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The ab-plane optical conductivity of the Weyl semimetal TaP is calculated from the band structure and compared to the experimental data. The overall agreement between theory and experiment is found to be best when the Fermi level is slightly (20 to 60 meV) shifted upwards in the calculations. This confirms a small unintentional doping of TaP, reported earlier, and allows a natural explanation of the strong low-energy (50 meV) peak seen in the experimental ab-plane optical conductivity: this peak originates from transitions between the almost parallel non-degenerate electronic bands split by spin-orbit coupling. The temperature evolution of the peak can be reasonably well reproduce by calculations using an analog of the Mott formula., Deutsche Forschungsgemeinschaft

The Wiedemann-Franz (WF) law relates the electronic component of the thermal conductivity to the electrical conductivity in metals through the Lorenz number. The WF law has proven to be remarkably robust, however violations have been observed in many topological materials. In this work, we report thermoelectric measurements conducted on Heusler, ferromagnetic, Weyl semimetal Co$_2$MnAl which shows a drastic, temperature dependent violation of the WF law below 300 K. We then discuss our result in the context of known physical explanations for WF law violation. Both the magnitude and temperature dependence of the violation in Co2MnAl are extreme, indicating that there may be more than one effect contributing to the violation in this system., Comment: 8 pages, 5 figures

Recently, new states of matter like superconducting or topological quantum states were found in transition metal dichalcogenides (TMDs) and manifested themselves in a series of exotic physical behaviors. Such phenomena have been demonstrated to exist in a series of transition metal tellurides including MoTe2, WTe2, and alloyed MoxW1-xTe2. However, the behaviors in the alloy system have been rarely addressed due to their difficulty in obtaining atomic layers with controlled composition, albeit the alloy offers a great platform to tune the quantum states. Here, we report a facile CVD method to synthesize the MoxW1-xTe2 with controllable thickness and chemical composition ratios. The atomic structure of a monolayer MoxW1-xTe2 alloy was experimentally confirmed by scanning transmission electron microscopy. Importantly, two different transport behaviors including superconducting and Weyl semimetal states were observed in Mo-rich Mo0.8W0.2Te2 and W-rich Mo0.2W0.8Te2 samples, respectively. Our results show that the electrical properties of MoxW1-xTe2 can be tuned by controlling the chemical composition, demonstrating our controllable CVD growth method is an efficient strategy to manipulate the physical properties of TMDCs. Meanwhile, it provides a perspective on further comprehension and sheds light on the design of devices with topological multicomponent TMDC materials.

The chiral anomaly is the predicted breakdown of chiral symmetry in a Weyl semimetal with monopoles of opposite chirality when an electric field is applied parallel to a magnetic field. It occurs because of charge pumping between monopoles of opposite chirality. Experimental observation of this fundamental effect is plagued by concerns about the current pathways. Here we demonstrate the thermal chiral anomaly, energy pumping between monopoles, in topological insulator bismuth–antimony alloys driven into an ideal Weyl semimetal state by a Zeeman field, with the chemical potential pinned at the Weyl points and in the absence of any trivial Fermi surface pockets. The experimental signature is a large enhancement of the thermal conductivity in an applied magnetic field parallel to the thermal gradient. This work demonstrates both pumping of energy and charge between the two Weyl points of opposite chirality and that they are related by the Wiedemann–Franz law. A thermal signature of the chiral anomaly is reported in an ideal Weyl semimetal.

Topological magnetic semimetals, like Co3Sn2S2 and Co2MnGa, display exotic transport properties, such as large intrinsic anomalous (AHE) due to uncompensated Berry curvature. The highly symmetric XPt3 compounds exhibit anti-crossing gapped nodal lines, a driving mechanism in the intrinsic Berry curvature Hall effects. Uniquely, these compounds contain two sets of gapped nodal lines that harmoniously dominate the Berry curvature in this complex multi band system. We calculate a maximum AHE of 1965 S cm-1 in the CrPt3 by first principles electronic structure. We have grown high-quality CrPt3 thin films with perpendicular magnetic anisotropy by magnetron sputtering and measured a robust AHE of 1750 S cm−1 for different sputtering growth conditions. Additionally, the cubic films display an easy magnetic axis along [111] direction. The facile and scalable fabrication of these materials is prime candidates for integration into topological devices. Topological magnetic semimetals can realise large intrinsic anomalous Hall effects using the characteristics of their electronic band structure and Berry curvature. Here, the authors predict an anomalous Hall effect for cubic CrPt3 using first principle calculations and confirm the results experimentally.

Using density functional theory, we propose the (MnSb2Te4)·(Sb2Te3)n family of stoichiometric van der Waals compounds that harbor multiple topologically nontrivial magnetic phases. In the ground state, the first three members of the family (n = 0, 1, 2) are 3D antiferromagnetic topological insulators, while for n ≥ 3 a special phase is formed, in which a nontrivial topological order coexists with a partial magnetic disorder in the system of the decoupled 2D ferromagnets, whose magnetizations point randomly along the third direction. Furthermore, due to a weak interlayer exchange coupling, these materials can be field-driven into the FM Weyl semimetal (n = 0) or FM axion insulator states (n ≥ 1). Finally, in two dimensions, we reveal these systems to show intrinsic quantum anomalous Hall and AFM axion insulator states, as well as quantum Hall state, achieved under external magnetic field. Our results demonstrate that MnSb2Te4 is not topologically trivial as was previously believed that opens possibilities of realization of a wealth of topologically nontrivial states in the (MnSb2Te4)·(Sb2Te3)n family., M.M.O. acknowledges the support by the Spanish Ministerio de Ciencia e Innovación (Grant No. PID2019-103910GB-I00). E.V.C. acknowledges the Saint Petersburg State University Project for Scientific Investigations (ID No. 51126254, https://spin.lab.spbu.ru). S.V.E. acknowledges support by the Russian Science Foundation (Grant No. 18-12-00169-p). Y.M.K. acknowledges support from a government research assignment for ISPMS SB RAS (Project FWRW-2019-0032) for the FLEUR calculations. I.P.R. acknowledges support from the Ministry of Education and Science of the Russian Federation (State Task No. 0721-2020-0033) for the tight-binding calculations. A.E. acknowledges support from OeAD Grant No. HR 07/2018 for the HUTSEPOT calculations.

Weyl semimetals are extraordinary systems where exotic phenomena such as Fermi arcs, pseudo-gauge fields and quantum anomalies arise from topological band degeneracy in crystalline solids for electrons and metamaterials for photons and phonons. On the other hand, higher-order topological insulators unveil intriguing multidimensional topological physics beyond the conventional bulk-edge correspondences. However, it is unclear whether higher-order topology can emerge in Weyl semimetals. Here, we report the experimental discovery of higher-order Weyl semimetals in its phononic analog which exhibit topologically-protected boundary states in multiple dimensions. We create the physical realization of the higher-order Weyl semimetal in a chiral phononic crystal with uniaxial screw symmetry. Using near-field spectroscopies, we observe the chiral Fermi arcs on the surfaces and a new type of hinge arc states on the hinge boundaries. These topological boundary arc states link the projections of Weyl points in different dimensions and directions, and hence demonstrate higher-order multidimensional topological physics in Weyl semimetals. Our study establishes the fundamental connection between higher-order topology and Weyl physics in crystalline materials and unveils a new horizon of higher-order topological semimetals where unprecedented materials such as higher-order topological nodal-lines may emerge.

Heusler compounds are a family of materials with high tunability due to their structure and lots of states or properties have been discovered in it. Topological semimetals (TSM) are a new phase of quantum matter that many materials have been reported to have this phase, including Heusler compounds. In this review, basic concepts of Heusler compounds and main properties of three TSMs are first reviewed, followed by analysis of topological semimetal states in Heusler compounds. In the end, the most suitable TSM state in Heusler compound is given.

In recent days, study of topological Weyl semimetals have become an active branch of physics and materials science because they led to realization of the Weyl fermions and exhibited protected Fermi arc surface states. Therefore, topological Weyl semimetals TaX (X = P, As) are important electronic systems to investigate both from the point of view of fundamental physics and potential applications. In this work, we have studied the structural, elastic, mechanical, electronic, bonding, acoustic, thermal and optical properties of TaX (X = P, As) in detail via first-principles method using the density functional theory. A comprehensive study of elastic constants and moduli shows that both TaP and TaAs possesses low to medium level of elastic anisotropy (depending on the measure), reasonably good machinability, mixed bonding characteristics with ionic and covalent contributions, brittle nature and relatively high Vickers hardness with a low Debye temperature and melting temperature. The minimum thermal conductivities and anisotropies of TaX (X = P, As) are calculated. Bond population analysis supports the bonding nature as predicted by the elastic parameters. The bulk electronic band structure calculations reveal clear semi-metallic features with quasi-linear energy dispersions in certain sections of the Brillouin zone near the Fermi level. A pseudogap in the electronic energy density of states at the Fermi level separating the bonding and the antibonding states indicates significant electronic stability of tetragonal TaX (X = P, As).The reflectivity spectra show almost non-selective behavior over a wide range of photon energy encompassing visible to mid-ultraviolet regions. High reflectivity over wide spectral range makes TaX suitable as reflecting coating. TaX (X = P, As) are very efficient absorber of ultraviolet radiation. Both the compounds are moderately optically anisotropic owing to the anisotropic nature of the electronic band structure. The refractive indices are very high in the infrared to visible range. All the energy dependent optical parameters show metallic features and are in complete accord with the underlying bulk electronic density of states calculations.

The nonlinear Hall effect (NLHE), the phenomenon in which a transverse voltage can be produced without a magnetic field, provides a potential alternative for rectification or frequency doubling1,2. However, the low-temperature detection of the NLHE limits its applications3,4. Here, we report the room-temperature NLHE in a type-II Weyl semimetal TaIrTe4, which hosts a robust NLHE due to broken inversion symmetry and large band overlapping at the Fermi level. We also observe a temperature-induced sign inversion of the NLHE in TaIrTe4. Our theoretical calculations suggest that the observed sign inversion is a result of a temperature-induced shift in the chemical potential, indicating a direct correlation of the NLHE with the electronic structure at the Fermi surface. Finally, on the basis of the observed room-temperature NLHE in TaIrTe4 we demonstrate the wireless radiofrequency (RF) rectification with zero external bias and magnetic field. This work opens a door to realizing room-temperature applications based on the NLHE in Weyl semimetals. Broken inversion symmetry in a type-II Weyl semimetal TaIrTe4 enables observation of the room-temperature nonlinear Hall effect as well as wireless radiofrequency rectification.

Topological Weyl semimetals have attracted considerable interest because they manifest underlying physics and device potential in spintronics. Large anomalous Hall effect (AHE) in non-collinear antiferromagnets (AFMs) represents a striking Weyl phase, which is associated with Bloch-band topological features. In this work, we report robust AHE and Lifshitz transition in high-quality Weyl semimetal Mn3Ge thin film, comprising stacked Kagome lattice and chiral antiferromagnetism. We successfully achieved giant AHE in our Mn3Ge film, with a strong Berry curvature enhanced by the Weyl phase. The enormous coercive field HC in our AHE curve at 5 K reached an unprecedented 5.3 T among hexagonal Mn3X systems. Our results provide direct experimental evidence of an electronic topological transition in the chiral AFMs. The temperature was demonstrated to play an efficient role in tuning the carrier concentration, which could be quantitatively determined by the two-band model. The electronic band structure crosses the Fermi energy level and leads to the reversal of carrier type around 50 K. The results not only offer new functionality for effectively modulating the Fermi level location in topological Weyl semimetals but also present a promising route of manipulating the carrier concentration in antiferromagnetic spintronic devices.

Weyl semimetals have drawn considerable attention for their exotic topological properties in many research fields. When in combination with s-wave superconductors, the supercurrent can be carried by their topological surface channels, forming junctions mimicking the behavior of Majorana bound states. Here, we present a transmon-like superconducting quantum intereference device (SQUID) consisting of lateral junctions made of Weyl semimetal Td-MoTe2 and superconducting leads of niobium nitride (NbN). The SQUID is coupled to a readout cavity made of molybdenum rhenium (MoRe), whose response at high power reveals the existence of the constituting Josephson junctions (JJs). The loop geometry of the circuit allows the resonant frequency of the readout cavity to be tuned by the magnetic flux. We demonstrate a JJ made of MoTe2 and a flux-tunable transmon-like circuit based on Weyl semimetals. Our study provides a platform to utilize topological materials in SQUID-based quantum circuits for potential applications in quantum information processing.

We theoretically propose and numerically demonstrate a radiative thermal router based on magnetic Weyl semimetals. In designing the thermal router, we utilize two unique properties of optical gyrot...

The Weyl semimetal (WSM), which hosts pairs of Weyl points and accompanying Berry curvature in momentum space near Fermi level, is expected to exhibit novel electromagnetic phenomena. Although the large optical/electronic responses such as nonlinear optical effects and intrinsic anomalous Hall effect (AHE) have recently been demonstrated indeed, the conclusive evidence for their topological origins has remained elusive. Here, we report the gigantic magneto-optical (MO) response arising from the topological electronic structure with intense Berry curvature in magnetic WSM Co3Sn2S2. The low-energy MO spectroscopy and the first-principles calculation reveal that the interband transitions on the nodal rings connected to the Weyl points show the resonance of the optical Hall conductivity and give rise to the giant intrinsic AHE in dc limit. The terahertz Faraday and infrared Kerr rotations are found to be remarkably enhanced by these resonances with topological electronic structures, demonstrating the novel low-energy optical response inherent to the magnetic WSM. The evidence of topological origin for the recently observed anomalous Hall effect remains elusive. Here, the authors report that the resonance of the optical Hall conductivity resulted from topological electronic structure gives rise to the large intrinsic anomalous Hall effect in the magnetic Weyl semimetal Co3Sn2S2.

The recently discovered topological quantum materials (TQMs) have electronic structures that can be characterized by certain topological invariants. In these novel materials, the unusual bulk and surface electrons not only give rise to many exotic physical phenomena but also foster potential new technological applications. To characterize the unusual electronic structures of these new materials, investigators have used angle-resolved photoemission spectroscopy (ARPES) as an effective experimental tool to directly visualize the unique bulk and surface electronic structures of TQMs. In this review, we first give a brief introduction of TQMs and ARPES, which is followed by examples of the application of ARPES to different TQMs ranging from topological insulators to Dirac and Weyl semimetals. We conclude with a brief perspective of the current development of ARPES and its potential application in the study of TQMs.

Weyl semimetal Td-WTe2 has attracted extensive attention due to its abundant physical properties. Further study on Td-WTe2 crystal structure is helpful for its understanding and modification in app...

Using scanning tunneling microscopy/spectroscopy (STM/STS), we examine quasiparticle scattering and interference properties at the surface of WTe2. WTe2, layered transition metal dichalcogenide, is predicted to be a type-II Weyl semimetal. The Weyl fermion states in WTe2 emerge as topologically protected touching points of electron and hole pockets, and Fermi arcs connecting them can be visible in the spectral function on the surface. To probe the properties of surface states, we have conducted low-temperature STM/STS (at 2.7 K) on the surfaces of WTe2 single crystals. We visualize the surface states of WTe2 with atomic scale resolution. Clear surface states emerging from the bulk electron pocket have been identified and their connection with the bulk electronic states shows good agreement with calculations. We show the interesting double resonance peaks in the local density of states appearing at localized impurities. The low-energy resonant peak occurs near the Weyl point above the Fermi energy and it may be mixed with the surface state of Weyl points, which makes it difficult to observe the topological nature of the Weyl semimetal WTe2.

A Weyl semimetal (WSM)\ is a three-dimensional topological phase of matter where pairs of nondegenerate bands cross at isolated points in the Brillouin zone called Weyl nodes. Near these points, the electronic dispersion is gapless and linear. A magnetic field $B$ changes this dispersion into a set of Landau levels which are dispersive along the direction of the magnetic field only. The $n=0$ Landau level is special since its dispersion$\ $is linear and unidirectional. The presence of this chiral level distinguishes Weyl from Schr\"{o}dinger fermions. In this paper, we study the quantum oscillations of the orbital magnetization and magnetic susceptibility in Weyl semimetals. We generalise earlier works% \cite{Mikitik2019} on these De Haas-Van Alphen oscillations by considering the effect of a tilt of the Weyl nodes. We study how the fundamental period of the oscillations in the small $B$ limit and the strength of the magnetic field $B_{1}$ required to reach the quantum limit are modified by the magnitude and orientation of the tilt vector $\mathbf{t}$. We show that the magnetization from a single node is finite in the $B\rightarrow 0$ limit. Its sign depends on the product of the chirality and sign of the tilt component along the magnetic field direction. We also study the magnetic oscillations from a pair of Weyl nodes with opposite chirality and with opposite or identical tilt. Our calculation shows that these two cases lead to a very different behavior of the magnetization in the small and large $B$ limits. We finally consider the effect of an energy shift $\pm \Delta _{0}$ of a pair of Weyl nodes on the magnetic oscillations. We assume a constant density of carriers so that both nodes share a common Fermi level. Our calculation can easily be extended to a WSM with an arbitrary number of pairs of Weyl nodes., Comment: 15 pages with 11 figures

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.

Topological optics is an emerging research area in which various topological and geometrical ideas are being proposed to design and manipulate the behaviors of photons. Here, the photonic spin Hall effect on the surfaces of topological Weyl semimetal (WSM) films was studied. Our results show that the spin-dependent splitting (i.e. photonic spin Hall shifts) induced by the spin-orbit interaction is little sensitive to the tilt αt of Weyl nodes and the chemical potential μ in type-I WSM film. In contrast, photonic spin Hall shifts in both the in-plane and transverse directions present versatile dependent behaviors on the αt and μ in type-II WSM film. In particular, the largest in-plane and transverse spin Hall shifts appear at the tilts between −2 and −3, which are ~40 and ~10 times of the incident wavelength, respectively. Nevertheless, the largest spin Hall shifts for type-II WSM film with positive αt are only several times of incident wavelength. Moreover, the photonic spin Hall shifts also exhibit different variation trends with decreasing the chemical potential for different signs of αt in type-II WSM films. This dependence of photonic spin Hall shifts on tilt orientation in type-II WSM films has been explained by time-reversal-symmetry-breaking Hall conductivities in WSMs.

Weyl semimetals (WSMs) exhibit an electronic structure governed by linear band dispersions and degenerate (Weyl) points that lead to exotic physical phenomena. While WSMs were established in bulk monopnictide compounds several years ago, the growth of thin films remains a challenge. Here, we report the bottom-up synthesis of single-crystalline NbP and TaP thin films, 9 to 70 nm thick, by means of molecular beam epitaxy. The as-grown epitaxial films feature a phosphorus-rich stoichiometry, a tensile-strained unit cell, and a homogeneous surface termination, unlike their bulk crystal counterparts. These properties result in an electronic structure governed by topological surface states as directly observed using in situ momentum photoemission microscopy, along with a Fermi-level shift of -0.2 eV with respect to the intrinsic chemical potential. Although the Fermi energy of the as-grown samples is still far from the Weyl points, carrier mobilities close to 103 cm2/(V s) have been measured at room temperature in patterned Hall-bar devices. The ability to grow thin films of Weyl semimetals that can be tailored by doping or strain, is an important step toward the fabrication of functional WSM-based devices and heterostructures.

Lasing behavior of optically active planar topological Weyl semimetal (TWS) is investigated in view of the Kerr and Faraday rotations. Robust topological character of TWS is revealed by the presence of Weyl nodes and relevant surface conductivities. We focus our attention on the surfaces where no Fermi arcs are formed, and thus Maxwell equations contain topological terms. We explicitly demonstrate that two distinct lasing modes arise because of the presence of effective refractive indices which lead to the birefringence phenomena. Transfer matrix is constructed in such a way that reflection and transmission amplitudes involve $2\times2$ matrix-valued components describing the bimodal character of the TWS laser. We provide associated parameters of the topological laser system yielding the optimal impacts. We reveal that gain values corresponding to the lasing threshold display a quantized behavior, which occurs due to topological character of the system. Our proposal is supported by the corresponding graphical demonstrations. Our observations and predictions suggest a concrete way of forming TWS laser and coherent perfect absorber; and are awaited to be confirmed by an experimental realization based on our computations., Comment: 17 pages, 9 figures, 3 tables

Based on ab initio calculations and the Wannier-based tight-binding method, we studied the topological electronic properties and strain modulation of transition metal monochalcogenides (TMM) Mo2XY (X, Y = S, Se, Te, X ≠ Y). These materials are nodal line semimetals in the absence of spin–orbit coupling (SOC). The presence of SOC turns them into Weyl semimetals with 24 Weyl nodes located in the kz ≠ 0 planes and related by time-reversal, rotation C3z, and mirror symmetries. The maximal separation between two neighboring Weyl points with opposite chirality is of the order of magnitude of 0.10 A−1, which can be readily accessed by angle-resolved photoemission spectroscopy (ARPES). The Weyl semimetal phase shows great robustness and demonstrates different responses under uniaxial and biaxial strain. Intriguingly, the location of the Weyl point changes significantly, resulting in a striking modulation of topological properties under in-plane biaxial strain. Our finding provides a realistic and promising platform for studying and manipulating the behavior of Weyl fermions in experiments.

Bulk 1T$^\prime$-MoTe$_2$ shows a structural phase transition from 1T$^\prime$ to Weyl semimetallic (WSM) $ T_{d} $ phase at $\sim$ 240 K. This phase transition and transport properties in the two phases have not been investigated on ultra-thin crystals. Here we report electrical transport, $1/f$ noise and Raman studies in ultra-thin 1T$^\prime$-MoTe$_2$ ($\sim$ 5 to 16 nm thick) field-effect transistors (FETs) devices as a function of temperature. The electrical resistivities for thickness 16 nm and 11 nm show maxima at temperatures 208 K and 178 K, respectively, making a transition from semiconducting to semi-metallic phase, hitherto not observed in bulk samples. Raman frequencies and linewidths for 11nm thick crystal show change around 178 K, attributed to additional contribution to the phonon self-energy due to enhanced electron-phonon interaction in the WSM phase. Further, the resistivity at low-temperature shows an upturn below 20 K along with the maximum in the power spectral density of the low frequency $1/f$ noise. The latter rules out the metal-insulator transition (MIT) being responsible for the upturn of resistivity below 20 K. The low temperature resistivity follows $\rho \propto 1/T$, changing to $\rho \propto T$ with increasing temperature supports electron-electron interaction physics at electron-hole symmetric Weyl nodes below 20 K. These observations will pave the way to unravel the properties of WSM state in layered ultra-thin van der Waals materials.

Recent progress in understanding the electronic band topology and emergent topological properties encourage us to reconsider the band structure of well-known materials including elemental substances. Controlling such a band topology by external field is of particular interest from both fundamental and technological viewpoints. Here we report possible signatures of the pressure-induced topological phase transition from a semiconductor to a Weyl semimetal in elemental tellurium probed by transport measurements. Pressure variation of the periods of Shubnikov–de Haas oscillations, as well as oscillation phases, shows an anomaly around the pressure theoretically predicted for topological phase transition. This behavior is consistent with the pressure-induced band deformation and resultant band-crossing effect. Moreover, effective cyclotron mass is reduced toward the critical pressure, potentially reflecting the emergence of massless linear dispersion. The present result paves the way for studying the electronic band topology in well-known compounds and topological phase transition by the external field.

The search for unconventional superconductivity in Weyl semimetal materials is currently an exciting pursuit, since such superconducting phases could potentially be topologically non-trivial and host exotic Majorana modes. The layered material TaIrTe4 is a newly predicted time-reversal invariant type II Weyl semimetal with the minimum number of Weyl points. Here, we report the discovery of surface superconductivity in Weyl semimetal TaIrTe4. Our scanning tunneling microscopy/spectroscopy (STM/STS) visualizes Fermi arc surface states of TaIrTe4 that are consistent with the previous angle-resolved photoemission spectroscopy results. By a systematic study based on STS at ultralow temperature, we observe uniform superconducting gaps on the sample surface. The superconductivity is further confirmed by electrical transport measurements at ultralow temperature, with an onset transition temperature (Tc) up to 1.54 K being observed. The normalized upper critical field h*(T/Tc) behavior and the stability of the superconductivity against the ferromagnet indicate that the discovered superconductivity is unconventional with the p-wave pairing. The systematic STS, and thickness- and angular-dependent transport measurements reveal that the detected superconductivity is quasi-1D and occurs in the surface states. The discovery of the surface superconductivity in TaIrTe4 provides a new novel platform to explore topological superconductivity and Majorana modes., Surface superconductivity detected in Weyl semimetals provides a new platform to explore topological superconductivity and Majorana modes.

Photo sensing and energy harvesting based on exotic properties of quantum materials and new operation principles have great potentials to break the fundamental performance limit of conventional photodetectors and solar cells. As topological nontrivial materials, Weyl semimetals have demonstrated novel optoelectronic properties that promise potential applications in photo detection and energy harvesting arising from their gapless linear dispersion near Weyl nodes and Berry field enhanced nonlinear optical effect at the vicinity of Weyl nodes. In this work, we demonstrate robust photocurrent generation from charge separation of photoexctied electron-hole pairs at the edge of Td-WTe2, a type-II Weyl semimetal, due to crystalline-symmetry breaking along certain crystal fracture directions and possibly enhanced by robust fermi-arc type surface states. Using scanning photocurrent microscopy (SPCM) measurements, we further demonstrate that the edge current response is robust over a wide excitation photon energy. We find that this robust feature is highly generic, and shall arise universally in a wide class of quantum materials with similar crystal symmetries. In addition, possible connections between these edge photocurrents and topological properties of Weyl semimetals are explored. The robust and generic edge current response demonstrated in this work provides a new type of charge separation mechanism for photosensing and energy harvesting over broad wavelength range.

Weyl semimetals exhibit interesting electronic properties due to their topological band structure. In particular, large anomalous Hall and anomalous Nernst signals are often reported, which allow f...

Theoretical calculations and experimental observations show MoTe2 is a type II Weyl semimetal, along with many members of transition metal dichalcogenides family. We have grown highly crystalline large-area MoTe2 thin films on Si/SiO2 substrates by chemical vapor deposition. Very uniform, continuous, and smooth films were obtained as confirmed by scanning electron microscopy and atomic force microscopy analyses. Measurements of the temperature dependence of longitudinal resistivity and current-voltage characteristics at different temperature are discussed. Unsaturated, positive quadratic magnetoresistance of the as-grown thin films has been observed from 10 K to 200 K. Hall resistivity measurements confirm the majority charge carriers are hole., 11 Pages, 7 Figures, This is published in Journal of Materials Research as a first view article. Volume, issue number are not yet assigned. That will be provided once those are available

An axion insulator is a correlated topological phase, which is predicted to arise from the formation of a charge-density wave in a Weyl semimetal1,2 -that is, a material in which electrons behave as massless chiral fermions. The accompanying sliding mode in the charge-density-wave phase - the phason - is an axion3,4 and is expected to cause anomalous magnetoelectric transport effects. However, this axionic charge-density wave has not yet been experimentally detected. Here we report the observation of a large positive contribution to the magnetoconductance in the sliding mode of the charge-density-wave Weyl semimetal (TaSe4)2I for collinear electric and magnetic fields. The positive contribution to the magnetoconductance originates from the anomalous axionic contribution of the chiral anomaly to the phason current, and is locked to the parallel alignment of the electric and magnetic fields. By rotating the magnetic field, we show that the angular dependence of the magnetoconductance is consistent with the anomalous transport of an axionic charge-density wave. Our results show that it is possible to find experimental evidence for axions in strongly correlated topological condensed matter systems, which have so far been elusive in any other context.

Magnetic Weyl semimetals Weyl semimetals (WSMs)—materials that host exotic quasiparticles called Weyl fermions—must break either spatial inversion or time-reversal symmetry. A number of WSMs that break inversion symmetry have been identified, but showing unambiguously that a material is a time-reversal-breaking WSM is tricky. Three groups now provide spectroscopic evidence for this latter state in magnetic materials (see the Perspective by da Silva Neto). Belopolski et al. probed the material Co 2 MnGa using angle-resolved photoemission spectroscopy, revealing exotic drumhead surface states. Using the same technique, Liu et al. studied the material Co 3 Sn 2 S 2 , which was complemented by the scanning tunneling spectroscopy measurements of Morali et al. These magnetic WSM states provide an ideal setting for exotic transport effects. Science , this issue p. 1278 , p. 1282 , p. 1286 ; see also p. 1248

Much of the dramatic growth in research on topological materials has focused on topologically protected surface states. While the domain walls of topological materials such as Weyl semimetals with broken inversion or time-reversal symmetry can provide a hunting ground for exploring topological interfacial states, such investigations have received little attention to date. Here, utilizing in-situ cryogenic transmission electron microscopy combined with first-principles calculations, we discover intriguing domain-wall structures in MoTe2, both between polar variants of the low-temperature(T) Weyl phase, and between this and the high-T high-order topological phase. We demonstrate how polar domain walls can be manipulated with electron beams and show that phase domain walls tend to form superlattice-like structures along the c axis. Scanning tunneling microscopy indicates a possible signature of a conducting hinge state at phase domain walls. Our results open avenues for investigating topological interfacial states and unveiling multifunctional aspects of domain walls in topological materials., 6 figures

All-electric magnetization manipulation at low power is a prerequisite for a wide adoption of spintronic devices. Materials such as heavy metals1–3 or topological insulators4,5 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 structure6,7; 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 coupling6,9 and non-trivial band structures10 with a large spin polarization protected by time-reversal symmetry in both the surface and bulk states9–11. 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. The Weyl semimetal WTe2 possesses strong spin–orbit coupling and time-reversal-protected spin polarization in surface and bulk states. In a WTe2/permalloy heterostructure, WTe2 can act as a spin current source that enables magnetization switching at low current densities.

Surface Fermi arcs (SFAs), the unique open Fermi-surfaces (FSs) discovered recently in topological Weyl semimetals (TWSs), are unlike closed FSs in conventional materials and can give rise to many exotic phenomena, such as anomalous SFA-mediated quantum oscillations, chiral magnetic effects, three-dimensional quantum Hall effect, non-local voltage generation and anomalous electromagnetic wave transmission. Here, by using in-situ surface decoration, we demonstrate successful manipulation of the shape, size and even the connections of SFAs in a model TWS, NbAs, and observe their evolution that leads to an unusual topological Lifshitz transition not caused by the change of the carrier concentration. The phase transition teleports the SFAs between different parts of the surface Brillouin zone. Despite the dramatic surface evolution, the existence of SFAs is robust and each SFA remains tied to a pair of Weyl points of opposite chirality, as dictated by the bulk topology., Surface Fermi arcs (SFAs) are characteristic features of a topological Weyl semimetal but there is no easy way to manipulate them so far. Here, the authors report manipulation of the shape, size and connections of SFAs in a Weyl semimetal NbAs, leading to an unusual topological Lifshitz transition.

Demonstration of van der Waals (vdW) semiconductor/metal heterostructures (SMHs) based on transition metal dichalcogenides has been a central approach in high-speed electronics by introducing ultrafast carrier dynamics. In this regard, a Weyl semimetal WTe2 is of great interest due to its vdW layered nature, low work function, and superior electrical properties. However, little is still known about its heterostructures, and a few picoseconds photocarrier lifetimes hinder its applications in high-speed electronics. Here, we propose a SMH: semimetallic Td phase WTe2 with its sister compound of semiconducting 2H phase MoTe2. Time-resolved terahertz (THz) spectroscopy demonstrated that WTe2 exhibited the significantly shorter carrier lifetimes of sub-picosecond when forming a junction with MoTe2. We provided explicit characteristic signatures, revealing charge transfer across the interface and the subsequent interlayer exciton decay. This work not only offers the extension of the THz detection scope of ultrafast phenomena from atomically thin materials but also provides a building block of vertical SMHs for high-speed electronic devices with sub-picosecond photocarrier lifetimes.

The discovery of topological insulators with unique metallic surface states has added a new dimension in the field of electronics and spintronics. Following the initial breakthrough, the topologically non-trivial materials have been extended to various categories namely weak topological insulator, topological crystalline insulator, Dirac semimetal and Weyl semimetal. Interestingly, most of the topological materials are also good candidates for thermoelectric energy conversion because both demand similar material features such as heavy constituent elements, narrow band gap, and high spin-orbit coupling. Heavy metal chalcogenides satisfy these criteria and present a nice platform for exploring both thermoelectrics and topological insulation. Here, we present a brief overview of both topological materials and thermoelectrics with examples based on heavy metal chalcogenides from the perception of a solid-state chemist. Last part of this review deals with the major challenges at the intersection of these two fields and possible solutions to establish the strong correlation between topological and thermoelectric materials.

Niobium Arsenic (NbAs), presenting some novel properties, is a high-profile topological material in condensed matter physics. Our study focuses on the NbAs after hydrogen (H) and helium (He) radiation. There are a variety of defects induced. We investigate into the electronic properties of defects by analyzing electronic dispersion curves and density of states based on the DFT method. Meanwhile the formation energy of these defects are calculated. The impacts of radiation are various in different defects. Defects of interstitial H and He atom do not affect the topological properties of NbAs at low defect concentration. However, the other defects destruct part of the Weyl points, especially in the defects of vacancy and substitution As for Nb. In addition, Fermi levels of these defects shift and the degenerate bands are separate. From the perspective of formation energy, the defect of H interstitial is the easiest one to be formed in all the defects.

A Weyl semimetal with strong electron-phonon interaction can show axionic coupling in its insulator state at low temperatures, owing to the formation of a charge density wave (CDW). Such a CDW emerges in the linear-chain-compound Weyl semimetal ${\mathrm{Ta}}_{2}{\mathrm{Se}}_{8}\mathrm{I}$ below 263 K, resulting in the appearance of the dynamical condensed-matter axion quasiparticle. In this paper, we demonstrate that the interchain coupling in ${\mathrm{Ta}}_{2}{\mathrm{Se}}_{8}\mathrm{I}$ can be varied to suppress the CDW formation with pressure, while retaining the Weyl semimetal phase at high temperatures. Above 17 GPa, the Weyl semimetal phase does not survive, and we induce superconductivity, due to the amorphization of the iodine sublattice. Structurally, the quasi-one-dimensional Ta-Se chains remain intact and provide a channel for superconductivity. We highlight that our results show a near-complete suppression of the gap induced by the axionic charge density wave at pressures inaccessible to previous studies. Including this CDW phase, our experiments and theoretical predictions and analysis reveal the complete phase diagram of ${\mathrm{Ta}}_{2}{\mathrm{Se}}_{8}\mathrm{I}$ and its relationship to the nearby superconducting state. The results demonstrate ${\mathrm{Ta}}_{2}{\mathrm{Se}}_{8}\mathrm{I}$ to be a distinctively versatile platform for exploring correlated topological states.

Recent works have provided evidence that an axial anomaly can arise in Weyl semimetals. If this is the case, then the electromagnetic response of Weyl semimetals should be governed by the equations of axion electrodynamics. These equations capture both the chiral magnetic and anomalous Hall effects in the limit of linear response, while at higher orders their solutions can provide detectable electromagnetic signatures of the anomaly. In this work, we consider three versions of axion electrodynamics that have been proposed in the Weyl semimetal literature. These versions differ in the form of the chiral magnetic term and in whether or not the axion is treated as a dynamical field. In each case, we look for solutions to these equations for simple sample geometries subject to applied external fields. We find that in the case of a linear chiral magnetic term generated by a non-dynamical axion, self-consistent solutions can generally be obtained. In this case, the magnetic field inside of the Weyl semimetal can be magnified significantly, providing a testable signature for experiments. Self-consistent solutions can also be obtained for dynamical axions, but only in cases where the chiral magnetic term vanishes identically. Finally, for a nonlinear form of the chiral magnetic term frequently considered in the literature, we find that there are no self-consistent solutions aside from a few special cases.

We investigate the surface plasmon polariton dispersion and optical spectra of a thin film of tilted Weyl semimetal. Tilted Weyl semimetals possess tilted Weyl cones at the Weyl nodes and are categorized to type-I with closed Fermi surfaces and type-II with overtilted Weyl cones and open Fermi surfaces. We find that the surface plasmon polariton dispersion of this system is nonreciprocal even in the absence of the external magnetic field. Moreover, we demonstrate that the tilt parameter has a profound effect in controlling this nonreciprocity. We reveal that the thin film of type-II Weyl semimetal hosts the surface plasmon polariton modes with the negative group velocity. Furthermore, we show that the angular optical spectra of this structure are highly asymmetric and this angular asymmetry in the absorptivity and reflectivity depends profoundly on the tilt parameter of the tilted Weyl semimetal. These exciting features propose employing the tilted Weyl semimetals in optical sensing devices, optical data storage, and devices for quantum information processing.

The physical realization of Chern insulators is of fundamental and practical interest, as they are predicted to host the quantum anomalous Hall (QAH) effect and topologically protected chiral edge states which can carry dissipationless current. Current realizations of the QAH state often require complex heterostructures and sub-Kelvin temperatures, making the discovery of intrinsic, high temperature QAH systems of significant interest. In this work we show that time-reversal symmetry breaking Weyl semimetals, being essentially stacks of Chern insulators with inter-layer coupling, may provide a new platform for the higher temperature realization of robust chiral edge states. We present combined scanning tunneling spectroscopy and theoretical investigations of the magnetic Weyl semimetal, Co3Sn2S2. Using modeling and numerical simulations we find that depending on the strength of the interlayer coupling, chiral edge states can be localized on partially exposed kagome planes on the surfaces of a Weyl semimetal. Correspondingly, our dI/dV maps on the kagome Co3Sn terraces show topological states confined to the edges which display linear dispersion. This work provides a new paradigm for realizing chiral edge modes and provides a pathway for the realization of higher temperature QAH effect in magnetic Weyl systems in the two-dimensional limit., Magnetic Weyl semimetals in the 2D limit may behave like 2D Chern insulators and host the quantum anomalous Hall effect at high temperatures. Here, the authors report the observation of linearly dispersing topological states confined to the edges of the kagome Co3Sn terraces in the magnetic Weyl system Co3Sn2S2.

$\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ has been proposed to have magnetic topological states as a potential Weyl semimetal. We synthesized single crystals of $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ and systematically investigated their structural and physical properties. $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ has an isostructural septuple-layered structure that is similar to the magnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ but possesses transpositional Mn and Sb atoms between the sublayers. Magnetic and specific-heat measurements revealed a ferrimagnetic phase transition with a Curie temperature ${T}_{\mathrm{C}}$ of \ensuremath{\sim}31 K in $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$, which originates from the interaction of the interexchanged ${\mathrm{Mn}}^{2+}$ ions. As the temperature decreases below ${T}_{\mathrm{C}}$, negative longitudinal magnetoresistance and anomalous Hall effect are observed, implying a non-negligible connection between the magnetism and expected transport carriers that may be driven by topological bands. Our results indicate that ferrimagnetic $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ provides insights for further studies on magnetic topological materials.

Since the discovery of the Harper-Hofstadter model, it has been known that condensed matter systems with periodic modulations can be promoted to non-trivial topological states with emergent gauge fields in higher dimensions. In this work, we develop a general procedure to compute the gauge fields in higher dimensions associated to low-dimensional systems with periodic (charge- and spin-) density wave modulations. We construct two-dimensional (2D) models with modulations that can be promoted to higher-order topological phases with $U(1)$ and $SU(2)$ gauge fields in 3D. Corner modes in our 2D models can be pumped by adiabatic sliding of the phase of the modulation, yielding hinge modes in the promoted models. We also examine a 3D Weyl semimetal (WSM) gapped by charge-density wave (CDW) order, possessing quantum anomalous Hall (QAH) surface states. We show that this 3D system is equivalent to a 4D nodal line system gapped by a $U(1)$ gauge field with a nonzero second Chern number. We explain the recently identified interpolation between inversion-symmetry protected phases of the 3D WSM gapped by CDWs using the corresponding 4D theory. Our results can extend the search for (higher-order) topological states in higher dimensions to density wave systems., Comment: v2: approximately published version, fixed typos and references, and added extensions to arbitrary orbital positions. 21+37 pages, 5+10 figures. v1: 20+33 pages, 5+10 figures

Noncentrosymmetric PrAlGe is considered to be a magnetic Weyl semimetal that breaks both time-reversal and inversion symmetry, in which the Weyl nodes can be moved by magnetization. However, the magnetic interaction of this system remains poorly understood. In this work we perform a systematical investigation on magnetic properties and critical behaviors in single-crystal PrAlGe. The temperature, field, and angle dependence of magnetization reveal a strong magnetic anisotropy along the $c$ axis and absolute isotropic characteristic in the $ab$ plane. By fitting to the magnetic entropy change $[\mathrm{\ensuremath{\Delta}}{S}_{M}(T,H)]$ around the critical temperature ${T}_{C}=16$ K, critical exponents $\ensuremath{\beta}=0.136(6), \ensuremath{\gamma}=1.801(7)$, and $\ensuremath{\delta}=14.2(6)$ are obtained. Based on the obtained critical exponents, $\mathrm{\ensuremath{\Delta}}{S}_{M}(T,H)$ and $M(T,H)$ curves are scaled into universality curves under the framework of universality principle. The critical behaviors and exponents suggest that the magnetic interaction in PrAlGe is of a two-dimensional Ising type, revealing a uniaxial magnetic interaction along the $c$ axis. However, the ordering moments are tilted from the $c$ axis, which causes antiferromagnetism in the $ab$ plane. The clarification of the magnetic interaction and magnetic structure is of significance for unveiling its interplay with topological properties in this system.

Nodal ring semimetals are a class of topological material characterized by a one-dimensional circular region of band crossing in momentum space. The presence of spin-orbit coupling, whether extrinsic or intrinsic, may change the parent nodal ring phase to a Weyl semimetal, Dirac semimetal, or topological insulator child phase. We investigate second harmonic generation and circular photogalvanic effect in the mid-infrared region of nodal ring materials where spin-orbit coupling produces a Weyl semimetal child phase (such as in $\mathrm{Zr}{\mathrm{Te}}_{5}$ and $\mathrm{Ca}{\mathrm{P}}_{3}$). Spin-orbit coupling breaks the symmetries protecting the nodal ring, inducing a nontrivial Berry curvature which gives rise to colossal photocurrents up to the order of ${10}^{3} \ensuremath{\mu}\mathrm{A}/{\text{V}}^{2}$ at the interband harmonic. Our results are found to be rather robust to parameters such as Fermi level, residual scattering rate, and the number of Weyl points. However, decreasing temperature tends to destroy the harmonic peaks and changing the nodal ring radius drastically alters the harmonic condition, shifting the peak frequency. Equivalent calculations and experiments have been carried out for intrinsic Weyl semimetals such as TaAs where the photocurrents calculated and observed were at least one order of magnitude smaller, highlighting that the parent nodal ring phase enhances these optical nonlinear phenomena.

Layered type-II Weyl semimetals, such as WTe2, MoTe2, and TaIrTe4 have been demonstrated as a supreme photodetection material with topologically enhanced responsivity and specific sensitivity to the orbital angular momentum of light. Toward future device applications with high performance and ultrafast response, it is necessary to understand the dynamical processes of hot carriers and transient electronic properties of these materials under photoexcitation. In this work, mid-infrared ultrafast spectroscopy is performed to study the dynamical evolution of the anisotropic response of TaIrTe4. The dynamical relaxation of photoexcited carriers exhibits three exponential decay components relating to optical/acoustic phonon cooling and subsequent heat transfer to the substrate. The ultrafast transient dynamics imply that TaIrTe4 is an ideal material candidate for ultrafast optoelectronic applications, especially in the long-wavelength region. The angle-resolved measurement of transient reflection reveals that the reflectivity becomes less anisotropic in the quasi-equilibrium state, indicating a reduction in the anisotropy of dynamical conductivity in presence of photoexcited hot carriers. The results are indispensable in material engineering for polarization-sensitive optoelectronics and high field electronics.

The intrinsic antiferromagnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ undergoes a metamagnetic transition in a $c$-axis magnetic field. It has been predicted that ferromagnetic $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ is an ideal Weyl semimetal with a single pair of Weyl nodes. Here we report measurements of quantum oscillations detected in the field-induced ferromagnetic phase of $\mathrm{Mn}{\mathrm{Bi}}_{2\ensuremath{-}x}{\mathrm{Sb}}_{x}{\mathrm{Te}}_{4}$, where Sb substitution tunes the majority carriers from electrons to holes. Single-frequency Shubnikov--de Haas oscillations were observed in a wide range of Sb concentrations ($0.54\ensuremath{\le}x\ensuremath{\le}1.21$). The evolution of the oscillation frequency and the effective mass shows reasonable agreement with the Weyl semimetal band structure of ferromagnetic $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ predicted by density functional calculations. Intriguingly, the quantum oscillation frequency shows a strong temperature dependence, indicating that the electronic structure depends sensitively on magnetism.

Weyl semimetals, a class of 3D topological materials, exhibit a unique electronic structure featuring linear band crossings and disjoint surface states (Fermi-arcs). While first demonstrations of topologically driven phenomena have been realized in bulk crystals, efficient routes to control the electronic structure have remained largely unexplored. Here, a dramatic modification of the electronic structure in epitaxially grown NbP Weyl semimetal thin films is reported, using in situ surface engineering and chemical doping strategies that do not alter the NbP lattice structure and symmetry, retaining its topological nature. Through the preparation of a dangling-bond-free, P-terminated surface which manifests in a surface reconstruction, all the well-known trivial surface states of NbP are fully suppressed, resulting in a purely topological Fermi-arc dispersion. In addition, a substantial Fermi-energy shift from -0.2 to 0.3 eV across the Weyl points is achieved by surface chemical doping, unlocking access to the hitherto unexplored n-type region of the Weyl spectrum. These findings constitute a milestone toward surface-state and Fermi-level engineering of topological bands in Weyl semimetals, and, while there are still challenges in minimizing doping-driven disorder and grain boundary density in the films, they do represent a major advance to realize device heterostructures based on Weyl physics.

Controlling the anomalous Hall effect (AHE) in magnetic topological materials is an important property. Because of the close relationship between anomalous Hall conductance (AHC) and topological band (strong Berry curvature), AHC can be effectively tuned by magnetic field combined with strong spin-orbit interaction and special band structure. In this work, we observed a magnetic field driving the nonmonotonic magnetic field dependence of anomalous Hall resistivity and the sign change in magnetic-field-induced Weyl semimetal HoPtBi. The tunable ranges of the AHC and the anomalous Hall angle are $\ensuremath{-}75\ensuremath{\sim}73\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Omega}}}^{--1}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{--1}$ and $\ensuremath{-}12.3\ensuremath{\sim}9.1%$, respectively. Anisotropic measurements identified the magnetic field is the key factor in controlling the additional Hall term sign. Further analysis indicated that it originated from the field-induced shift of the Weyl points via exchange splitting of bands near the Fermi level. The large tunable effect of the magnetic field on the electronic band structure provides a path to tune the topological properties in this system. These findings suggest that HoPtBi is a good platform for tuning the Berry phase and AHC with the magnetic field.

There is tremendous interest in measuring the strong electron–phonon interactions seen in topological Weyl semimetals. The semimetal NbIrTe4 has been proposed to be a Type-II Weyl semimetal with 8 pairs of opposite Chirality Weyl nodes which are very close to the Fermi energy. We show using polarized angular-resolved micro-Raman scattering at two excitation energies that we can extract the phonon mode dependence of the Raman tensor elements from the shape of the scattering efficiency versus angle. This van der Waals semimetal with broken inversion symmetry and 24 atoms per unit cell has 69 possible phonon modes of which we measure 19 modes with frequencies and symmetries consistent with Density Functional Theory calculations. We show that these tensor elements vary substantially in a small energy range which reflects a strong variation of the electron–phonon coupling for these modes.