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Emerging from the intricate interplay of topology and magnetism, the giant anomalous Hall effect (AHE) is the most known topological property of the recently discovered kagomé ferromagnetic Weyl semimetal Co3Sn2S2 with the magnetic Co atoms arranged on a kagomé lattice. Here it is reported that the AHE in Co3Sn2S2 can be fine‐tuned by an applied magnetic field orientated within ≈2° of the kagomé plane, while beyond this regime, it stays unchanged. Particularly, it can vanish in magnetic fields parallel to the kagomé plane and even decrease in magnetic fields collinear with the spin direction. This tunable AHE can be attributed to local spin switching enabled by the geometrical frustration of the magnetic kagomé lattice, revealing that spins in a kagomé ferromagnet change their switching behavior as the magnetic field approaches the kagomé plane. These results also suggest a versatile way to tune the properties of a kagomé magnet. [ABSTRACT FROM AUTHOR]

Weyl semimetals (WSMs) have recently attracted considerable research attention because of their remarkable optical and electrical properties. In this study, we investigate the near-field radiative heat transfer (NFRHT) between graphene-covered Weyl slabs, particularly focusing on the supported coupled surface plasmon polaritons (SPPs). Unlike bare Weyl slabs where the epsilon-near-zero (ENZ) effect contributes the most to the NFRHT, adding a monolayer graphene sheet yields coupled SPPs, i.e., the coupling of graphene SPPs (GSPPs) and Weyl SPPs (WSPPs), which dominates the NFRHT. The graphene sheet greatly suppresses the ENZ effect by compressing the parallel wavevector, thereby enabling the heat transfer coefficient (HTC) to be significantly changed. Further, for the graphene-covered magnetic Weyl slab configuration, an increase in the number of Weyl nodes suppresses the SPP coupling and ENZ effect, thereby weakening the NFRHT with a regulation ratio of 4.4 whereas an increase in the Fermi level slightly influences the NFRHT. Several typical heterostructures are also proposed for comparison, and results show that a mono-cell structure has the largest total HTC. Our findings will facilitate the understanding of surface plasmon-coupled radiative heat transfer and enable opportunities in energy harvesting and thermal management at the nanoscale based on WSM-based systems. [ABSTRACT FROM AUTHOR]

Fano resonance (FR) and the triggered nonreciprocal thermal radiation (NTR) in a Weyl semimetal (WSM)‐ incorporated Kretschmann configuration has been observed and analyzed. Employing the anisotropic permittivity tensor, the electromagnetic responses of the proposed layered structure have been studied, and we illustrate that FR and strong nonreciprocity is attributed to the coupling between the planar waveguide mode and surface plasmon polaritons in WSM. It is shown that absorptivity (α) is not consistent with emissivity (e) in the considered angular range, which confirms near‐complete violation of Kirchhoff's law with nonmagnetic WSM. To analyze the physical origin of NTR, we performed numerical calculations for the α, e, nonreciprocity (η), as well as the magnetic field inside each layer. The compre‐hensive investigation of η with regard to geometric parameters suggests great potential for the practical use of perfect absorption and NTR over a wide range of frequencies and layer thicknesses. [ABSTRACT FROM AUTHOR]

The LD conversion offers great potential in providing an additional degree of freedom that may be beneficial to novel polarization‐wavelength‐selective detectors and optical switching. However, conventional crystals with optical anisotropy typically show unidirectional LD over a broad wavelength range and the materials with LD conversion behavior remain scarce. Here, by combining angle‐resolved polarized optical absorption measurements and theoretical calculations, this study demonstrates that quasi‐1D Weyl semimetal (TaSe4)2I single crystal shows unique LD conversion phenomenon at the wavelength of 1064 nm. Meanwhile, (TaSe4)2I persists giant linear optical anisotropy due to the intrinsic feature of 1D [TaSe4] chains along the c‐axis. Additionally, the (TaSe4)2I nanoribbon exhibits anisotropic second harmonic generation (SHG). These results hold significant importance to facilitate the development of topological photonics and novel photonic devices applications based on topological semimetals. [ABSTRACT FROM AUTHOR]

Near-field radiative heat transfer (NFRHT) between electronic components has become an important factor in the development of high-performance optoelectronic devices. Here, a metamaterial containing graded particles is proposed and measured. Two NFRHT peaks are excited by the local surface plasmon mode and surface plasmon mode. The results show that the filling factor and gradient factor have important effects on the NFRHT properties of the metamaterial. These measurement results provide the experimental basis for the application of gradient metamaterials in optoelectronic devices. [ABSTRACT FROM AUTHOR]

The non-reciprocal behavior of defect modes of a one-dimensional photonic crystal with two Weyl-semimetal-based defects is investigate by applying the transfer matrix method. The transmission spectrum of the structure exhibits two distinct defect modes. The study shows that these defect modes can be adjusted by altering the thickness of the Weyl semimetal layers and the modulus of the axial vector $ \vec{b} $ b → for both forward- and backward-impinging cases. The peak transmission and frequency of the defect modes are found to be dependent on the thickness of the Weyl semimetal layers, the magnitude of the axial vector $ \vec{b} $ b → , and the impinging direction of the incoming beam. Additionally, the quality factor of the defect modes changes with the thickness of the Weyl semimetal layers and the magnitude of the axial vector $ \vec{b} $ b → but is largely unaffected by reversing the impinging direction of the incoming beam. [ABSTRACT FROM AUTHOR]

NbAs is a Weyl semimetal and belongs to the group of topological phases that exhibit distinct quantum and topological attributes. Topological phases have a fundamentally different response to external perturbations, such as magnetic fields. To obtain insights into the response of such phases to pressure, we conducted a comprehensive study on the pressure-induced electronic and structural transitions in NbAs. We used micro-X-ray diffraction (XRD) and micro-X-ray spectroscopy (XAS) techniques to elucidate the changes at different atomic and electronic length scales (local, medium, and bulk) as combined with theoretical calculations. High-pressure XRD measurements revealed a rather common compression behavior up to ~12 GPa that could be fitted to an equation of state formalism with a bulk modulus of K 0 = 179.6 GPa. Complementary Nb K-edge XAS data unveiled anomalies at pressure intervals of ~12–15 and ~25–26 GPa in agreement with previous literature data from XRD studies. We attribute these anomalies to a previously reported topological Lifshitz transition and the tetragonal-to-hexagonal phase transition, respectively. Analysis of EXAFS results revealed slight changes in the mean next-nearest neighbor distance Nb–As(1) (~2.6 Å) at ~15 GPa, while the second nearest neighboring bond Nb–Nb(1) (~3.4 Å) shows a pronounced anomaly. This indicates that the electronic changes across the Lifshitz transition are accommodated first in the medium-range atomic structure and then at the local range and bulk. The variances of these bonds show anomalous but progressive evolutions close to the tetragonal-to-hexagonal transition at ~25 GPa, which allowed us to derive the evolution of vibration properties in this material. We suggest a prominent displacive character of the I 4 1 m d → P 6 ¯ m 2 transition facilitated by phonon modes. [ABSTRACT FROM AUTHOR]

Weyl semimetal showing open-arc surface states is a prominent example of topological quantum matter in three dimensions. With the bulk-boundary correspondence present, nontrivial surface--bulk hybridization is inevitable but less understood. Spectroscopies have been often limited to verifying the existence of surface Fermi arcs, whereas its spectral shape related to the hybridization profile in energy--momentum space is not well studied. We present an exactly solvable formalism at the surface for a wide range of prototypical Weyl semimetals. The resonant surface state and the bulk influence coexist as a surface--bulk hybrid and are treated in a unified manner. Directly accessible to angle- resolved photoemission spectroscopy, we analytically reveal universal information about the system obtained from the spectroscopy of resonant topological states. We systematically find inhomogeneous and anisotropic singular responses around the surface--bulk merging borderline crossing Weyl points, highlighting its critical role in the Weyl topology. The response in scanning tunneling spectroscopy is also discussed. The results will provide much-needed insight into the surface--bulk-coupled physical properties and guide in-depth spectroscopic investigation of the nontrivial hybrid in many topological semimetal materials. [ABSTRACT FROM AUTHOR]

Weyl semimetals resulting from either inversion (P) or time-reversal (T) symmetry breaking have been revealed to show the record-breaking large optical response due to intense Berry curvature of Weyl-node pairs. Different classes of Weyl semimetals with both P and T symmetry breaking potentially exhibit optical magnetoelectric (ME) responses, which are essentially distinct from the previously observed optical responses in conventional Weyl semimetals, leading to the versatile functions such as directional dependence for light propagation and gyrotropic effects. However, such optical ME phenomena of (semi)metallic systems have remained elusive so far. Here, we show the large nonlinear optical ME response in noncentrosymmetric magnetic Weyl semimetal PrAlGe, in which the polar structural asymmetry and ferromagnetic ordering break P and T symmetry. We observe the giant second harmonic generation (SHG) arising from the P symmetry breaking in the paramagnetic phase, being comparable to the largest SHG response reported in Weyl semimetal TaAs. In the ferromagnetically ordered phase, it is found that interference between this nonmagnetic SHG and the magnetically induced SHG emerging due to both P and T symmetry breaking results in the magnetic field switching of SHG intensity. Furthermore, such an interference effect critically depends on the light-propagating direction. The corresponding magnetically induced nonlinear susceptibility is significantly larger than the prototypical ME material, manifesting the existence of the strong nonlinear dynamical ME coupling. The present findings establish the unique optical functionality of P-and T-symmetry broken ME topological semimetals. [ABSTRACT FROM AUTHOR]

Despite theoretical expectations for 2D Weyl semimetals (WSMs), realizing stable 2D topological semimetal states experimentally is currently a great challenge. Here, 2D WSM states achieved by a thickness‐dependent topological phase transition from 3D Dirac semimetal to 2D WSM in molecular‐beam‐epitaxy‐grown Bi0.96Sb0.04 thin films are reported. 2D weak anti‐localization (WAL) and chiral anomaly arise in the Bi0.96Sb0.04 films for thicknesses below ≈10 nm, supporting 2D Weyl semimetallic transport in the films. This is particularly evident from magnetoresistance (MR) measurements which show cusp structures at around B = 0, indicating WAL, and negative MR, typical of chiral anomaly, only for layers with thicknesses below ≈10 nm. The temperature dependencies of the dephasing length for various thicknesses are consistent with those of the MR. Analysis based on second harmonic generation, terahertz emission, Seebeck/Hall effects, Raman scattering, X‐ray diffraction, and X‐ray photoemission demonstrates that the Dirac‐ to Weyl‐semimetal phase transition for films thinner than ≈10 nm is induced by inversion‐symmetry breaking due to the lattice‐mismatch strain between the Bi0.96Sb0.04 film and substrate. The realization of 2D WSMs is particularly significant for applications in high‐speed electronics, spintronics, and quantum computations due to their high mobility, chiral spin, and topologically‐protected quantum qubits. [ABSTRACT FROM AUTHOR]

The advancement of 6G technology relies on the development of high-performance terahertz detectors that can operate at room temperature. These detectors are crucial for Internet of Things (IoT) applications, which require sensitive environmental sensing and efficient reception of 6G signals. One significant research focus is on detection technology with high responsiveness and low equivalent noise power for 6G signals, which experience high losses in the air. To meet the demand for ultra-sensitive detectors in 6G technology, we have employed several techniques. Firstly, we prepared a large area of Weyl-semimetal layer through magnetron sputtering. Secondly, we obtained a high-quality Weyl-semimetal active layer by carefully controlling the annealing conditions. Next, a thin nano-Au layer was introduced as a micro-cavity reflection layer to enhance the device's detection rate. Additionally, we incorporated an electromagnetic induction well to improve carrier confinement and enhance the detection sensitivity. This proposed high-performance terahertz detector, with its potential for industrial production, offers a valuable technical solution for the advancement of 6G technology. [ABSTRACT FROM AUTHOR]

The presence of π gauge flux enabled by positive and negative hopping amplitude can lead to Möbius bands, which was recently demonstrated in both realistic acoustic and photonic lattices, twisted at k = π. Here, we show that the artificial gauge flux configuration can be achieved by exploiting the interactions between photonic s and p orbital-like fundamental modes in circular and peanut-shaped waveguides, respectively. By manipulating the interplay between the gauge fields and the crystal symmetry, we show that breaking the primitive translation symmetry through lattice site dimerization and deformation can cause the original Dirac semimetal phase, characterized by a four-fold Dirac point at the Brillouin zone center, to transform into various topological phases. The designed photonic waveguide array supports topological phases such as Möbius insulator and Weyl-like semimetal phases. Noticeably different to the existing cases, we explicitly show that the twisting Möbius bands cross each other at k = 0 due to the lattice gauging with alternating sign, which results in distinct beam dynamics excitation. We also present Weyl-like flat-band edge states in such photonics waveguide arrays. Our results suggest that such s − p hybridized photonic waveguide array servers as a convenient and flexible platform for studying topological physics, particularly in simulating the effects of gauge field in alternative configuration. [ABSTRACT FROM AUTHOR]

This study presents a thorough analysis of the electronic structures of the TaPxAs1−x series of compounds, which are of significant interest due to their potential as topological materials. Using a combination of first principles and Wannier‐based tight‐binding methods, this study investigates both the bulk and surface electronic structures of the compounds for varying compositions (x = 0, 0.25, 0.50, 0.75, 1), with a focus on their topological properties. By using chirality analysis, (111) surface electronic structure analysis, and surface Fermi arcs analysis, it is established that the TaPxAs1−x compounds exhibit topologically nontrivial behavior, characterized as Weyl semimetals (WSMs). The effect of spin–orbit coupling (SOC) on the topological properties of the compounds is further studied. In the absence of SOC, the compounds exhibit linearly dispersive fourfold degenerate points in the first Brillouin zone (FBZ) resembling Dirac semimetals. However, the introduction of SOC induces a phase transition to WSM states, with the number and position of Weyl points (WPs) varying depending on the composition of the alloy. For example, TaP has 12 WPs in the FBZ. The findings provide novel insights into the electronic properties of TaPxAs1−x compounds and their potential implications for the development of topological materials for various technological applications. [ABSTRACT FROM AUTHOR]

Near-field radiative heat transfer has attracted increasing attention in recent years in the development and manufacturing of thermal devices analogous to the building blocks of current microelectronics. In this work, we study theoretically a near-field thermal transistor operating at room temperature. The source and drain were assumed as indium antimonide (InSb) plates, whereas the gate as a Weyl semimetal (WSM). Numerical results computed using the fluctuational electrodynamics framework indicate that the modulation and/or amplification of the heat flux in the considered transistor can be achieved by modifying the gate temperature, and by the action of an external magnetic field upon the system. Results obtained in this work make the proposed near-field thermal transistor a suitable candidate for the contactless devices for the heat flux control and thermal management at nanoscale. [ABSTRACT FROM AUTHOR]

The YbCoC2 compound, which crystallizes in a base-centered orthorhombic unit cell in the  A m m 2  space group CeNiC2 structure, is unique among Yb-based compounds due to the highest magnetic ordering temperature of  T N = 27  K. Magnetization measurements have made it possible to plot the H-T magnetic phase diagram and determine the magnetocaloric effect of this recently discovered high-temperature heavy-fermion compound, YbCoC2. YbCoC2 undergoes spin transformation to the spin-polarized state through a metamagnetic transition in an external magnetic field. The transition is found to be of the first order. The dependencies of magnetic entropy change  Δ S m (T) —have segments with positive and negative magnetocaloric effects for  Δ H ≤ 6  T. For  Δ H = 9  T, the magnetocaloric effect becomes positive, with a maximum  Δ S m (T)  value of 4.1 J (kg K)−1 at  T N  and a refrigerant capacity value of 56.6 J kg−1. [ABSTRACT FROM AUTHOR]

Weyl semimetal is a unique topological phase with topologically protected band crossings in the bulk and robust surface states called Fermi arcs. Weyl nodes always appear in pairs with opposite chiralities, and they need to have either time‐reversal or inversion symmetry broken. When the time‐reversal symmetry is broken the minimum number of Weyl points (WPs) is two. If these WPs are located at the Fermi level, they form an ideal Weyl semimetal (WSM). In this study, intrinsic ferromagnetic (FM) EuCd2As2 are grown, predicted to be an ideal WSM and studied its electronic structure by angle‐resolved photoemission spectroscopy, and scanning tunneling microscopy which agrees closely with the first principles calculations. Moreover, anomalous Hall conductivity and Nernst effect are observed, resulting from the non‐zero Berry curvature, and the topological Hall effect arising from changes in the band structure caused by spin canting produced by magnetic fields. These findings can help realize several exotic quantum phenomena in inorganic topological materials that are otherwise difficult to assess because of the presence of multiple pairs of Weyl nodes. [ABSTRACT FROM AUTHOR]

Rich nontrivial topological phases rooted in the interplay between magnetism and topology in the layered antiferromagnet EuCd2As2 have captured vast attention, especially the ideal Weyl semimetal state realized in the spin‐polarized ferromagnetic (FM) structure driven by a moderate external magnetic field. In this work, combining magnetization, magneto‐transport, and structure measurements under high pressure and first principles calculations, this study finds that the pressure can drive the in‐plane antiferromagnetic structure of EuCd2As2 across an intermediate in‐plane FM structure then into the out‐of‐plane FM structure. This study also finds butterfly‐shaped MR and anomalous Hall effect under large pressure, which may support the pressure‐driven FM state. High‐pressure angle‐dispersive X‐ray diffraction and X‐ray absorption near‐edge spectroscopy measurements exclude structure transition and/or change of Eu2+ valence state as sources for the magnetic phase transitions. Alternatively, apparently reduced axial ratio (c/a) and compressed Eu‐layer space distance should play important roles. Interestingly, the calculations unveil that the out‐of‐plane FM structure hosts only one pair of Weyl nodes around the Fermi level, suggesting that pressure can be an alternative way to realize the ideal Weyl semimetal state in EuCd2As2 and will be useful for exploring exotic topological properties in such layered magnetic topological phase with strongly competing magnetic exchanges. [ABSTRACT FROM AUTHOR]

The itinerant 4 d ferromagnetic perovskite SrRuO3 [bulk Curie temperature (T C)Â =Â 165 K] has been studied extensively for many decades because of the unique nature of its ferromagnetism, metallicity, chemical stability, and compatibility with other perovskite-structured oxides. Recently, SrRuO3 has been gathering renewed interest as a magnetic Weyl semimetal and a two-dimensional ferromagnetic system. Ultra-high-quality SrRuO3 film growth techniques, combining oxide molecular beam epitaxy technology and a statistical machine learning method, have revealed new physics and physical properties of SrRuO3 as a magnetic Weyl semimetal, such as quantum transport of Weyl fermions and high-mobility two-dimensional carriers from surface Fermi arcs. This review summarizes the methods of growing ultra-high-quality SrRuO3 films and the novel physics found in them. In addition, progress in crystal structure analyses and the electrical and magnetic properties of SrRuO3 over the last decade will also be discussed. [ABSTRACT FROM AUTHOR]

We present polarized Raman spectroscopy of the topological semimetal Co3Sn2S2, which was recently shown to host a Weyl semimetal phase. Stokes Raman spectra were obtained with the incident light parallel to the c‐axis of Co3Sn2S2. Two major phonon Raman peaks were observed at 289 and 386 cm−1 over continuous background emission signals. The intensity of the low‐wavenumber (289 cm−1) peak showed no polarization dependence. The high‐wavenumber (386 cm−1) peak and the continuous background signal were strongly polarized in the incident light polarization direction. These responses were almost independent of the in‐plane crystal orientation to the incident polarization, as is the manifestation of the D3d point group symmetry of the unit cell of Co3Sn2S2. According to the group theory and Raman tensor analyses, the low‐ and high‐wavenumber Raman signals are attributed to Γ point phonon modes with Eg and A1g symmetries, respectively. Furthermore, line shape analyses revealed that the high‐wavenumber A1g mode exhibited asymmetric peak feature well described by the Breit–Wigner–Fano function. These results suggest the Fano resonance between the A1g phonon scattering with the continuous electronic background associated with low energy excitations near the Fermi energy. The clarified phonon energies and symmetries, as well as the electronic contribution to the Raman scattering, will not only be useful as a fingerprint to readily verify the experimentally grown or theoretically calculated crystal structure but also suggest importance of Raman spectroscopy as an effective tool to study low energy excitations and their interactions in Co3Sn2S2. [ABSTRACT FROM AUTHOR]

Topology is now securely established as a means to explore and classify electronic states in crystalline solids. This review provides a gentle but firm introduction to topological electronic band structure suitable for new researchers in the field. I begin by outlining the relevant concepts from topology, then give a summary of the theory of non-interacting electrons in periodic potentials. Next, I explain the concepts of the Berry phase and Berry curvature, and derive key formulae. The remainder of the article deals with how these ideas are applied to classify crystalline solids according to the topology of the electronic states, and the implications for observable properties. Among the topics covered are the role of symmetry in determining band degeneracies in momentum space, the Chern number and Z 2 topological invariants, surface electronic states, two- and three-dimensional topological insulators, and Weyl and Dirac semimetals [ABSTRACT FROM AUTHOR]

In this work, we theoretically investigate the superluminal and subluminal reflected group delay in an air-Weyl semimetal film-Weyl semimetal substrate layered structure by using the $4\times $ 4 transfer matrix method. We show a tunable transition between positive and negative group delays of reflection pulse in this layered structure by controlling the properties of the Weyl Semimetal layer, and we also reveal its mechanism to modify the propagation properties of the light pulse reflected from this structure. It is demonstrated that the reflected group delays are tunable from positive to negative values and vice versa, which are adjusted by tuning the incident angle, thickness of Weyl semimetal film, tilt parameter, and Fermi energy. The control of the magnitude and the sign of the reflected group delay of light propagation represent a key point in slow and fast light technologies. Our results are helpful in controlling pulse propagations and are useful for the design of Weyl semimetal-based delay devices. [ABSTRACT FROM AUTHOR]

High-performance terahertz wave detectors at room temperature are still urgently required for a wide range of applications. The available technologies, however, are plagued by low sensitivity, narrow spectral bandwidth, complicated structure, and high noise equivalent power (NEP). Here, we have demonstrated a Weyl semimetal surface plasmon-enhanced high-performance terahertz wave detectors which are based on microdisk array deposited WTe2 nanofilm epitaxially grown on GaN substrate for room temperature operation. With the microdisk array combined the WTe2 layer, strong terahertz wave surface plasmon polaritons can be generated at the WTe2–air interfaces, which results in significant improvement in detecting performance. For the 40 μm diameter microdisk array, a detectivity (D*) of 5.52 × 1012 cm Hz1/2 pW−1 at 0.1 THz is achieved at room temperature. In addition, the responsivity (RA) of 8.78 A W−1 is also obtained. Such high-performance millimeter and terahertz wave photodetectors are useful for wide applications such as high capacity communications, walk-through security, biological diagnosis, spectroscopy, and remote sensing. [ABSTRACT FROM AUTHOR]

The characteristic energies of elementary excitations and collective modes in many quantum materials lie mostly in the terahertz (THz) frequency range, which provides a wide space for the development of THz optical materials and devices. In particular, topological Weyl semimetal Mn3Sn is a noncollinear antiferromagnet with anomalous THz properties, which is strongly affected by thermal energy and external magnetic fields. Despite the explosive growth of the research on magnetic Weyl semimetals recently, its nanoscale structure for applications in THz optical devices remains to be explored. Here, we fabricated nanostructured Mn3+0.53Sn films with different crystal orientations, driven by Ostwald Ripening (OR) on (0001) c-Al2O3. A huge anisotropic THz response manifested a firm link between the optical properties of Weyl antiferromagnet Mn3Sn and its contrivable spin structures. The topological properties of Mn3Sn are robustly protected in its nanostructures. This work can provide a new horizon for the fabrication of a nanostructured magnetic Weyl alloy and for its potential applications in subwavelength high-performance THz devices. [ABSTRACT FROM AUTHOR]

We consider the semiclassical quantization condition for the energy of an electron in a magnetic field in the case when the electron orbit lies on a Fermi-surface pocket surrounding the Weyl point of a topological semimetal and analyze the constant γ appearing in this condition. It is shown that this constant has the universal value, γ = 0, independent on the tilt of the Weyl spectrum. Since the constant γ for an extremal cross section of the Fermi surface determines the phase of quantum oscillations, this result explains why measurements of the phase permit one to find Weyl points in crystals even though the extremal cross section of the pocket does not pass through this point, and the appropriate Berry phase of the orbit differs from π. [ABSTRACT FROM AUTHOR]

Weyl semimetal-based photodetectors have attracted great attention due to their high performance in fast, self-powered, and ultra-broad-band photodetection. However, the inherent large dark current of the semimetal hinders further improvement of their performance. Thus, it is urgent to utilize a van der Waals (vdW) heterojunction strategy to effectively decrease the dark current and separate the carriers. Herein, a vertical Schottky junction photodetector based on Weyl semimetal TaIrTe4 and n-Si nanostructures has been studied. The junction presents a high photoresponsivity of 910 mA W–1, a specific detectivity of ∼1.04 × 1011 jones, and a fast response speed of 15.1/18.2 μs under 808 nm irradiation. Furthermore, a stable and reproducible broad-band detection (325–2000 nm) is achieved, due to the efficient NIR light absorption of TaIrTe4. In particular, the device presents impressive responsivities of 14/1.32/0.45 mA W–1 under 1310/1550/2000 nm light, respectively. Notably, these excellent performances of the TaIrTe4/Si nanostructures are superior to those of most of the previously explored 2D materials/Si-based devices and are comparable to those of several commercial silicon photodiode sensors. It is believed that the above results can provide ideas for the research of Weyl semimetals in the application of high-performance nanoscale optoelectronic devices. [ABSTRACT FROM AUTHOR]

The Berry curvature in Weyl semimetal (WSM) induces chiral surface states through Fermi arcs, giving rise to an anomalous intrinsic Hall response. The Weyl semimetals are conducting in the bulk; we study the surface-bulk scattering mechanism and build an original Boltzmann equation for the surface electron transport in Weyl semimetals. We derive a general formula for the anomalous Hall conductivity (AHC) as a function of chemical potential, surface-bulk coupling strength and sample size, which is valid for both diffusive and ballistic limits. The ballistic limit is beyond the Green's function as the ladder approximation deteriorates at small chemical potential, showing unique advantages of our Boltzmann method. Moreover, we investigate the energy dependence of anomalous Hall conductivity and find it non-monotonic with temperature. [ABSTRACT FROM AUTHOR]

Weyl points, which are the degenerate points in three-dimensional momentum space, have been widely studied in the photonic system, and show some intriguing phenomena such as topologically protected surface states and chiral anomalies. Type-I Weyl systems possess a complete bandgap, and topologically protected surface states can be excited without disturbing the bulk states. In this work, we investigate the influence of the sign of coupling coefficient on the topological property of the system and find that type-I Weyl points can be realized by introducing a negative coupling between the stacking layers of the designed photonic crystal. We propose a new strategy to construct a type-I Weyl system by stacking the hexagonal photonic lattice. Different from the topological nontrivial photonic system with a positive coefficient, the negative couplings in the photonic system are realized by adding another resonating site between stacking layers. We theoretically demonstrate that the effective coupling between the resonating sites in adjacent layers sign-flips through the judicious design of the nearest coupling strength and eigenfrequency of the additional sites. The surface states at opposite boundaries of the proposed system have opposite group velocities, which is the feature of type-I Weyl points. Our study provides a new method of exploring topologically protected photonic systems and developing possible topological devices. [ABSTRACT FROM AUTHOR]

Co3Sn2S2 is a recently identified magnetic Weyl semimetal in Shandite compounds. Upon cooling, Co3Sn2S2 undergoes a ferromagnetic transition with c-axis polarized moments (∼0.3 µB/Co) around TC = 175 K, followed by another magnetic anomaly around TA ≈ 140 K. A large intrinsic anomalous Hall effect is observed in the magnetic state below TC with a maximum of anomalous Hall angle near TA. Here, we report an elastic neutron scattering on the crystalline lattice of Co3Sn2S2 in a magnetic field up to 10 T. A strongly anisotropic magnetoelastic response is observed-while only a slight enhancement of the Bragg peaks is observed when B//c. The in-plane magnetic field (B//ab) dramatically suppresses the Bragg peak intensity probably by tilting the moments and lattice toward the external field direction. The in-plane magnetoelastic response commences from TC, and as it is further strengthened below TA, it becomes nonmonotonic against the field between TA and TC because of the competition from another in-plane magnetic order. These results suggest that a magnetic field can be employed to tune the Co3Sn2S2 lattice and its related topological states. [ABSTRACT FROM AUTHOR]

A magnetic Weyl semimetal is a recent focus of extensive research as it may exhibit large and robust transport phenomena associated with topologically protected Weyl points in momentum space. Since a magnetic texture provides a handle for the configuration of the Weyl points and its transport response, understanding of magnetic dynamics forms the basis for future control of a topological magnet. Mn3Sn is an example of an antiferromagnetic Weyl semimetal that exhibits a large response comparable to the one observed in ferromagnets despite a vanishingly small magnetization. The noncollinear spin order in Mn3Sn can be viewed as a ferroic order of cluster magnetic octupole and breaks the time‐reversal symmetry, stabilizing Weyl points and the significantly enhanced Berry curvature near the Fermi energy. Herein, the first observation of time‐resolved octupole oscillation in Mn3Sn is reported. In particular, the giant effective damping of the octupole dynamics is found, and it is feasible to conduct an ultrafast switching at <10 ps, a hundred times faster than the case of spin‐magnetization in a ferromagnet. Moreover, high domain wall velocity over 10 km s−1 is theoretically predicted. This work paves the path toward realizing ultrafast electronic devices using the topological antiferromagnet. [ABSTRACT FROM AUTHOR]

Developing a technology to electrically manipulate a Weyl semimetal state is a vital step for designing a nonvolatile memory using topologically robust properties. Recently, such manipulation is realized for the first time in the antiferromagnetic Weyl semimetal Mn3Sn using the readout signal of anomalous Hall effect in the Mn3Sn/heavy metal (Pt, W) heterostructures. Here, it is reported that the switching of Hall signal can be significantly enhanced by 1) removing the buffer layer of Ru to adjust the crystal orientation of Mn3Sn, and 2) annealing after deposition of the heavy metal to change the interfacial condition. The switching of the Hall resistance is 0.35 Ω in the Mn3Sn/W sample, which becomes one order of magnitude larger than the previously reported value using Ru/Mn3Sn/Pt heterostructures. Moreover, by increasing the read current, it is found that the readout voltage may go well beyond 1 mV, a milestone for future applications in memory technology. [ABSTRACT FROM AUTHOR]

The discovery of type‐II Weyl semimetal states in layered transition metal dichalcogenides Td‐WTe2 indicates great potential for novel electronic, spintronic, and quantum devices. Theoretically, the interaction between the topological states and the magnetic ordered states of Td‐WTe2 enables the modulation of Weyl semimetal states by an external magnetic field. However, currently, ferromagnetism in layered Td‐WTe2 is still elusive and rarely observed. In this research, ferromagnetic order into WTe2 using magnetic chromium (Cr) doping with a two‐step Te flux strategy is introduced. The Curie temperature (Tc) and the ferromagnetic moment are well tuned with a Cr doping concentration. The Tc of the Cr‐doped layered Td‐WTe2 could be regulated from 182 to 283 K, which is close to room temperature. The saturation magnetic moment could be changed from 2.26 to 4.20 emu g–1, which is stronger than most values reported for these materials. Most intriguingly, the Cr‐doped layered Td‐WTe2 single crystals still exhibit semimetallic behavior and they possess very large magnetoresistance with obvious Shubnikov de Haas (SdH) quantum oscillations and an anomalous Hall effect. The findings offer feasible ways to induce and tune ferromagnetic orders in layered Td‐WTe2 and thus to control its topological phase with external magnetic fields. [ABSTRACT FROM AUTHOR]

Nontrivial topology in condensed-matter systems enriches quantum states of matter to go beyond either the classification into metals and insulators in terms of conventional band theory or that of symmetry-broken phases by Landau's order parameter framework. So far, focus has been on weakly interacting systems, and little is known about the limit of strong electron correlations. Heavy fermion systems are a highly versatile platform to explore this regime. Here we report the discovery of a giant spontaneous Hall effect in the Kondo semimetal Ce3Bi4Pd3 that is noncentrosymmetric but preserves time-reversal symmetry. We attribute this finding to Weyl nodes--singularities of the Berry curvature--that emerge in the immediate vicinity of the Fermi level due to the Kondo interaction. We stress that this phenomenon is distinct from the previously detected anomalous Hall effect in materials with broken time-reversal symmetry; instead, it manifests an extreme topological response that requires a beyond-perturbation-theory description of the previously proposed nonlinear Hall effect. The large magnitude of the effect in even tiny electric and zero magnetic fields as well as its robust bulk nature may aid the exploitation in topological quantum devices. [ABSTRACT FROM AUTHOR]

We discuss the magnetic and topological properties of bulk crystals and quasi-two-dimensional (quasi-2D) thin films formed by stacking intrinsic magnetized topological insulator (for example, Mn (SbxBi1-x)2X4 with X = Se,Te) septuple layers and topological insulator quintuple layers in arbitrary order. Our analysis makes use of a simplified model that retains only Dirac cone degrees of freedom on both surfaces of each septuple or quintuple layer. We demonstrate the model's applicability and estimate its parameters by comparing with ab initio density-functional theory (DFT) calculations. We then employ the coupled Dirac cone model to provide an explanation for the dependence of thin-film properties, particularly the presence or absence of the quantum anomalous Hall effect, on film thickness, magnetic configuration, and stacking arrangement, and to comment on the design ofWeyl superlattices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

WTe2, as a type-II Weyl semimetal, has 2D Fermi arcs on the (001) surface in the bulk and 1D helical edge states in its monolayer. These features have recently attracted wide attention in condensed matter physics. However, in the intermediate regime between the bulk and monolayer, the edge states have not been resolved owing to its closed band gap which makes the bulk states dominant. Here, we report the signatures of the edge superconductivity by superconducting quantum interference measurements in multilayer WTe2 Josephson junctions and we directly map the localized supercurrent. In thick WTe2 (⁠|$\sim 60{\rm{\ nm}})$|⁠ , the supercurrent is uniformly distributed by bulk states with symmetric Josephson effect (⁠|$| {I_c^ + (B)} | {=} | {I_c^ - (B)} |\ $|⁠). In thin WTe2 (10 nm), however, the supercurrent becomes confined to the edge and its width reaches up to |$1.4{\rm{\ \mu m\ }}$| and exhibits non-symmetric behavior |$| {I_c^ + (B)} | \ne | {I_c^ - (B)} |$|⁠. The ability to tune the edge domination by changing thickness and the edge superconductivity establishes WTe2 as a promising topological system with exotic quantum phases and a rich physics. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The holographic duality allows to construct and study models of strongly coupled quantum matter via dual gravitational theories. In general such models are characterized by the absence of quasiparticles, hydrodynamic behavior and Planckian dissipation times. One particular interesting class of quantum materials are ungapped topological semimetals which have many interesting properties from Hall transport to topologically protected edge states. We review the application of the holographic duality to this type of quantum matter including the construction of holographic Weyl semimetals, nodal line semimetals, quantum phase transition to trivial states (ungapped and gapped), the holographic dual of Fermi arcs and how new unexpected transport properties, such as Hall viscosities arise. The holographic models promise to lead to new insights into the properties of this type of quantum matter. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 (T c) up to 1.54 K being observed. The normalized upper critical field h *(T/T c) 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

As paradigmatic phenomena, Hall effects have inspired tremendous studies of symmetry and topology in condensed matter physics. Intriguingly, a second-order nonlinear Hall effect was recently proposed in noncentrosymmetric materials even in the presence of time-reversal symmetry. This effect originates from the Berry curvature dipole of electronic band structures and was predicted in Dirac and Weyl materials such as transition metal dichalcogenides and Weyl semimetals. Although it has been experimentally verified in some transition metal dichalcogenides, no obvious observation of such effects has been reported in Weyl semimetals. Taking advantage of the ideal Weyl phase with no coexisting trivial bands at the Fermi level, we use strained HgTe as a concrete example to show that the ideal Weyl semimetal is a promising platform for demonstrating the nonlinear Hall effect. Based on numerical calculations of the Berry curvature dipole, it was found that the magnitude of nonlinear Hall effect can be simply engineered by in-plane strain. Our work provides a versatile platform with high tunability, which could greatly facilitate the study of nonlinear Hall effect in three-dimensional topological materials. [ABSTRACT FROM AUTHOR]

The BiSb alloy is of special significance m topological insulator research. Here we focus on Bi0.96 Sb0.04 alloy m which the conduction band edge just touches the valence band edge. First increasing and then decreasing of the resistance of Bi0.96 Sb0.04alloy shows its typical semimetal behavior. Transport measurement shows that the Shubnikov de Haas oscillation occurs m longitudinal magnetoresistance and transverse Hall resistance with opposite phase. Further investigation of longitudinal magnetoresistance (MR) shows that when magnetic field B is along with the electric current, the longitudinal magnetoresistance is negative at some magnetic field from 0.5 to 1.5 T. which is resulted from the chiral anomaly of Weyl fermions splitted from dirac cone by the magnetic field. More interestingly, the negative part of the longitudinal MR is not just a monotonic decrease, but with a kind of oscillation. This could be an approach for acquiring Weyl Fermions and Weyl semimetals m Bi0.96 Sb0.04 single crystal alloy. [ABSTRACT FROM AUTHOR]

We review recent advances in strongly correlated oxides as thermoelectric materials in pursuit of energy harvesting. We discuss two topics: one is the enhancement of the ordinary thermoelectric properties by controlling orbital degrees of freedom and orbital fluctuation not only in bulk but also at the interface of correlated oxides. The other topic is the use of new phenomena driven by spin-orbit coupling (SOC) of materials. In 5d electron oxides, we show some SOC-related transport phenomena, which potentially contribute to energy harvesting. We outline the current status and a future perspective of oxides as thermoelectric materials. [ABSTRACT FROM AUTHOR]

Topological Weyl semimetals (TWSs) with pairs of Weyl points and topologically protected Fermi arc states have broadened the classification of topological phases and provide superior platform for study of topological superconductivity. Here we report the nontrivial superconductivity and topological features of sulfurdoped Td-phase MoTe2 with enhanced Tc compared with type-II TWS MoTe2. It is found that Td-phase S-doped MoTe2 (MoTe2-xSx, x~0.2) is a two-band s-wave bulk superconductor (~0.13 meV and 0.26 meV), where the superconducting behavior can be explained by the s+- pairing model. Further, measurements of the quasi-particle interference (QPI) patterns and a comparison with band-structure calculations reveal the existence of Fermi arcs in MoTe2-xSx . More interestingly, a relatively large superconducting gap (~1.7 meV) is detected by scanning tunneling spectroscopy on the sample surface, showing a hint of topological nontrivial superconductivity based on the pairing of Fermi arc surface states. Our work demonstrates that the Td-phase MoTe2-xSx is not only a promising topological superconductor candidate but also a unique material for study of s+- superconductivity. [ABSTRACT FROM AUTHOR]

Topological materials ranging from topological insulators to Weyl and Dirac semimetals form one of the most exciting current fields in condensed-matter research. Many half-Heusler compounds, RPtBi (R = rare earth), have been theoretically predicted to be topological semimetals. Among various topological attributes envisaged in RPtBi, topological surface states, chiral anomaly, and planar Hall effect have been observed experimentally. Here, we report an unusual intrinsic anomalous Hall effect (AHE) in the antiferromagnetic Heusler Weyl semimetal compounds GdPtBi and NdPtBi that is observed over a wide temperature range. In particular, GdPtBi exhibits an anomalous Hall conductivity of up to 60 Ω-1⋅cm-1 and an anomalous Hall angle as large as 23%. Muon spin-resonance (μSR) studies of GdPtBi indicate a sharp antiferromagnetic transition (TN) at 9 K without any noticeable magnetic correlations above TN. Our studies indicate that Weyl points in these half-Heuslers are induced by a magnetic field via exchange splitting of the electronic bands at or near the Fermi energy, which is the source of the chiral anomaly and the AHE. [ABSTRACT FROM AUTHOR]

Insulating states can be topologically nontrivial, a well-established notion that is exemplified by the quantum Hall effect and topological insulators. By contrast, topological metals have not been experimentally evidenced until recently. In systems with strong correlations, they have yet to be identified. Heavy-fermion semimetals are a prototype of strongly correlated systems and, given their strong spin-orbit coupling, present a natural setting to make progress. Here, we advance a Weyl-Kondo semimetal phase in a periodic Anderson model on a noncentrosymmetric lattice. The quasiparticles near the Weyl nodes develop out of the Kondo effect, as do the surface states that feature Fermi arcs. We determine the key signatures of this phase, which are realized in the heavy-fermion semimetal Ce3Bi4Pd3. Our findings provide the much-needed theoretical foundation for the experimental search of topological metals with strong correlations and open up an avenue for systematic studies of such quantum phases that naturally entangle multiple degrees of freedom. [ABSTRACT FROM AUTHOR]

Motivated by the chiral anomaly steering negative longitudinal magnetoresistance in GdBiPt under external magnetic field, we studied the electronic structures of GdBi with paramagnetism, antiferromagnetism and ferromagnetism by first-principles calculations with modified Becke and Johnson local density approximation plus Hubbard . Our calculated results reveal that paramagnetic GdBi is semiconducting, while the antiferromagnetic GdBi is a topological nontrivial compensation metal. We also predict the presence of a pair of Weyl fermions in ferromagnetic GdBi and GdSb. The band crossing along the direction of magnetization is protected by the fourfold rotation symmetry, and the topological charge associating with each band crossing point is . [ABSTRACT FROM AUTHOR]

Generalized Dirac monopoles in momentum space are constructed in even dimensions from the Weyl Hamiltonian in terms of Green's functions. In dimensions, the (unit) charge of the monopole is equal to both the winding number and the Chern number, expressed as the integral of the Berry curvature. Based on the equivalence of the Chern and winding numbers, a chirally coupled and Lorentz invariant field theory action is studied for the Weyl semimetal phase. At the one loop order, the effective action yields both the chiral magnetic effect and the anomalous Hall effect. The Chern number appears as a coefficient in the conductivity, thus emphasizes the role of topology. The anomalous contribution of chiral fermions to transport phenomena is reflected as the gauge anomaly with the Pfaffian invariant . Relevance of monopoles and Chern numbers for the semiclassical chiral kinetic theory is also discussed. [ABSTRACT FROM AUTHOR]

As novel topological phases in correlated electron systems, we have found two examples of non-ferromagnetic states that exhibit a large anomalous Hall effect. One is the chiral spin liquid compound, which exhibits a spontaneous Hall effect in a spin liquid state due to spin ice correlation. The other is the chiral antiferromagnetsandthat exhibit a large anomalous Hall effect at room temperature. The latter shows a sign change of the anomalous Hall effect by a small change in the magnetic field by a few 100 G, which should be useful for various applications. We will discuss that the magnetic Weyl metal states are the origin for such a large anomalous Hall effect observed in both the spin liquid and antiferromagnet that possess almost no magnetisation. [ABSTRACT FROM PUBLISHER]