Weyl Semimetal WTe2 Research Articles

Integrated mid-infrared sensing and ultrashort lasers based on wafer-level Td-WTe2 Weyl semimetal

There is an urgent need for infrared (IR) detection systems with high-level miniaturization and room-temperature operation capability. The rising star of two-dimensional (2D) semimetals with extraordinary optoelectronic properties can fulfill these criteria. However, the formidable challenges with regard to large-scale patterning and substrate-selective requirements limit material deposition options for device fabrication. Here, we report a convenient and straightforward eutectic-tellurization transformation method for the wafer-level synthesis of 2D type-II Weyl semimetal WTe2. The non-cryogenic WTe2/Si Schottky junction device displays an ultrawide detection range covering 10.6 μm with a high detectivity of ∼109 Jones in the mid-infrared (MIR) region and a short response time of 1.3 μs. The detection performance has surpassed most reported IR sensors. On top of that, on-chip device arrays based on Schottky junction display an outstanding MIR imaging capability without cryogenic cooling, and 2D WTe2 Weyl semimetal can serve as a saturable absorber for stable Q-switched and mode-locked laser operation applications. Our work offers a viable route for wafer-scale vdW preparation of 2D semimetals, showcasing their intriguing potential in on-chip integrated MIR detection systems and ultrafast laser photonics.

Spatiotemporal determination of photoinduced strain in a Weyl semimetal.

The application of dynamic strain holds the potential to manipulate topological invariants in topological quantum materials. This study investigates dynamic structural deformation and strain modulation in the Weyl semimetal WTe2, focusing on the microscopic regions with static strain defects. The interplay of static strain fields, at local line defects, with dynamic strain induced from photo-excited coherent acoustic phonons results in the formation of local standing waves at the defect sites. The dynamic structural distortion is precisely determined utilizing ultrafast electron microscopy with nanometer spatial and gigahertz temporal resolutions. Numerical simulations are employed to interpret the experimental results and explain the mechanism for how the local strain fields are transiently modulated through light-matter interaction. This research provides the experimental foundation for investigating predicted phenomena such as the mixed axial-torsional anomaly, acoustogalvanic effect, and axial magnetoelectric effects in Weyl semimetals, and paves the road to manipulate quantum invariants through transient strain fields in quantum materials.

Erratum to: Self-enhancing photothermal conversion of 2D Weyl semimetal WTe2 with topological surface states for efficient solar vapor generation

Erratum to: Self-enhancing photothermal conversion of 2D Weyl semimetal WTe2 with topological surface states for efficient solar vapor generation

Self-Powered Broadband Photodetectors Based on Large-Scale Continuous Films of Weyl Semimetal WTe2

Self-Powered Broadband Photodetectors Based on Large-Scale Continuous Films of Weyl Semimetal WTe₂

Magnetization switching driven by spin-orbit torque of Weyl semimetal WTe<sub>2</sub>

The Wely semimetal WTe<sub>2</sub> exhibits significant spin-orbit coupling characteristics and can generate unconventional spin current with out-of-plane polarization, which has become a hotspot in recent years. Meanwhile, WTe<sub>2</sub> also has high charge-spin conversion efficiency, allowing perpendicular magnetization to be switched deterministically without the assistance of an external magnetic field, which is critical for the high-density integration of low-power magnetic random-access memories. The purpose of this paper is to review the recent advances in the research on spin orbit torque in heterostructures composed of WTe<sub>2</sub> and ferromagnetic layers, focusing on progress of research on the detection and magnetization switching in the spin orbit torque of heterojunctions composed of WTe<sub>2</sub> prepared by different methods (e.g. mechanical exfoliation and chemical vapor deposition) and ferromagnetic layers such as conventional magnets (e.g, FeNi and CoFeB, etc.) and two-dimensional magnets (e.g. Fe<sub>3</sub>GeTe<sub>2</sub>, etc.). Finally, the prospect of related research is discussed.

Highly selective photothermal conversion of CO2 to ethylene using hierarchical boxwood ball-like Weyl semimetal WTe2 catalysts

WTe2 is developed for efficient photothermal CO2 conversion to C2H4 for the first time. DFT calculations and in situ FTIR spectroscopy reveal that both W and Te as active sites promote C–C coupling to form C2H4 by suitable W–Te atomic distance.

Self-enhancing photothermal conversion of 2D Weyl semimetal WTe2 with topological surface states for efficient solar vapor generation

Self-enhancing photothermal conversion of 2D Weyl semimetal WTe2 with topological surface states for efficient solar vapor generation

The vertical longitudinal magnetoresistance in a van der Waals thin film of WTe2

We report the magneto-transport measurements of thin film devices of the topological Weyl semimetal WTe2 with the applied current along and vertical to the in-plane directions. The device is composed of a van der Waals thin film of WTe2 sandwiched between top and bottom Au electrodes. At low temperatures, we observe not only a large unsaturated magnetoresistance and Shubnikov–de Haas oscillations with current in the in-plane direction but also a saturated vertical longitudinal magnetoresistance and quantum oscillations with current in the out-of-plane direction in a thin film of WTe2. Our work provides insight into the origin of the unsaturated magnetoresistance in WTe2 and may inspire non-planar engineering to reach higher integration in spintronics.

Structural and electronic properties of Weyl semimetal WTe2 under high pressure

The structural and electronic properties of WTe2 are studied from ambient pressure to 30.0 ​GPa. The pressure-induced structural phase transition is found from Td phase to 1T′ phase above 10.0 ​GPa by lattice offset, which is also reflected at approximately 11.2 ​GPa based on the discontinuous electrical parameters. A pair of hole wave and electron wave packets of Td phase gradually overlap revealing the reversible Td-to-1T′ phase structural transition. WTe2 is observed to undergo semimetal-to-semiconductor transition owing to the calculated energy band up to 15 ​GPa, which is also observed at 11.2 ​GPa according to the resistivity-temperature curves under compression and decompression conditions. The electronic density of states at the Fermi level is mainly contributed by the d orbital of W and the p and s orbital of Te. The photoconductivity property is shown by the theoretical and experimental methods.

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.

Investigating active area dependent high performing photoresponse through thin films of Weyl Semimetal WTe2

WTe2 is one of the wonder layered materials, displays interesting overlapping of electron–hole pairs, opening of the surface bandgap, anisotropy in its crystal structure and very much sought appealing material for room temperature broadband photodection applications. Here we report the photoresponse of WTe2 thin films and microchannel devices fabricated on silicon nitride substrates. A clear sharp rise in photocurrent observed under the illumination of visible (532 nm) and NIR wavelengths (1064 nm). The observed phoresponse is very convincing and repetitive for ON /OFF cycles of laser light illumination. The channel length dependence of photocurrent is noticed for few hundred nanometers to micrometers. The photocurrent, rise & decay times, responsivity and detectivity are studied using different channel lengths. Strikingly microchannel gives few orders of greater responsivity compared to larger active area investigated here. The responsivity and detectivity are observed as large as 29 A/W and 3.6 × 108 Jones respectively. The high performing photodetection properties indicate that WTe2 can be used as a broad band material for future optoelectronic applications.

Resonant Second-Harmonic Generation as a Probe of Quantum Geometry.

Nonlinear responses are actively studied as probes of topology and band geometric properties of solids. Here, we show that second harmonic generation serves as a probe of the Berry curvature, quantum metric, and quantum geometric connection. We generalize the theory of second harmonic generation to include Fermi surface effects in metallic systems, and finite scattering timescale. In doped materials the Fermi surface and Fermi sea cause all second harmonic terms to exhibit resonances, and we identify two novel contributions to the second harmonic signal: a double resonance due to the Fermi surface and a higher-order pole due to the Fermi sea. We discuss experimental observation in the monolayer of time reversal symmetric Weyl semimetal WTe_{2} and the parity-time reversal symmetric topological antiferromagnet CuMnAs.

Enhanced superconductivity with interlayer spacing dependent T c in intercalated Weyl semimetal MoTe2

Manipulating the strength of the interlayer coupling is an effective strategy to induce intriguing properties in layered materials. Recently, enhanced superconductivity has been reported in Weyl semimetal MoTe2 and WTe2 via ionic liquid (IL) cation intercalation. However, how the superconductivity enhancement depends on the interlayer interaction still remains elusive. Here by inserting IL cations with different sizes into MoTe2 through this strategy, we are able to tune the interlayer spacing of the intercalated MoTe2 samples and reveal the dependence of superconducting transition temperature T c on the interlayer spacing. Our results show that T c increases with the interlayer spacing, suggesting that the weakened interlayer coupling plays an important role in the superconductivity. Interestingly, the intercalation induced superconductivity shows a high Ginzburg–Landau anisotropy, which suggests a quasi-two-dimensional nature of the superconductivity where the adjacent superconducting layers are coupled through Josephson tunnelling.

Ultrahigh transverse thermoelectric power factor in flexible Weyl semimetal WTe2

Topological semimetals are well known for their interesting physical properties, while their mechanical properties have rarely received attention. With the increasing demand for flexible electronics, we explore the great potential of the van der Waals bonded Weyl semimetal WTe2 for flexible thermoelectric applications. We find that WTe2 single crystals have an ultrahigh Nernst power factor of ~3 Wm−1K−2, which outperforms the conventional Seebeck power factors of the state-of-the-art thermoelectric semiconductors by 2–3 orders of magnitude. A unique band structure that hosts compensated electrons and holes with extremely high mobilities is the primary mechanism for this huge Nernst power factor. Moreover, a large Ettingshausen signal of ~5 × 10−5 KA−1m is observed at 23.1 K and 9 T. In this work, the combination of the exceptional Nernst–Ettingshausen performance and excellent mechanical transformative ability of WTe2 would be instructive for flexible micro-/nano-thermoelectric devices.

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 resistance1-4, higher-than-ballistic conduction5-7, Poiseuille flow in narrow channels8-10 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.

Josephson junctions of Weyl semimetal WTe2 induced by spontaneous nucleation of PdTe superconductor

We report on the fabrication of Josephson junction devices with weak links utilizing the Weyl and higher-order topological semimetal WTe2. We show that the WTe2/Pd contact annealed at a low temperature of 80 °C did not exhibit superconducting properties because neither WTe2 nor Pd are superconductors in the ground state. Upon 180 °C annealing, spontaneous formation of superconducting PdTe due to Pd diffusion enabled us to obtain the interface between WTe2 and superconductor suitable for the Josephson junction. This result is a facile technique to make a Josephson junction and induce Cooper pairs into topological telluride semimetals.

Site-Dependent Local Spin Susceptibility and Low-Energy Excitation in a Weyl Semimetal WTe2

Site-dependent local spin susceptibility is investigated with $^{125}$Te nuclear magnetic resonance in a Weyl semimetal WTe$_2$. The nuclear spin-lattice relaxation rate $1/T_1T$ shows a dependence of the square of temperature $T$ at high temperatures, followed by a constant behavior below 50 K. The temperature dependence features Weyl fermions appearing around the linearly crossing bands. The Knight shift $K$ scales to the square root of $1/T_1T$, corroborating a predominant spin contribution in low-lying excitation. The observed dependence of $K$ and $1/T_1T$ on the four Te sites shows the site-dependent electron correlation and density of states. The angular profile of the NMR spectrum gives the anisotropic hyperfine coupling tensor, consistent with $5p$ hole occupations on Te sites.

A Gapped Phase in Semimetallic Td -WTe2 Induced by Lithium Intercalation.

The Weyl semimetal WTe2 has shown several correlated electronic behaviors, such as the quantum spin Hall effect, superconductivity, ferroelectricity, and a possible exciton insulator state, all of which can be tuned by various physical and chemical approaches. Here, a new electronic phase in WTe2 induced by lithium intercalation is discovered. The new phase exhibits an increasing resistivity with decreasing temperature and its carrier density is almost two orders of magnitude lower than the carrier density of the semimetallic Td phase, probed by in situ Hall measurements as a function of lithium intercalation. The theoretical calculations predict the new lithiated phase to be a potential charge density wave (CDW) phase with a bandgap of ≈0.14eV, in good agreement with the in situ transport data. The new phase is structurally distinct from the initial Td phase, characterized by polarization-angle-dependent Raman spectroscopy, and large lattice distortions close to 6% are predicted in the new phase. This finding of a new gapped phase in a 2D semimetal demonstrates electrochemical intercalation as a powerful tuning knob for modulating electron density and phase stability in 2D materials.

Superior peroxidase mimetic activity induced by topological surface states of Weyl semimetal WTe2

Nanozymes, as promising alternatives that integrate the advantage of natural enzymes and nanomaterials are attracted enormous interest due to their low cost, environmental tolerance, high stability, as well as significant catalytic activity. Two-dimensional materials, transition metal dichalcogenides (TMDs) harbor the potential as peroxidase mimic which is attributed to the active edge sites and surface electron transfer capability. However, a challenge faced in peroxidase mimetic of TMDs is the low catalytic activity which originates from the inert structures. Here, we, for the first time, report that the typical Weyl semimetal WTe2 possesses the superior peroxidase-like performance toward H2O2, which derives from the two-step electron-pathway. We demonstrate a novel functionality of the Weyl semimetal through introducing topologically protected surface states (TSSs) for regulating the electron transfer processes. The underlying mechanism for TSSs promoting enzyme mimetic catalysis is attributed to the effective electron bath provided by the robust TSSs. Experiments and the first-principles calculations show that TSSs of WTe2 which serve as effective electron baths are directly involved in the two-step electron transfer process. In the first step, TSSs accept the electrons donated by the substrate which further enhances the substrate’s absorption. Upon H2O2 adsorption, the electrons are transferred out of TSSs and injected into the absorbed H2O2 orbitals. These findings offer a new linkage between the topological matters with TSSs and nanomaterials for enzyme mimicking.

Thermal stability study of Weyl semimetal WTe2 by polarized micro-Raman scattering

Weyl semimetal Td-phase WTe2, a novel topological matter, possesses non-trivial band structures with strong spin-orbit coupling and thus exhibits a pronounced charge-to-spin conversion efficiency, holding promises as a superior spin current source for spin memory and logic devices. Good thermal stability of WTe2 is crucial for device fabrication and performance, which should be compatible with the semiconductor technology usually involving high temperature process of ∼350–400 °C. However, the thermal stability of this novel material has not been very clear yet. Here, we study the exfoliated Td-WTe2 flakes by in-situ polarized micro-Raman scattering with a much higher heating temperature up to 550 °C. We find that the structure and composition of WTe2 flakes can remain unchanged up to 425 °C. However, around 450 °C, the composition of WTe2 flakes starts to change due to the decomposition and sublimation of Te atoms in WTe2. Our study demonstrates that Weyl semimetal WTe2 shows good thermal stability and has great potentials for the energy-efficient spintronic device application.