear transport measurements and allow us to attribute the inversion to temperature-induced changes in Fermi surface convexity. We also report a large anisotropy of nonlinear magnetoresistance in WTe₂, due to its low symmetry of Fermi surfaces. The good agreement between experiments and theoretical modeling reveals the critical role of Fermi surface topology and convexity on the nonlinear magneto-response. These results lay a new path to explore ramifications of distinct fermiology for nonlinear transport in condensed-matter.

Quantum Oscillations of the Positive Longitudinal Magnetoconductivity: A Fingerprint for Identifying Weyl Semimetals

Weyl semimetals (WSMs) host charged Weyl fermions as emergent quasiparticles. We develop a unified analytical theory for the anomalous positive longitudinal magnetoconductivity (LMC) in a WSM, which bridges the gap between the classical and ultraquantum approaches. More interestingly, the LMC is found to exhibit periodic-in-1/B quantum oscillations, originating from the oscillations of the nonequilibrium chiral chemical potential. The quantum oscillations, superposed on the positive LMC, are a remarkable fingerprint of a WSM phase with a chiral anomaly, whose observation is a valid criteria for identifying a WSM material. In fact, such quantum oscillations were already observed by several experiments.

Negative longitudinal magnetoresistance in gallium arsenide quantum wells

Negative longitudinal magnetoresistances (NLMRs) have been recently observed in a variety of topological materials and often considered to be associated with Weyl fermions that have a defined chirality. Here we report NLMRs in non-Weyl GaAs quantum wells. In the absence of a magnetic field the quantum wells show a transition from semiconducting-like to metallic behaviour with decreasing temperature. We observe pronounced NLMRs up to 9 Tesla at temperatures above the transition and weak NLMRs in low magnetic fields at temperatures close to the transition and below 5 K. The observed NLMRs show various types of magnetic field behaviour resembling those reported in topological materials. We attribute them to microscopic disorder and use a phenomenological three-resistor model to account for their various features. Our results showcase a contribution of microscopic disorder in the occurrence of unusual phenomena. They may stimulate further work on tuning electronic properties via disorder/defect nano-engineering.

The attribution of negative longitudinal magnetoresistance (NLMR) in Weyl metals to a chiral anomaly is already challenged. Here, NLMR resembling that of Weyl metals is demonstrated in a non-Weyl-metal GaAs quantum well originating from different types of disorder.

An ultrafast symmetry switch in a Weyl semimetal

Topological quantum materials exhibit fascinating properties^1-3, with important applications for dissipationless electronics and fault-tolerant quantum computers^4,5. Manipulating the topological invariants in these materials would allow the development of topological switching applications analogous to switching of transistors⁶. Lattice strain provides the most natural means of tuning these topological invariants because it directly modifies the electron-ion interactions and potentially alters the underlying crystalline symmetry on which the topological properties depend^7-9. However, conventional means of applying strain through heteroepitaxial lattice mismatch¹⁰ and dislocations¹¹ are not extendable to controllable time-varying protocols, which are required in transistors. Integration into a functional device requires the ability to go beyond the robust, topologically protected properties of materials and to manipulate the topology at high speeds. Here we use crystallographic measurements by relativistic electron diffraction to demonstrate that terahertz light pulses can be used to induce terahertz-frequency interlayer shear strain with large strain amplitude in the Weyl semimetal WTe₂, leading to a topologically distinct metastable phase. Separate nonlinear optical measurements indicate that this transition is associated with a symmetry change to a centrosymmetric, topologically trivial phase. We further show that such shear strain provides an ultrafast, energy-efficient way of inducing robust, well separated Weyl points or of annihilating all Weyl points of opposite chirality. This work demonstrates possibilities for ultrafast manipulation of the topological properties of solids and for the development of a topological switch operating at terahertz frequencies.

Superconductivity in Potassium-Intercalated T d-WTe2

To realize a topological superconductor is one of the most attracting topics because of its great potential in quantum computation. In this study, we successfully intercalate potassium (K) into the van der Waals gap of type II Weyl semimetal WTe₂ and discover the superconducting state in K _xWTe₂ through both electrical transport and scanning tunneling spectroscopy measurements. The superconductivity exhibits an evident anisotropic behavior. Moreover, we also uncover the coexistence of superconductivity and the positive magnetoresistance state. Structural analysis substantiates the negligible lattice expansion induced by the intercalation, therefore suggesting K-intercalated WTe₂ still hosts the topological nontrivial state. These results indicate that the K-intercalated WTe₂ may be a promising candidate to explore the topological superconductor.

Spin-momentum locking and spin-orbit torques in magnetic nano-heterojunctions composed of Weyl semimetal WTe2

Spin–orbit torque has recently been intensively investigated for the purposes of manipulating the magnetization in magnetic nano-devices and understanding fundamental physics. Therefore, the search for novel materials or material combinations that exhibit a strong enough spin-torque effect has become one of the top priorities in this field of spintronics. Weyl semimetal, a new topological material that features open Fermi arc with strong spin–orbit coupling and spin–momentum locking effect, is naturally expected to exhibit an enhanced spin-torque effect in magnetic nano-devices. Here we observe a significantly enhanced spin conductivity, which is associated with the field-like torque at low temperatures. The enhancement is obtained in the b-axis WTe₂/Py bilayers of nano-devices but not observed in the a-axis of WTe₂/Py nano-devices, which can be ascribed to the enhanced spin accumulation by the spin–momentum locking effect of the Fermi arcs of the Weyl semimetal WTe₂.

The Fermi arcs, topological surface states of Weyl semimetals can enable the intriguing spin control and facilitate topological spintronics. Here the authors report the spin-orbit torque at the interface of WTe₂/Py and attribute it to the enhanced spin accumulation by the spin-momentum locking effect of the Fermi arcs of WTe₂.

Oxidation-Induced Topological Phase Transition in Monolayer 1T'-WTe2

Monolayer (ML) tungsten ditelluride (WTe₂) is a well-known quantum spin Hall (QSH) insulator with topologically protected gapless edge states, thus promising dissipationless electronic devices. However, experimental findings exhibit the fast oxidation of ML WTe₂ in ambient conditions. To reveal the changes of topological properties of WTe₂ arising from oxidation, we systematically study the surface oxidation reaction of ML 1T'-WTe₂ using first-principles calculations. The calculated results indicate that the fast oxidation of WTe₂ originates from the existence of H₂O in air, which significantly promotes the oxidation of ML 1T'-WTe₂. More importantly, this low-coverage oxidized WTe₂ loses its topological features and is changed into a trivial insulator. Furthermore, we propose a fully oxidized ML WTe₂ that can still possess the QSH insulator states. The topological phase transition induced by oxidation provides exotic insight into understanding the topological features of layered transition-metal dichalcogenide materials.

Galvanomagnetic properties of the putative type-II Dirac semimetal PtTe2

Platinum ditelluride has recently been characterized, based on angle-resolved photoemission spectroscopy data and electronic band structure calculations, as a possible representative of type-II Dirac semimetals. Here, we report on the magnetotransport behavior (electrical resistivity, Hall effect) in this compound, investigated on high-quality single-crystalline specimens. The magnetoresistance (MR) of PtTe₂ is large (over 3000% at T = 1.8 K in B = 9 T) and unsaturated in strong fields in the entire temperature range studied. The MR isotherms obey a Kohler’s type scaling with the exponent m = 1.69, different from the case of ideal electron-hole compensation. In applied magnetic fields, the resistivity shows a low-temperature plateau, characteristic of topological semimetals. In strong fields, well-resolved Shubnikov – de Haas (SdH) oscillations with two principle frequencies were found, and their analysis yielded charge mobilities of the order of 10³ cm² V⁻¹ s⁻¹ and rather small effective masses of charge carriers, 0.11 m_e and 0.21 m_e. However, the extracted Berry phases point to trivial character of the electronic bands involved in the SdH oscillations. The Hall effect data corroborated a multi-band character of the electrical conductivity in PtTe₂, with moderate charge compensation.

Inversion symmetry breaking induced triply degenerate points in orderly arranged PtSeTe family materials

k paths exactly with [Formula: see text] symmetry allow to find triply degenerate points (TDPs) in band structures. The paths that host the type-II Dirac points in PtSe₂ family materials also have the [Formula: see text] spatial symmetry. However, due to Kramers degeneracy (the systems have both inversion symmetry and time reversal symmetry), the crossing points in them are Dirac ones. In this work, based on symmetry analysis, first-principles calculations, and [Formula: see text] method, we predict that PtSe₂ family materials should undergo topological transitions if the inversion symmetry is broken, i.e. the Dirac fermions in PtSe₂ family materials split into TDPs in PtSeTe family materials (PtSSe, PtSeTe, and PdSeTe) with orderly arranged S/Se (Se/Te). It is different from the case in high-energy physics that breaking inversion symmetry I leads to the splitting of Dirac fermion into Weyl fermions. We also address a possible method to achieve the orderly arranged in PtSeTe family materials in experiments. Our study provides a real example that Dirac points transform into TDPs, and is helpful to investigate the topological transition between Dirac fermions and TDP fermions.

Evidence for a Dirac nodal-line semimetal in SrAs3

Dirac nodal-line semimetals with the linear bands crossing along a line or loop, represent a new topological state of matter. Here, by carrying out magnetotransport measurements and performing first-principle calculations, we demonstrate that such a state has been realized in high-quality single crystals of SrAs₃. We obtain the nontrivial π Berry phase by analysing the Shubnikov-de Haas quantum oscillations. We also observe a robust negative longitudinal magnetoresistance induced by the chiral anomaly. Accompanying first-principles calculations identifies that a single hole pocket enclosing the loop nodes is responsible for these observations.

Tuning the electrical transport of type II Weyl semimetal WTe2 nanodevices by Mo doping

We fabricated nanodevices from MoxW1-xTe2 (x = 0, 0.07, 0.35), and conducted a systematic comparative study of their electrical transport. Magnetoresistance measurements show that Mo doping can significantly suppress mobility and magnetoresistance. The results for the analysis of the two band model show that doping with Mo does not break the carrier balance. Through analysis of Shubnikov-de Haas oscillations, we found that Mo doping also has a strong suppressive effect on the quantum oscillation of the sample, and the higher the ratio of Mo, the fewer pockets were observed in our experiments. Furthermore, the effective mass of electron and hole increases gradually with increasing Mo ratio, while the corresponding quantum mobility decreases rapidly.

Evidence for topological type-II Weyl semimetal WTe2

Recently, a type-II Weyl fermion was theoretically predicted to appear at the contact of electron and hole Fermi surface pockets. A distinguishing feature of the surfaces of type-II Weyl semimetals is the existence of topological surface states, so-called Fermi arcs. Although WTe₂ was the first material suggested as a type-II Weyl semimetal, the direct observation of its tilting Weyl cone and Fermi arc has not yet been successful. Here, we show strong evidence that WTe₂ is a type-II Weyl semimetal by observing two unique transport properties simultaneously in one WTe₂ nanoribbon. The negative magnetoresistance induced by a chiral anomaly is quite anisotropic in WTe₂ nanoribbons, which is present in b-axis ribbon, but is absent in a-axis ribbon. An extra-quantum oscillation, arising from a Weyl orbit formed by the Fermi arc and bulk Landau levels, displays a two dimensional feature and decays as the thickness increases in WTe₂ nanoribbon.

Exotic transport properties of type-II Weyl semimetals have been predicted but are yet to be experimentally evidenced. Here, Li et al. report evidences of an anisotropy of negative magnetoresistance and a quantum oscillation arising from the predicted Weyl orbit in the type-II Weyl semimetal WTe₂.

Tuning the electrical transport of type II Weyl semimetal WTe2 nanodevices by Ga+ ion implantation

Here we introduce lattice defects in WTe₂ by Ga+ implantation (GI), and study the effects of defects on the transport properties and electronic structures of the samples. Theoretical calculation shows that Te Frenkel defects is the dominant defect type, and Raman characterization results agree with this. Electrical transport measurements show that, after GI, significant changes are observed in magnetoresistance and Hall resistance. The classical two-band model analysis shows that both electron and hole concentration are significantly reduced. According to the calculated results, ion implantation leads to significant changes in the band structure and the Fermi surface of the WTe₂. Our results indicate that defect engineering is an effective route of controlling the electronic properties of WTe₂ devices.

Composition and temperature-dependent phase transition in miscible Mo1-xWxTe2 single crystals

Transition metal dichalcogenides (TMDs) WTe2 and MoTe2 with orthorhombic Td phase, being potential candidates as type-II Weyl semimetals, are attracted much attention recently. Here we synthesized a series of miscible Mo1-xWxTe2 single crystals by bromine vapor transport method. Composition-dependent X-ray diffraction and Raman spectroscopy, as well as composition and temperature-dependent resistivity prove that the tunable crystal structure (from hexagonal (2H), monoclinic (β) to orthorhombic (Td) phase) can be realized by increasing W content in Mo1-xWxTe2. Simultaneously the electrical property gradually evolves from semiconductor to semimetal behavior. Temperature-dependent Raman spectroscopy proves that temperature also can induce the structural phase transition from β to Td phase in Mo1-xWxTe2 crystals. Based on aforementioned characterizations, we map out the temperature and composition dependent phase diagram of Mo1-xWxTe2 system. In addition, a series of electrical parameters, such as carrier type, carrier concentration and mobility, have also been presented. This work offers a scheme to accurately control structural phase in Mo1-xWxTe2 system, which can be used to explore type-II Weyl semimetal, as well as temperature/composition controlled topological phase transition therein.