Signature of Strong Spin-Orbital Coupling in the Large Nonsaturating Magnetoresistance Material WTe2

Polarization dependent light propagation in [Formula: see text] multilayer structure

[Formula: see text] is one of the exciting and outstanding semimetallic members of TMDCs, which has attracted immense attention for manipulating light propagation due to its inherent optical anisotropy and hyperbolic characteristic in the infrared frequency range. We investigate the dependence of the reflectance and transmittance of structures with a single and double [Formula: see text] thin film in terms of frequency and polarization angle of the incident wave. We find rich behaviors in the optical response of these structures due to their anisotropic permittivity tensors. Furthermore, we analyze the polarization state of transmitted and reflected waves through these structures. We demonstrate that these structures provide the ability to achieve desired polarization rotation for outgoing waves by tuning the frequency and polarization angle of the incident wave with respect to the principal axes of [Formula: see text] thin film. In particular, we elucidate the essential relevance of the optical response and polarization rotation of the double thin film structure to the in-plain twist angle of [Formula: see text] thin films. We explain that this structure permits comprehensive control of the polarization rotation of the outgoing waves by adjusting the twist angle of thin films. The proposed structure can be employed as an efficient light manipulator with the aim of application in communication, imaging, and information processing.

Sizable spin-to-charge conversion in PLD-grown amorphous (Mo, W)Te2-xfilms

We report on the spin-to-charge conversion (SCC) in Mo_0.25W_0.75Te_2-x(MWT)/Y₃Fe₅O₁₂(YIG) heterostructures at room temperature. The centimeter-scale amorphous MWT films are deposited on liquid-phase-epitaxial YIG by pulsed laser deposition technique. The significant SCC voltage is measured in the MWT layer with a sizable spin Hall angle of ∼0.021 by spin pumping experiments. The control experiments by inserting MgO or Ag layer between MWT and YIG show that the SCC is mainly attributed to the inverse spin Hall effect rather than the thermal or interfacial Rashba effect. Our work provides a novel spin-source material for energy-efficient topological spintronic devices.

A two-dimensional topological nodal-line material MgN4 with extremely large magnetoresistance

Using first-principles calculations, we predict a stable two-dimensional atomically thin material MgN₄. This material has a perfect intrinsic electron-hole compensation characteristic with high carrier mobility, making it a promising candidate material with extremely large magnetoresistance. As the magnetic field increases, the magnetoresistance of the monolayer MgN₄ will show a quadratic dependence on the strength of the magnetic field without saturation. Furthermore, nontrivial topological properties are also found in this material. In the absence of spin-orbit coupling, the monolayer MgN₄ belongs to a topological nodal-line material, in which the band crossings form a closed saddle-shape nodal-ring near the Fermi level in the Brillouin zone. Once the spin-orbit coupling is considered, a small local energy gap is opened along the nodal ring, resulting in a topological insulator defined on a curved Fermi surface with ₂ = 1. The combination of two-dimensional single-atomic-layer thickness, an extremely large magnetoresistance effect, and topological non-trivial properties in the monolayer MgN₄ makes it an excellent platform for designing novel multi-functional devices.

Phase transitions and suppression of magnetoresistance in WTe2-xSexsystem

X-ray diffraction, Raman spectroscopy, and electrical resistivity measurements on polycrystalline WTe2-xSe_x(0 ⩽ x ⩽ 0.8) reveal aT_d-1T'structural phase transition and suppression of magnetoresistance atx = 0.2. These phenomena are consistent with the pressure phase diagram of WTe₂. However, chemical pressure due to substitution of smaller Se ion cannot generate pressure required for the phase transition. Strain induced by sample inhomogeneity is believed to be a trigger to the behaviors. In agreement with previous predictions and reports, a mixed phase of1T'and 2Hstructures was also detected in Se-rich samples. Coincidentally atx = 0.2, electrical resistivity analysis suggests a phase transition from a metallic phase to a nonmetallic phase that is possibly a topological-insulating phase.

Thickness and defect dependent electronic, optical and thermoelectric features of [Formula: see text]

Transition metal dichalcogenides (TMDs) receive significant attention due to their outstanding electronic and optical properties. In this study, we investigate the electronic, optical, and thermoelectric properties of single and few layer [Formula: see text] in detail utilizing first-principles methods based on the density functional theory (DFT). Within the scope of both PBE and HSE06 including spin orbit coupling (SOC), the simulations predict the electronic band gap values to decrease as the number of layers increases. Moreover, spin-polarized DFT calculations combined with the semi-classical Boltzmann transport theory are applied to estimate the anisotropic thermoelectric power factor (Seebeck coefficient, S) for [Formula: see text] in both the monolayer and multilayer limit, and S is obtained below the optimal value for practical applications. The optical absorbance of [Formula: see text] monolayer is obtained to be slightly less than the values reported in literature for 2H TMD monolayers of [Formula: see text], [Formula: see text], and [Formula: see text]. Furthermore, we simulate the impact of defects, such as vacancy, antisite and substitution defects, on the electronic, optical and thermoelectric properties of monolayer [Formula: see text]. Particularly, the Te-[Formula: see text] substitution defect in parallel orientation yields negative formation energy, indicating that the relevant defect may form spontaneously under relevant experimental conditions. We reveal that the electronic band structure of [Formula: see text] monolayer is significantly influenced by the presence of the considered defects. According to the calculated band gap values, a lowering of the conduction band minimum gives rise to metallic characteristics to the structure for the single Te(1) vacancy, a diagonal Te line defect, and the Te(1)-[Formula: see text] substitution, while the other investigated defects cause an opening of a small positive band gap at the Fermi level. Consequently, the real ([Formula: see text]) and imaginary ([Formula: see text]) parts of the dielectric constant at low frequencies are very sensitive to the applied defects, whereas we find that the absorbance (A) at optical frequencies is less significantly affected. We also predict that certain point defects can enhance the otherwise moderate value of S in pristine [Formula: see text] to values relevant for thermoelectric applications. The described [Formula: see text] monolayers, as functionalized with the considered defects, offer the possibility to be applied in optical, electronic, and thermoelectric devices.

Pressure-Induced Dimensional Crossover in a Kagome Superconductor

The recently discovered kagome superconductors AV_{3}Sb_{5} exhibit tantalizing high-pressure phase diagrams, in which a new domelike superconducting phase emerges under moderate pressure. However, its origin is as yet unknown. Here, we carried out the high-pressure electrical measurements up to 150 GPa, together with the high-pressure x-ray diffraction measurements and first-principles calculations on CsV_{3}Sb_{5}. We find the new superconducting phase to be rather robust and inherently linked to the interlayer Sb2-Sb2 interactions. The formation of Sb2-Sb2 bonds at high pressure tunes the system from two-dimensional to three-dimensional and pushes the p_{z} orbital of Sb2 upward across the Fermi level, resulting in enhanced density of states and increase of T_{C}. Our work demonstrates that the dimensional crossover at high pressure can induce a topological phase transition and is related to the abnormal high-pressure T_{C} evolution. Our findings should apply for other layered materials.

Two-dimensional ferroelasticity and ferroelastic strain controllable anisotropic transport properties in CuTe monolayer

Two-dimensional ferroelastic (2D-FE) materials where FE strain originates from the lattice deformation associated with spontaneous FE phase transition, hold great promise as miniaturized shape-memory devices. Moreover, the structural anisotropy within the low-symmetry 2D-FE materials can usually lead to intrinsic anisotropy in their electronic or transport properties as well. As a result, the strong coupling of FE strain with the anisotropic electronic structure or electric-/thermoelectric-transport will largely extend the functionality and device applications for 2D-FE materials. In the current work, after performing comprehensive first-principles calculations in combination with transport simulations based on the Boltzmann formalism, we identify the experimentally synthesizable CuTe monolayer as a new 2D-FE material whose anisotropic electric- and thermoelectric-transport properties can be effectively manipulated by FE strain. Typically, CuTe monolayers that can be potentially exfoliated from the synthesized van der Waals (vdW) layered CuTe bulk are predicted to exhibit the room temperature stable ferroelasticity and large axial FE strain (up to 18.4%) created by the in-plane orthorhombic lattice deformation. Owing to the planar orientation dependent metallic vs. nearly semiconducting electronic structure, highly anisotropic electric conductivity and thermopower coefficient can be obtained along the two planar principal axes of the CuTe monolayer. To simulate the more realistic experimental scenarios, coherent formation of FE domain walls and domain-wall motion assisted FE switching have also been evaluated in CuTe multi-domain configurations. Based on the transverse thermoelectric effect inherent in anisotropic CuTe monolayers, the schematic model for obtaining the FE strain controllable electric current within CuTe multi-domain configurations has been proposed, which can be verified experimentally.

Spin-Orbit Torque in Van der Waals-Layered Materials and Heterostructures

Spin-orbit torque (SOT) opens an efficient and versatile avenue for the electrical manipulation of magnetization in spintronic devices. The enhancement of SOT efficiency and reduction of power consumption are key points for the implementation of high-performance SOT devices, which strongly rely on the spin-orbit coupling (SOC) strength and magnetic properties of ferromagnetic/non-magnetic heterostructures. Recently, van der Waals-layered materials have shown appealing properties for use in efficient SOT applications. On the one hand, transition-metal dichalcogenides, topological insulators, and graphene-based heterostructures possess appreciable SOC strength. This feature can efficiently converse the charge current into spin current and result in large SOT. On the other hand, the newly discovered layered magnetic materials provide ultra-thin and gate-tunable ferromagnetic candidates for high-performance SOT devices. In this review, the latest advancements of SOT research in various layered materials are summarized. First, a brief introduction of SOT is given. Second, SOT studies of various layered materials and heterostructures are summarized. Subsequently, progresses on SOT-induced magnetization switching are presented. Finally, current challenges and prospects for future development are suggested.

Possible Routes to Obtain Enhanced Magnetoresistance in a Driven Quantum Heterostructure with a Quasi-Periodic Spacer

In this work, we perform a numerical study of magnetoresistance in a one-dimensional quantum heterostructure, where the change in electrical resistance is measured between parallel and antiparallel configurations of magnetic layers. This layered structure also incorporates a non-magnetic spacer, subjected to quasi-periodic potentials, which is centrally clamped between two ferromagnetic layers. The efficiency of the magnetoresistance is further tuned by injecting unpolarized light on top of the two sided magnetic layers. Modulating the characteristic properties of different layers, the value of magnetoresistance can be enhanced significantly. The site energies of the spacer is modified through the well-known Aubry-André and Harper (AAH) potential, and the hopping parameter of magnetic layers is renormalized due to light irradiation. We describe the Hamiltonian of the layered structure within a tight-binding (TB) framework and investigate the transport properties through this nanojunction following Green's function formalism. The Floquet-Bloch (FB) anstaz within the minimal coupling scheme is introduced to incorporate the effect of light irradiation in TB Hamiltonian. Several interesting features of magnetotransport properties are represented considering the interplay between cosine modulated site energies of the central region and the hopping integral of the magnetic regions that are subjected to light irradiation. Finally, the effect of temperature on magnetoresistance is also investigated to make the model more realistic and suitable for device designing. Our analysis is purely a numerical one, and it leads to some fundamental prescriptions of obtaining enhanced magnetoresistance in multilayered systems.

Studying local Berry curvature in 2H-WSe2 by circular dichroism photoemission utilizing crystal mirror plane

It was recently reported that circular dichroism in angle-resolved photoemission spectroscopy (CD-ARPES) can be used to observe the Berry curvature in 2H-WSe₂ (Cho et al. in Phys Rev Lett 121:186401, 2018). In that study, the mirror plane of the experiment was intentionally set to be perpendicular to the crystal mirror plane, such that the Berry curvature becomes a symmetric function about the experimental mirror plane. In the present study, we performed CD-ARPES on 2H-WSe₂ with the crystal mirror plane taken as the experimental mirror plane. Within such an experimental constraint, two experimental geometries are possible for CD-ARPES. The Berry curvature distributions for the two geometries are expected to be antisymmetric about the experimental mirror plane and exactly opposite to each other. Our experimental CD intensities taken with the two geometries were found to be almost opposite near the corners of the 2D projected hexagonal Brillouin zone (BZ) and were almost identical near the center of the BZ. This observation is well explained by taking the Berry curvature or the atomic orbital angular momentum (OAM) into account. The Berry curvature (or OAM) contribution to the CD intensities can be successfully extracted through a comparison of the CD-ARPES data for the two experimental geometries. Thus, the CD-ARPES experimental procedure described provides a method for mapping Berry curvature in the momentum space of topological materials, such as Weyl semimetals.

Synthesis of Semiconducting 2H-Phase WTe2 Nanosheets with Large Positive Magnetoresistance

Tungsten ditelluride (WTe₂) is provoking immense interest because of its unique electronic properties, but studies about its semiconducting hexagonal (2H) phase are quite rare. Herein, we report the synthesis of semiconducting 2H WTe₂ nanosheets with large positive magnetoresistance, for the first time, by a simple lithium-intercalation-assisted exfoliation strategy. Systematic characterizations including high-resolution transmission electron microscopy, X-ray diffraction, and Raman and X-ray photoelectron spectroscopies provide clear evidence to distinguish the structure of 2H WTe₂ nanosheets from the orthorhombic (Td) phase bulk counterpart. The corresponding electronic phase transition from metal to semiconductor is also confirmed by density of states calculation, optical absorption, and electrical transport property measurements. Besides, the 2H WTe₂ nanosheets exhibit large positive magnetoresistance with values of up to 29.5% (10 K) and 16.2% (300 K) at 9 T. Overall, these findings open up a promising avenue into the exploration of WTe₂-based materials in the semiconductor field.

Giant, unconventional anomalous Hall effect in the metallic frustrated magnet candidate, KV3Sb5

The anomalous Hall effect (AHE) is one of the most fundamental phenomena in physics. In the highly conductive regime, ferromagnetic metals have been the focus of past research. Here, we report a giant extrinsic AHE in KV3Sb5, an exfoliable, highly conductive semimetal with Dirac quasiparticles and a vanadium Kagome net. Even without report of long range magnetic order, the anomalous Hall conductivity reaches 15,507 Ω-1 cm-1 with an anomalous Hall ratio of ≈ 1.8%; an order of magnitude larger than Fe. Defying theoretical expectations, KV3Sb5 shows enhanced skew scattering that scales quadratically, not linearly, with the longitudinal conductivity, possibly arising from the combination of highly conductive Dirac quasiparticles with a frustrated magnetic sublattice. This allows the possibility of reaching an anomalous Hall angle of 90° in metals. This observation raises fundamental questions about AHEs and opens new frontiers for AHE and spin Hall effect exploration, particularly in metallic frustrated magnets.

Signatures of non-trivial band topology in LaAs/LaBi heterostructure

In this article, we investigate non-trivial topological features in a heterostructure of extreme magnetoresistance (XMR) materials LaAs and LaBi using density functional theory. The proposed heterostructure is found to be dynamically stable and shows bulk band inversion with non-trivial Z2topological invariant and a Dirac cone at the surface. In addition, its electron and hole carrier densities ratio is also calculated to investigate the possibility to possess XMR effect. Electrons and holes in the heterostructure are found to be nearly compensated, thereby facilitating it to be a suitable candidate for XMR studies.

Anisotropic Picosecond Spin-Photocurrent from Weyl Semimetal WTe2

The generation and detection of ultrafast spin current, preferably reaching a frequency up to terahertz, is the core of spintronics. Studies have shown that the Weyl semimetal WTe₂ is of great potential in generating spin currents. However, the prior studies have been limited to the static measurements with the in-plane spin orientation. In this work, we demonstrate a picosecond spin-photocurrent in a Td-WTe₂ thin film via a terahertz time domain spectroscopy with a circularly polarized laser excitation. The anisotropic dependence of the circular photogalvanic effect (CPGE) in the terahertz emission reveals that the picosecond spin-photocurrent is generated along the rotational asymmetry a-axis. Notably, the generated spins are aligned along the out-of-plane direction under the light normally incident to the film surface, which provides an efficient means to manipulate magnetic devices with perpendicular magnetic anisotropy. A spin-splitting band induced by intrinsic inversion symmetry breaking enables the manipulation of a spin current by modulating the helicity of the laser excitation. Moreover, CPGE nearly vanishes at a transition temperature of ∼175 K due to the carrier compensation. Our work provides an insight into the dynamic behavior of the anisotropic spin-photocurrent of Td-WTe₂ in terahertz frequencies and shows a great potential for the future development of terahertz-spintronic devices with Weyl semimetals.

Magnetoelastoresistance in WTe2: Exploring electronic structure and extremely large magnetoresistance under strain

Strain describes the deformation of a material as a result of applied stress. It has been widely employed to probe transport properties of materials, ranging from semiconductors to correlated materials. In order to understand, and eventually control, transport behavior under strain, it is important to quantify the effects of strain on the electronic bandstructure, carrier density, and mobility. Here, we demonstrate that much information can be obtained by exploring magnetoelastoresistance (MER), which refers to magnetic field-driven changes of the elastoresistance. We use this powerful approach to study the combined effect of strain and magnetic fields on the semimetallic transition metal dichalcogenide [Formula: see text] We discover that WTe₂ shows a large and temperature-nonmonotonic elastoresistance, driven by uniaxial stress, that can be tuned by magnetic field. Using first-principle and analytical low-energy model calculations, we provide a semiquantitative understanding of our experimental observations. We show that in [Formula: see text], the strain-induced change of the carrier density dominates the observed elastoresistance. In addition, the change of the mobilities can be directly accessed by using MER. Our analysis also reveals the importance of a heavy-hole band near the Fermi level on the elastoresistance at intermediate temperatures. Systematic understanding of strain effects in single crystals of correlated materials is important for future applications, such as strain tuning of bulk phases and fabrication of devices controlled by strain.

All-electric magnetization switching and Dzyaloshinskii-Moriya interaction in WTe2/ferromagnet heterostructures

All-electric magnetization manipulation at low power is a prerequisite for a wide adoption of spintronic devices. Materials such as heavy metals^1-3 or topological insulators^4,5 provide good charge-to-spin conversion efficiencies. They enable magnetization switching in heterostructures with either metallic ferromagnets or with magnetic insulators. Recent work suggests a pronounced Edelstein effect in Weyl semimetals due to their non-trivial band structure^6,7; the Edelstein effect can be one order of magnitude stronger than it is in topological insulators or Rashba systems. Furthermore, the strong intrinsic spin Hall effect from the bulk states in Weyl semimetals can contribute to the spin current generation⁸. The Td phase of the Weyl semimetal WTe₂ (WTe₂ hereafter) possesses strong spin-orbit coupling^6,9 and non-trivial band structures¹⁰ with a large spin polarization protected by time-reversal symmetry in both the surface and bulk states^9-11. Atomically flat surfaces, which can be produced with high quality¹², facilitate spintronic device applications. Here, we use WTe₂ as a spin current source in WTe₂/Ni₈₁Fe₁₉ (Py) heterostructures. We report field-free current-induced magnetization switching at room temperature. A charge current density of ~2.96 × 10⁵ A cm^-2 suffices to switch the magnetization of the Py layer. With the charge current along the b axis of the WTe₂ layer, the thickness-dependent charge-to-spin conversion efficiency reaches 0.51 at 6-7 GHz. At the WTe₂/Py interface, a Dzyaloshinskii-Moriya interaction (DMI) with a DMI constant of -1.78 ± 0.06 mJ m^-2 induces chiral domain wall tilting. Our study demonstrates the capability of WTe₂ to efficiently manipulate magnetization and sheds light on the role of the interface in Weyl semimetal/magnet heterostructures.

First principles investigation of topological phase in XMR material TmSb under hydrostatic pressure

In this article, we report emergence of topological phase in extremely large magnetoresistance (XMR) material TmSb under hydrostatic pressure using first principles calculations. We find that TmSb, a topologically trivial semimetal, undergoes a topological phase transition with band inversion at X point without breaking any symmetry under a hydrostatic pressure of 12 GPa. At 15 GPa, it again becomes topologically trivial with band inversion at [Formula: see text] as well as X point. We find that the pressures corresponding to the topological phase transitions are far below the pressure corresponding to structural phase transition at 25.5 GPa. The reentrant behaviour of topological quantum phase with hydrostatic pressure would help in finding a correlation between topology and XMR effects through experiments.

Time-resolved XUV ARPES with tunable 24-33 eV laser pulses at 30 meV resolution

High harmonic generation of ultrafast laser pulses can be used to perform angle-resolved photoemission spectroscopy (ARPES) to map the electronic band structure of materials with femtosecond time resolution. However, currently it is difficult to reach high momenta with narrow energy resolution. Here, we combine a gas phase extreme ultraviolet (XUV) femtosecond light source, an XUV monochromator, and a time-of-flight electron analyzer to develop XUV-based time-resolved ARPES. Our technique can produce tunable photon energy between 24-33 eV with an unprecedented energy resolution of 30 meV and time resolution of 200 fs. This technique enables time-, energy- and momentum-resolved investigation of the nonequilibrium dynamics of electrons in materials with a full access to their first Brillouin zone. We evaluate the performance of this setup through exemplary measurements on various quantum materials, including WTe₂, WSe₂, TiSe₂, and Bi₂Sr₂CaCu₂O_8+δ.

Nonlinear Planar Hall Effect

An intriguing property of a three-dimensional (3D) topological insulator (TI) is the existence of surface states with spin-momentum locking, which offers a new frontier of exploration in spintronics. Here, we report the observation of a new type of Hall effect in a 3D TI Bi_{2}Se_{3} film. The Hall resistance scales linearly with both the applied electric and magnetic fields and exhibits a π/2 angle offset with respect to its longitudinal counterpart, in contrast to the usual angle offset of π/4 between the linear planar Hall effect and the anisotropic magnetoresistance. This novel nonlinear planar Hall effect originates from the conversion of a nonlinear transverse spin current to a charge current due to the concerted actions of spin-momentum locking and time-reversal symmetry breaking, which also exists in a wide class of noncentrosymmetric materials with a large span of magnitude. It provides a new way to characterize and utilize the nonlinear spin-to-charge conversion in a variety of topological quantum materials.

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.

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.

Experimental Observation of Hidden Berry Curvature in Inversion-Symmetric Bulk 2H-WSe_{2}

We investigate the hidden Berry curvature in bulk 2H-WSe_{2} by utilizing the surface sensitivity of angle resolved photoemission (ARPES). The symmetry in the electronic structure of transition metal dichalcogenides is used to uniquely determine the local orbital angular momentum (OAM) contribution to the circular dichroism (CD) in ARPES. The extracted CD signals for the K and K^{'} valleys are almost identical, but their signs, which should be determined by the valley index, are opposite. In addition, the sign is found to be the same for the two spin-split bands, indicating that it is independent of spin state. These observed CD behaviors are what are expected from Berry curvature of a monolayer of WSe_{2}. In order to see if CD-ARPES is indeed representative of hidden Berry curvature within a layer, we use tight binding analysis as well as density functional calculation to calculate the Berry curvature and local OAM of a monolayer WSe_{2}. We find that measured CD-ARPES is approximately proportional to the calculated Berry curvature as well as local OAM, further supporting our interpretation.

Spin-Resolved Electronic Response to the Phase Transition in MoTe_{2}

The semimetal MoTe_{2} is studied by spin- and angle-resolved photoemission spectroscopy across the centrosymmetry-breaking structural transition temperature of the bulk. A three-dimensional spin-texture is observed in the bulk Fermi surface in the low temperature, noncentrosymmetric phase that is consistent with first-principles calculations. The spin texture and two types of surface Fermi arc are not completely suppressed above the bulk transition temperature. The lifetimes of quasiparticles forming the Fermi arcs depend on thermal history and lengthen considerably upon cooling toward the bulk structural transition. The results indicate that a new form of polar instability exists near the surface when the bulk is largely in a centrosymmetric phase.

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.

Room-Temperature Nanoseconds Spin Relaxation in WTe2 and MoTe2 Thin Films

The Weyl semimetal WTe2 and MoTe2 show great potential in generating large spin currents since they possess topologically protected spin-polarized states and can carry a very large current density. In addition, the intrinsic non-centrosymmetry of WTe2 and MoTe2 endows with a unique property of crystal symmetry-controlled spin-orbit torques. An important question to be answered for developing spintronic devices is how spins relax in WTe2 and MoTe2. Here, a room-temperature spin relaxation time of 1.2 ns (0.4 ns) in WTe2 (MoTe2) thin film using the time-resolved Kerr rotation (TRKR) is reported. Based on ab initio calculation, a mechanism of long-lived spin polarization resulting from a large spin splitting around the bottom of the conduction band, low electron-hole recombination rate, and suppression of backscattering required by time-reversal and lattice symmetry operation is identified. In addition, it is found that the spin polarization is firmly pinned along the strong internal out-of-plane magnetic field induced by large spin splitting. This work provides an insight into the physical origin of long-lived spin polarization in Weyl semimetals, which could be useful to manipulate spins for a long time at room temperature.

Comparative study of the compensated semi-metals LaBi and LuBi: a first-principles approach

We have investigated the electronic structures of LaBi and LuBi, employing the full-potential all electron method as implemented in Wien2k. Using this, we have studied in detail both the bulk and the surface states of these materials. From our band structure calculations we find that LuBi, like LaBi, is a compensated semi-metal with almost equal and sizable electron and hole pockets. In analogy with experimental evidence in LaBi, we thus predict that LuBi will also be a candidate for extremely large magneto-resistance (XMR), which should be of immense technological interest. Our calculations reveal that LaBi, despite being gapless in the bulk spectrum, displays the characteristic features of a [Formula: see text] topological semi-metal, resulting in gapless Dirac cones on the surface, whereas LuBi only shows avoided band inversion in the bulk and is thus a conventional compensated semi-metal with extremely large magneto-resistance.

Pressure induced superconductivity bordering a charge-density-wave state in NbTe4 with strong spin-orbit coupling

Transition-metal chalcogenides host various phases of matter, such as charge-density wave (CDW), superconductors, and topological insulators or semimetals. Superconductivity and its competition with CDW in low-dimensional compounds have attracted much interest and stimulated considerable research. Here we report pressure induced superconductivity in a strong spin-orbit (SO) coupled quasi-one-dimensional (1D) transition-metal chalcogenide NbTe₄, which is a CDW material under ambient pressure. With increasing pressure, the CDW transition temperature is gradually suppressed, and superconducting transition, which is fingerprinted by a steep resistivity drop, emerges at pressures above 12.4 GPa. Under pressure p = 69 GPa, zero resistance is detected with a transition temperature T_c = 2.2 K and an upper critical field μ₀H_c2 = 2 T. We also find large magnetoresistance (MR) up to 102% at low temperatures, which is a distinct feature differentiating NbTe₄ from other conventional CDW materials.

Spin-Orbit Torques in NbSe2/Permalloy Bilayers

We present measurements of current-induced spin-orbit torques generated by NbSe₂, a fully metallic transition-metal dichalcogenide material, made using the spin-torque ferromagnetic resonance (ST-FMR) technique with NbSe₂/Permalloy bilayers. In addition to the out-of-plane Oersted torque expected from current flow in the metallic NbSe₂ layer, we also observe an in-plane antidamping torque with torque conductivity σ_S ≈ 10³ (ℏ/2e)(Ωm)^-1 and indications of a weak field-like contribution to the out-of-plane torque oriented opposite to the Oersted torque. Furthermore, in some samples we also measure an in-plane field-like torque with the form m̂ × ẑ, where m̂ is the Permalloy magnetization direction and ẑ is perpendicular to the sample plane. The size of this component varies strongly between samples and is not correlated with the NbSe₂ thickness. A torque of this form is not allowed by the bulk symmetries of NbSe₂ but is consistent with symmetry breaking by a uniaxial strain that might result during device fabrication.

Elastic and electronic tuning of magnetoresistance in MoTe2

Quasi-two-dimensional transition metal dichalcogenides exhibit dramatic properties that may transform electronic and photonic devices. We report on how the anomalously large magnetoresistance (MR) observed under high magnetic field in MoTe2, a type II Weyl semimetal, can be reversibly controlled under tensile strain. The MR is enhanced by as much as ~30% at low temperatures and high magnetic fields when uniaxial strain is applied along the a crystallographic direction and reduced by about the same amount when strain is applied along the b direction. We show that the large in-plane electric anisotropy is coupled with the structural transition from the 1T' monoclinic to the Td orthorhombic Weyl phase. A shift of the Td-1T' phase boundary is achieved by minimal tensile strain. The sensitivity of the MR to tensile strain suggests the possibility of a nontrivial spin-orbital texture of the electron and hole pockets in the vicinity of Weyl points. Our ab initio calculations show a significant orbital mixing on the Fermi surface, which is modified by the tensile strains.

Visualizing Type-II Weyl Points in Tungsten Ditelluride by Quasiparticle Interference

Weyl semimetals (WSMs) are classified into two types, type I and II, according to the topology of the Weyl point, where the electron and hole pockets touch each other. Tungsten ditelluride (WTe₂) has garnered a great deal of attention as a strong candidate to be a type-II WSM. However, the Weyl points for WTe₂ are located above the Fermi level, which has prevented us from identifying the locations and the connection to the Fermi arc surface states by using angle-resolved photoemission spectroscopy. Here, we present experimental proof that WTe₂ is a type-II WSM. We measured energy-dependent quasiparticle interference patterns with a cryogenic scanning tunneling microscope, revealing the position of the Weyl point and its connection with the Fermi arc surface states, in agreement with prior theoretical predictions. Our results provide an answer to this crucial question and stimulate further exploration of the characteristics of WSMs.

Observation of Room-Temperature Magnetoresistance in Monolayer MoS2 by Ferromagnetic Gating

Room-temperature magnetoresistance (MR) effect is observed in heterostructures of wafer-scale MoS₂ layers and ferromagnetic dielectric CoFe₂O₄ (CFO) thin films. Through the ferromagnetic gating, an MR ratio of -12.7% is experimentally achieved in monolayer MoS₂ under 90 kOe magnetic field at room temperature (RT). The observed MR ratio is much higher than that in previously reported nonmagnetic metal coupled with ferromagnetic insulator, which generally exhibited MR ratio of less than 1%. The enhanced MR is attributed to the spin accumulation at the heterostructure interface and spin injection to the MoS₂ layers by the strong spin-orbit coupling effect. The injected spin can contribute to the spin current and give rise to the MR by changing the resistance of MoS₂ layers. Furthermore, the MR effect decreases as the thickness of MoS₂ increases, and the MR ratio becomes negligible in MoS₂ with thickness more than 10 layers. Besides, it is interesting to find a magnetic field direction dependent spin Hall magnetoresistance that stems from a combination of the spin Hall and the inverse spin Hall effects. Our research provides an insight into exploring RT MR in monolayer materials, which should be helpful for developing ultrathin magnetic storage devices in the atomically thin limit.

Three-Dimensional Electronic Structure of the Type-II Weyl Semimetal WTe_{2}

By combining bulk sensitive soft-x-ray angular-resolved photoemission spectroscopy and first-principles calculations we explored the bulk electron states of WTe_{2}, a candidate type-II Weyl semimetal featuring a large nonsaturating magnetoresistance. Despite the layered geometry suggesting a two-dimensional electronic structure, we directly observe a three-dimensional electronic dispersion. We report a band dispersion in the reciprocal direction perpendicular to the layers, implying that electrons can also travel coherently when crossing from one layer to the other. The measured Fermi surface is characterized by two well-separated electron and hole pockets at either side of the Γ point, differently from previous more surface sensitive angle-resolved photoemission spectroscopy experiments that additionally found a pronounced quasiparticle weight at the zone center. Moreover, we observe a significant sensitivity of the bulk electronic structure of WTe_{2} around the Fermi level to electronic correlations and renormalizations due to self-energy effects, previously neglected in first-principles descriptions.

Magnetoresistance and Hall resistivity of semimetal WTe2 ultrathin flakes

This article reports the characterization of WTe₂ thin flake magnetoresistance and Hall resistivity. We found it does not exhibit magnetoresistance saturation when subject to high fields, in a manner similar to their bulk characteristics. The linearity of Hall resistivity in our devices confirms the compensation of electrons and holes. By relating experimental results to a classic two-band model, the lower magnetoresistance values in our samples is demonstrated to be caused by decreased carrier mobility. The dependence of mobility on temperature indicates the main role of optical phonon scattering at high temperatures. Our results provide more detailed information on carrier behavior and scattering mechanisms in WTe₂ thin films.

Angle-resolved photoemission spectroscopy for the study of two-dimensional materials

Quantum systems in confined geometries allow novel physical properties that cannot easily be attained in their bulk form. These properties are governed by the changes in the band structure and the lattice symmetry, and most pronounced in their single layer limit. Angle-resolved photoemission spectroscopy (ARPES) is a direct tool to investigate the underlying changes of band structure to provide essential information for understanding and controlling such properties. In this review, recent progresses in ARPES as a tool to study two-dimensional atomic crystals have been presented. ARPES results from few-layer and bulk crystals of material class often referred as “beyond graphene” are discussed along with the relevant developments in the instrumentation.

Quantum Effects and Phase Tuning in Epitaxial Hexagonal and Monoclinic MoTe2 Monolayers

Monolayer (ML) transition-metal dichalcogenides exist in different phases, such as hexagonal (2H) and monoclinic (1T') structures. They show very different properties: semiconducting for 2H-MoTe₂ and semimetallic for 1T'-MoTe₂. The formation energy difference between 2H- and 1T'-phase MoTe₂ is small, so there is a high chance of tuning the structures of MoTe₂ and thereby introducing applications of phase-change electronics. In this paper, we report the growth of both 2H- and 1T'-MoTe₂ MLs by molecular-beam epitaxy (MBE) and demonstrate its tenability by changing the conditions of MBE. We attribute the latter to an effect of Te adsorption. By scanning tunneling microscopy and spectroscopy, we reveal not only the atomic structures and intrinsic electronic properties of the two phases of MoTe₂ but also quantum confinement and quantum interference effects in the 2H- and 1T'-MoTe₂ domains, respectively, as effected by domain boundaries in the samples.

Tunable Positive to Negative Magnetoresistance in Atomically Thin WTe2

Transitional metal ditelluride WTe₂ has been extensively studied owing to its intriguing physical properties like nonsaturating positive magnetoresistance and being possibly a type-II Weyl semimetal. While surging research activities were devoted to the understanding of its bulk properties, it remains a substantial challenge to explore the pristine physics in atomically thin WTe₂. Here, we report a successful synthesis of mono- to few-layer WTe₂ via chemical vapor deposition. Using atomically thin WTe₂ nanosheets, we discover a previously inaccessible ambipolar behavior that enables the tunability of magnetoconductance of few-layer WTe₂ from weak antilocalization to weak localization, revealing a strong electrical field modulation of the spin-orbit interaction under perpendicular magnetic field. These appealing physical properties unveiled in this study clearly identify WTe₂ as a promising platform for exotic electronic and spintronic device applications.

Coexistence of topological Dirac fermions on the surface and three-dimensional Dirac cone state in the bulk of ZrTe5 single crystal

Although, the long-standing debate on the resistivity anomaly in ZrTe5 somewhat comes to an end, the exact topological nature of the electronic band structure remains elusive till today. Theoretical calculations predicted that bulk ZrTe5 to be either a weak or a strong three-dimensional (3D) topological insulator. However, the angle resolved photoemission spectroscopy and transport measurements clearly demonstrate 3D Dirac cone state with a small mass gap between the valence band and conduction band in the bulk. From the magnetization and magneto-transport measurements on ZrTe5 single crystal, we have detected both the signature of helical spin texture from topological surface state and chiral anomaly associated with the 3D Dirac cone state in the bulk. This implies that ZrTe5 hosts a novel electronic phase of material, having massless Dirac fermionic excitation in its bulk gap state, unlike earlier reported 3D topological insulators. Apart from the band topology, it is also apparent from the resistivity and Hall measurements that the anomalous peak in the resistivity can be shifted to a much lower temperature (T < 2 K) by controlling impurity and defects.

Spin splitting and reemergence of charge compensation in monolayer WTe2by 3d transition-metal adsorption

Transition-metal adsorption effectively modifies the intrinsic properties of monolayer WTe₂, where the electron–hole pockets reemerge with Ni adatom.

Tuning the magnetoresistance of ultrathin WTe2 sheets by electrostatic gating

The semimetallic, two-dimensional layered transition metal dichalcogenide WTe₂ has raised considerable interest due to its huge, non-saturating magnetoresistance. While for the origin of this effect, a close-to-ideal balance of electrons and holes has been put forward, the carrier concentration dependence of the magnetoresistance remains to be clarified. Here, we present a detailed study of the magnetotransport behaviour of ultrathin, mechanically exfoliated WTe₂ sheets as a function of electrostatic back gating. The carrier concentration and mobility, determined using the two band model and analysis of the Shubnikov-de Haas oscillations, indicate enhanced surface scattering for the thinnest sheets. By the back gate action, the magnetoresistance could be tuned by up to ∼100% for a ∼13 nm-thick WTe₂ sheet.

Direct Observation of a Long-Range Field Effect from Gate Tuning of Nonlocal Conductivity

We report the direct observation of a long-range field effect in WTe_{2} devices, leading to large gate-induced changes of transport through crystals much thicker than the electrostatic screening length. The phenomenon-which manifests itself very differently from the conventional field effect-originates from the nonlocal nature of transport in the devices that are thinner than the carrier mean free path. We reproduce theoretically the gate dependence of the measured classical and quantum magnetotransport, and show that the phenomenon is caused by the gate tuning of the bulk carrier mobility by changing the scattering at the surface. Our results demonstrate experimentally the possibility to gate tune the electronic properties deep in the interior of conducting materials, avoiding limitations imposed by electrostatic screening.

Compensated Semimetal LaSb with Unsaturated Magnetoresistance

By combining angle-resolved photoemission spectroscopy and quantum oscillation measurements, we performed a comprehensive investigation on the electronic structure of LaSb, which exhibits near-quadratic extremely large magnetoresistance (XMR) without any sign of saturation at magnetic fields as high as 40 T. We clearly resolve one spherical and one intersecting-ellipsoidal hole Fermi surfaces (FSs) at the Brillouin zone (BZ) center Γ and one ellipsoidal electron FS at the BZ boundary X. The hole and electron carriers calculated from the enclosed FS volumes are perfectly compensated, and the carrier compensation is unaffected by temperature. We further reveal that LaSb is topologically trivial but shares many similarities with the Weyl semimetal TaAs family in the bulk electronic structure. Based on these results, we have examined the mechanisms that have been proposed so far to explain the near-quadratic XMR in semimetals.

Coherent optical phonon oscillation and possible electronic softening in WTe2 crystals

A rapidly-growing interest in WTe2 has been triggered by the giant magnetoresistance effect discovered in this unique system. While many efforts have been made towards uncovering the electron- and spin-relevant mechanisms, the role of lattice vibration remains poorly understood. Here, we study the coherent vibrational dynamics in WTe2 crystals by using ultrafast pump-probe spectroscopy. The oscillation signal in time domain in WTe2 has been ascribed as due to the coherent dynamics of the lowest energy A1 optical phonons with polarization- and wavelength-dependent measurements. With increasing temperature, the phonon energy decreases due to anharmonic decay of the optical phonons into acoustic phonons. Moreover, a significant drop (15%) of the phonon energy with increasing pump power is observed which is possibly caused by the lattice anharmonicity induced by electronic excitation and phonon-phonon interaction.

Temperature-field phase diagram of extreme magnetoresistance

The recent discovery of extreme magnetoresistance (XMR) in LaSb introduced lanthanum monopnictides as a new platform to study this effect in the absence of broken inversion symmetry or protected linear band crossing. In this work, we report XMR in LaBi. Through a comparative study of magnetotransport effects in LaBi and LaSb, we construct a temperature-field phase diagram with triangular shape that illustrates how a magnetic field tunes the electronic behavior in these materials. We show that the triangular phase diagram can be generalized to other topological semimetals with different crystal structures and different chemical compositions. By comparing our experimental results to band structure calculations, we suggest that XMR in LaBi and LaSb originates from a combination of compensated electron-hole pockets and a particular orbital texture on the electron pocket. Such orbital texture is likely to be a generic feature of various topological semimetals, giving rise to their small residual resistivity at zero field and subject to strong scattering induced by a magnetic field.

Dramatically decreased magnetoresistance in non-stoichiometric WTe2 crystals

Recently, the layered semimetal WTe₂ has attracted renewed interest owing to the observation of a non-saturating and giant positive magnetoresistance (~10⁵%), which can be useful for magnetic memory and spintronic devices. However, the underlying mechanisms of the giant magnetoresistance are still under hot debate. Herein, we grew the stoichiometric and non-stoichiometric WTe₂ crystals to test the robustness of giant magnetoresistance. The stoichiometric WTe₂ crystals have magnetoresistance as large as 3100% at 2 K and 9-Tesla magnetic field. However, only 71% and 13% magnetoresistance in the most non-stoichiometry (WTe_1.80) and the highest Mo isovalent substitution samples (W_0.7Mo_0.3Te₂) are observed, respectively. Analysis of the magnetic-field dependent magnetoresistance of non-stoichiometric WTe₂ crystals substantiates that both the large electron-hole concentration asymmetry and decreased carrier mobility, induced by non-stoichiometry, synergistically lead to the decreased magnetoresistance. This work sheds more light on the origin of giant magnetoresistance observed in WTe₂.

Layer-dependent quantum cooperation of electron and hole states in the anomalous semimetal WTe2

The behaviour of electrons and holes in a crystal lattice is a fundamental quantum phenomenon, accounting for a rich variety of material properties. Boosted by the remarkable electronic and physical properties of two-dimensional materials such as graphene and topological insulators, transition metal dichalcogenides have recently received renewed attention. In this context, the anomalous bulk properties of semimetallic WTe₂ have attracted considerable interest. Here we report angle- and spin-resolved photoemission spectroscopy of WTe₂ single crystals, through which we disentangle the role of W and Te atoms in the formation of the band structure and identify the interplay of charge, spin and orbital degrees of freedom. Supported by first-principles calculations and high-resolution surface topography, we reveal the existence of a layer-dependent behaviour. The balance of electron and hole states is found only when considering at least three Te–W–Te layers, showing that the behaviour of WTe₂ is not strictly two dimensional.

Tungsten ditelluride is a semi-metallic two-dimensional material that has exhibited large magnetoresistance. Here, the authors use angle- and spin-resolved photoemission spectroscopy to investigate the band structure of this transition metal dichalcogenide and identify layer-dependent electronic behaviour.

Prediction of an arc-tunable Weyl Fermion metallic state in Mo(x)W(1-x)Te2

A Weyl semimetal is a new state of matter that hosts Weyl fermions as emergent quasiparticles. The Weyl fermions correspond to isolated points of bulk band degeneracy, Weyl nodes, which are connected only through the crystal's boundary by exotic Fermi arcs. The length of the Fermi arc gives a measure of the topological strength, because the only way to destroy the Weyl nodes is to annihilate them in pairs in the reciprocal space. To date, Weyl semimetals are only realized in the TaAs class. Here, we propose a tunable Weyl state in Mo(x)W(1-x)Te2 where Weyl nodes are formed by touching points between metallic pockets. We show that the Fermi arc length can be changed as a function of Mo concentration, thus tuning the topological strength. Our results provide an experimentally feasible route to realizing Weyl physics in the layered compound Mo(x)W(1-x)Te2, where non-saturating magneto-resistance and pressure-driven superconductivity have been observed.

Tuning magnetotransport in a compensated semimetal at the atomic scale

Either in bulk form, or in atomically thin crystals, layered transition metal dichalcogenides continuously reveal new phenomena. The latest example is 1T'-WTe2, a semimetal found to exhibit the largest known magnetoresistance in the bulk, and predicted to become a topological insulator in strained monolayers. Here we show that reducing the thickness through exfoliation enables the electronic properties of WTe2 to be tuned, which allows us to identify the mechanisms responsible for the observed magnetotransport down to the atomic scale. The longitudinal resistance and the unconventional magnetic field dependence of the Hall resistance are reproduced quantitatively by a classical two-band model for crystals as thin as six monolayers, whereas a crossover to an Anderson insulator occurs for thinner crystals. Besides establishing the origin of the magnetoresistance of WTe2, our results represent a complete validation of the classical theory for two-band electron-hole transport, and indicate that atomically thin WTe2 layers remain gapless semimetals.

Temperature-Induced Lifshitz Transition in WTe2

We use ultrahigh resolution, tunable, vacuum ultraviolet laser-based, angle-resolved photoemission spectroscopy (ARPES), temperature- and field-dependent resistivity, and thermoelectric power (TEP) measurements to study the electronic properties of WTe2, a compound that manifests exceptionally large, temperature-dependent magnetoresistance. The Fermi surface consists of two pairs of electron and two pairs of hole pockets along the X-Γ-X direction. Using detailed ARPES temperature scans, we find a rare example of a temperature-induced Lifshitz transition at T≃160  K, associated with the complete disappearance of the hole pockets. Our electronic structure calculations show a clear and substantial shift of the chemical potential μ(T) due to the semimetal nature of this material driven by modest changes in temperature. This change of Fermi surface topology is also corroborated by the temperature dependence of the TEP that shows a change of slope at T≈175  K and a breakdown of Kohler's rule in the 70-140 K range. Our results and the mechanisms driving the Lifshitz transition and transport anomalies are relevant to other systems, such as pnictides, 3D Dirac semimetals, and Weyl semimetals.