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Coupling Weyl quasiparticles and charge density waves (CDWs) can lead to fascinating band renormalization and many-body effects beyond band folding and Peierls gaps. For the quasi-one-dimensional chiral compound (TaSe4)2I with an incommensurate CDW transition at TC = 263 K, photoemission mappings thus far are intriguing due to suppressed emission near the Fermi level. Models for this unconventional behavior include axion insulator phases, correlation pseudogaps, polaron subbands, bipolaron bound states, etc. Our photoemission measurements show sharp quasiparticle bands crossing the Fermi level at T > TC, but for T < TC, these bands retain their dispersions with no Peierls or axion gaps at the Weyl points. Instead, occupied band edges recede from the Fermi level, opening a spectral gap. Our results confirm localization of quasiparticles (holes created by photoemission) is the key physics, which suppresses spectral weights over an energy window governed by incommensurate modulation and inherent phase defects of CDW.

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]

An approach to obtain easy-to-fabricate and spectrally selective nonreciprocal thermal emitter (NTE) has been proposed. The presented hybrid structure is composed of Weyl semimetal (WSM) film sandwiched by Ge/BaF2/Ge tri-layer and metal substrate. It has been shown that structural parameters hold tolerance on the order of hundreds of nanometers, which are highly favorable for fabricating with low cost and high performance. Multiband responses of the device can be realized through controlling the thickness of last Ge layer, which boosts the flexibility of preparation and potential applications. The giant nonreciprocity can be maintained with broad polar and azimuthal angle ranges. By manipulating the structural parameters, a remarkable result of nonreciprocal radiation is realized under both the transverse electric (TE) and transverse magnetic (TM) wave incidence. By changing the azimuthal angle (ϕi) of incidence, the nonreciprocity (η) can be effectively manipulated. The simulated η spectra is symmetric along the azimuthal angle ϕi=90°, and η=0 when ϕi=90°. Our results provide a much simpler way to achieve the multi-channel nonreciprocity and could lead to the advancement of power scavenging and conversion devices.

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]

Collective modes are responsible for the emergence of novel quantum phases in topological materials. In the quasi-one dimensional (1D) Weyl semimetal (TaSe4)2I, a charge density wave (CDW) opens band gaps at the Weyl points, thus turning the system into an axionic insulator. Melting the CDW would restore the Weyl phase, but 1D fluctuations extend the gapped regime far above the 3D transition temperature (T CDW = 263 K), thus preventing the investigation of this topological phase transition with conventional spectroscopic methods. Here we use a non-equilibrium approach: we perturb the CDW phase by photoexcitation, and we monitor the dynamical evolution of the band structure by time- and angle-resolved photoelectron spectroscopy. We find that, upon optical excitation, electrons populate the linearly dispersing states at the Fermi level (E F ), and fill the CDW gap. The dynamics of both the charge carrier population and the band gap renormalization (BGR) show a fast component with a characteristic time scale of a few hundreds femtoseconds. However, the BGR also exhibits a second slow component on the μs time scale. The combination of an ultrafast response and of persistent changes in the spectral weight at E F, and the resulting sensitivity of the linearly dispersing states to optical excitations, may explain the high performances of (TaSe4)2I as a material for broadband infrared photodetectors.

Weyl semimetals show special quantum states of matter, which have nontrivial topological features and very interesting and unique applications in the spintronics industry. One method of making these materials is the use of alloying method. In this paper, the alloys are constructed using the first-principles methods. After study of their stability, their structural, electronic, and topological properties have been studied. To study the structural and electronic properties of the alloys, the Wien2k package, based on density functional theory, has been used. Furthermore, the topological properties of the alloys have been calculated using the Wanniertools packages, based on the tight-binding method. Calculations show that the alloys, in the absence of spin-orbit coupling, have crossing points with fourfold degeneracy and band inversion. Therefore, they are topological Dirac semimetal. Considering the spin-orbit coupling, it is seen that alloys with concentrations of x = 0, 0.25, 0.5, 0.75 change to the normal semimetal by opening the band gap; but, the alloy with x = 1 concentration changes to the topological Weyl semimetal with 16 Weyl fermions couple. The full area of the first Brillouin zone (FBZ) was scanned to find the positions of the Weyl points. The results show that the Weyl points, with chirality of either +1 or -1, were scattered in the FBZ with the central symmetry, but all of them are far from the high-symmetry paths of FBZ. Furthermore, the surface state properties, like Fermi arcs, were calculated and studied for the NbSb Weyl semimetal using the Wanniertools computational packages.

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]

Nonreciprocal radiation, violating Kirchhoff’s law, has sparked significant interest due to its potential in energy conversion and capture technologies. In this paper, we introduce a hybrid structure combining graphene and Weyl semimetal to achieve tunable dual-channel nonreciprocal radiation through guided mode resonances. This structure overcomes the limitations of traditional single-channel systems by offering dual-channel nonreciprocal radiation, enhanced efficiency, and increased flexibility. The structure exhibits excellent wide-angle nonreciprocal characteristics. Furthermore, by investigating the impact of structural parameters, the structure demonstrates large parameter tolerance, facilitating practical fabrication. Finally, leveraging the tunable properties of graphene, we showcase the linear control behavior of the system. Our findings pave the way for advanced energy-harvesting and conversion structures in advanced thermal systems.

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 K0= 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 I41md→P6¯m2 transition facilitated by phonon modes.

Chiral Weyl fermions with linear energy-momentum dispersion in the bulk accompanied by Fermi-arc states on the surfaces prompt a host of enticing optical effects. While new Weyl semimetal materials keep emerging, the available optical probes are limited. In particular, isolating bulk and surface electrodynamics in Weyl conductors remains a challenge. We devised an approach to the problem based on near-field photocurrent imaging at the nanoscale and applied this technique to a prototypical Weyl semimetal TaIrTe4 As a first step, we visualized nano-photocurrent patterns in real space and demonstrated their connection to bulk nonlinear conductivity tensors through extensive modeling augmented with density functional theory calculations. Notably, our nanoscale probe gives access to not only the in-plane but also the out-of-plane electric fields so that it is feasible to interrogate all allowed nonlinear tensors including those that remained dormant in conventional far-field optics. Surface- and bulk-related nonlinear contributions are distinguished through their "symmetry fingerprints" in the photocurrent maps. Robust photocurrents also appear at mirror-symmetry breaking edges of TaIrTe4 single crystals that we assign to nonlinear conductivity tensors forbidden in the bulk. Nano-photocurrent spectroscopy at the boundary reveals a strong resonance structure absent in the interior of the sample, providing evidence for elusive surface states.

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.

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]

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]

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]

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]

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 Casimir effect is a quantum phenomenon induced by the zero-point energy of relativistic fields confined in a finite-size system. This effect for photon fields has been studied for a long time, while the realization of counterparts for fermion fields in Dirac/Weyl semimetals is an open question. We theoretically demonstrate the typical properties of the Casimir effect for relativistic electron fields in Dirac/Weyl semimetals and show the results from an effective Hamiltonian for realistic materials such as Cd3As2 and Na3Bi. We find an oscillation of the Casimir energy as a function of the thickness of the thin film, which stems from the existence of Dirac/Weyl nodes in momentum space. Experimentally, such an effect can be observed in thin films of semimetals, where the thickness dependence of thermodynamic quantities is affected by the Casimir energy.

The strong multi-band nonreciprocal radiation with Fibonacci multilayer is studied. It consists of a Weyl semimetal planar and a dielectric layer on a Fibonacci multilayer and a metal reflector. Here, without loss of generality, a quad-band nonreciprocal radiation effect is investigated as an illustration. The results show that four pairs of near-perfect absorption and emission are achieved that do not overlapped with each other, leading to perfect quad-channel nonreciprocal radiation. The underlying physical origin is revealed by investing the magnetic field magnitude distributions and is further verified by impedance matching theory. In addition, the influence of incident angle and geometric parameters on spectral nonreciprocity is investigated, and it is found that the performance can remain excellent in a wide range of geometric parameters, which reduces manufacturing costs. What’s more, the scheme can be easily expanded to achieve multi-band nonreciprocal radiation with the channel number larger than four by simply increasing the Fibonacci order. We believe that the designed structure should pave the way for the development of nonreciprocal thermal emitters with more advanced functions.

A novel scheme for achieving multichannel nonreciprocal thermal radiation, which is in the form of a photonic crystal heterostructure sandwiched between two symmetric dielectric spacers and Weyl semimetal film backed with a metal film, is proposed and studied. A dual-channel nonreciprocal thermal emitter is first designed and studied as an illustration. It is found that near-perfect nonreciprocal radiations are realized at two different wavelengths at the same time. The underlying physical origin of this phenomenon is disclosed by investigating the magnetic field distribution. In addition, the novel dual-channel nonreciprocal radiation performance is robust to variations in geometric parameters, which will benefit real fabrication and application. What's more, the scheme can be easily expanded to achieve multi-band nonreciprocal radiation with the channel number larger than two by simply increasing the sequence order of the photonic crystal heterostructure. The scheme proposed in this paper provides a new possibility to develop novel multichannel nonreciprocal thermal emitters.

The discovery of a significantly large anomalous Hall effect in the chiral antiferromagnetic system—Mn _3 Ge—indicates that the Weyl points are widely separated in phase space and positioned near the Fermi surface. In order to examine the effects of Fe substitution in Mn _3 Ge on the presence and location of the Weyl points, we synthesized (Mn $_{1-\alpha}$ Fe $_{\alpha})_3$ Ge ( $\alpha = 0-0.30$ ) compounds. The AHE was observed in compounds up to α = 0.22, but only within the temperature range where the magnetic structure remains the same as the Mn _3 Ge. Additionally, positive longitudinal magnetoconductance and planar Hall effect (PHE) were detected within the same temperature and doping range. These findings strongly suggest the existence of Weyl points in (Mn $_{1-\alpha}$ Fe $_{\alpha})_3$ Ge ( $\alpha = 0-0.22$ ) compounds. Further, we observed that with an increase in Fe doping fraction, there is a significant reduction in the magnitude of anomalous Hall conductivity, PHE, and positive longitudinal magnetoconductance, indicating that the Weyl points move further away from the Fermi surface. Consequently, it can be concluded that suitable dopants in the parent Weyl semimetals have the potential to tune the properties of Weyl points and the resulting anomalous electrical transport effects.

In recent years, non-Hermitian (NH) topological semimetals have garnered significant attention due to their unconventional properties. In this work, we explore one of the transport properties, namely the Hall conductance of a three-dimensional dissipative Weyl semi-metal formed as a result of the stacking of two-dimensional Chern insulators. We find that unlike Hermitian systems where the Hall conductance is quantized, in presence of non-Hermiticity, the quantized Hall conductance starts to deviate from its usual nature. We show that the non-quantized nature of the Hall conductance in such NH topological systems is intimately connected to the presence of exceptional points. We find that in the case of open boundary conditions, the transition from a topologically trivial regime to a non-trivial topological regime takes place at a different value of the momentum than that of the periodic boundary spectra. This discrepancy is solved by considering the non-Bloch case and the generalized Brillouin zone (GBZ). Finally, we present the Hall conductance evaluated over the GBZ and connect it to the separation between the Weyl nodes, within the non-Bloch theory.

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 (R A) 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 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.

The 6th generation (6G) of communication technology, defines 0.1–1 THz as the communication band. As a result, ultrafast modulation and multi-dimensional resolution coding technologies in the terahertz band pose a big challenge. In this work, a polarization-resolved ultrafast optical controlled terahertz modulator has been realized by combining tungsten ditelluride (WTe2) thin film with subwavelength metallic gratings. In the frequency range of 0.1–1.2 THz, which covers the 6G frequency band, the modulation depth of the device reaches up to 36.8% with a centimeter-scale effective area. An ultrafast response time of 0.95 ps is obtained due to the limitations of the grating for carrier degrees of freedom. In addition, the polarization sensitivity of terahertz waves polarized in the x- and y-direction is up to 0.77 due to the sub-wavelength grating structure. The results give a chance to develop the large-area, high-performance polarization-resolved ultrafast THz modulators based on Weyl semimetal, which could provide a reliable alternative for 6G communications equipment.

We theoretically set up a 4 × 4 magneto-optical matrix to investigate the transmittances induced by the forward and backward left hand circularly polarized (LCP) waves impinging the 4 μm thickness Weyl semimetal (WSM) material at normal incidence. Because of the different effective permittivity of the two LCP waves in WSM induced by the non-zero off-diagonal term, the two waves experience blocking transmission, bidirectional transmission and unidirectional transmission with the increase of the wavelength, respectively. Based on the wide wavelength of the unidirectional transmission, the broadband optical isolator can be realized. The threshold wavelength of the optical isolator can be regulated by changing the wavelength where the parallel component of the diagonal element is identical to the off-diagonal term. It is also found that the Fermi energy, tilt degree of Weyl cones and Weyl node separation reveal simple and available approaches to achieve the regulation. We believe that the adjustable broadband compact optical isolator can perform a significant role in fiber-optic communication system.

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]

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]

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]

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]

Non-reciprocal thermal radiation is crucial for energy transfer and storage applications. However, the range of angles of incidence limits the development of non-reciprocal thermal radiation. To address this problem, we have proposed and developed a non-reciprocal thermal radiation device with wide-angle characteristics, which operates in the mid-infrared spectral range. The radiator consists of a silicon-based periodic toroidal array, a Weyl semimetal (WSM), and a metal reflective layer Ag. A strong nonreciprocity of 0.88 is observed at 9.36 μm wavelength and 41.8° angle, while a nonreciprocity of 0.87 can be sustained within the range of 36°–74°. In addition, the nonreciprocity can be maintained above 0.8 in the spectral range of 9.3 μm ∼9.4 μm. Compared with the previous single working angle, our work has dramatically broadened the angle of energy absorption, and the non-reciprocal intensity can be maintained at a high level. Through a detailed analysis of the magnetic field modulus and impedance characteristics at non-reciprocal wavelengths, we reveal that cavity resonance excitation is the core physical process of mid-infrared wide-angle non-reciprocal radiation. In addition, changing the axial vector of the WSM can lead to a redshift of the non-reciprocal wave peaks. The proposed two-dimensional mid-infrared non-reciprocal emitter can realize wide-angle domain emission at specific wavelengths, which is promising for applications in energy storage and conversion systems such as solar cells and thermophotovoltaic cells. • Violation of Kirchhoff's law of thermal radiation in the case of zero magnetism using Weyl semimetals. • A periodic ring array structure based on cavity resonance modes is used. • Wide-angle non-reciprocal thermal radiation effects in specific wavelength bands are realized. [ABSTRACT FROM AUTHOR]

Weyl semimetals (WSMs) have lately attracted significant interest because of the intrinsic large gyrotropic optical response and strong chiral anomaly. This study presents a mid-infrared (mid-IR) lithography-free platform exhibits broadband near-unity circular dichroism (CD) with WSM-vanadium dioxide (VO 2) dual layers. The optical responses with mid-IR radiation have been explored with the generalized 4 × 4 transfer matrix algorithm. It is theoretically shown that the proposal possesses different absorption characteristics for right-handed circularly polarized (RCP) and left-handed circularly polarized (LCP) light. Theoretical results show that a CD in absorption reaching 0.936 and a g factor of 1.76 under 300 K temperature. The merit of the wide-angular chiral capacity can be maintained. The magnitude and resonance of CD can be tuned not only by adjusting the structural parameters, but also the insulator-to-metal transition (IMT) of VO 2 , which may facilitate the design of more efficient chiral optical devices. • Broadband and near-unity circular dichroism have been realized with WSM-VO 2 stack. • The merit of the wide-angular chiral capacity can be maintained. • Property of CD is strongly temperature dependent, enabling an efficient way for the control of chiral optical devices. [Display omitted] [ABSTRACT FROM AUTHOR]

We investigate non-reciprocal optical bistability (OB) of sandwiched structure comprising of two magnetic Weyl semimetal (WSM) layers and a nonlinear layer. The resonant tunneling mode in the sandwiched structure with a nonlinear layer creates favorable conditions for the emergence of OB. Besides, the inherent non-reciprocal nature of WSM enables non-reciprocal transmission of OB in the absence of an external magnetic field. By employing the transfer matrix method, we simulate the OB curves for forward and backward propagations, as well as analyze the non-reciprocity of OB. Furthermore, the impacts of various parameters on non-reciprocal OB are comprehensively discussed. The results demonstrate that the non-reciprocal OB curve and the width of the isolation of the non-reciprocal OB can be tuned by adjusting the thickness and the axial vector of the WSM, the temperature as well as the incident angle. This work would provide crucial insights for the design of high-performance non-reciprocal devices. [ABSTRACT FROM AUTHOR]

Nonreciprocal radiation can violate Kirchhoff's law and has gained significant attention due to the great potentials in radiative cooling, infrared sensing, thermal energy harvesting and thermophotovoltaic systems. In the practical application of nonreciprocal thermal emitters, not only the magnitude but also the sign of contrast (δ) between emissivity (e) and absorptivity (α) should be considered. Stacked epsilon-near-zero (ENZ)/Weyl semimetal (WSM)/ENZ nanocavity inserted with high refractive index dielectric layer (germanium. Ge) is proposed as a favorable platform to study and manage nonreciprocal thermal radiation (NTR). The emitter can be realized by lithography-free fabrication methods with fine control on the optical and geometrical parameters. We show that the designed scheme has the capability to tune the spectral bandwidth and resonance position of NTR. By changing the layer thickness and incidence angle (θ i) that will yield opposite signs for δ , the magnitude as well as the sign of δ can be effectively controlled. Moreover, the azimuthal angle (ϕ i) provide more freedom to manage the δ spectra. The distribution of absolute δ as functions of wavelength λ and ϕ i is symmetrical with ϕ i = 90 ∘ under both s- and p-polarizations. It is believed this approach heralds the potential of applying WSM to enable on-demand tunability of nonreciprocal radiative properties in the mid-infrared (mid-IR). [Display omitted] • A method for the manipulation of sign of nonreciprocity is proposed. • Near-unity nonreciprocity is realized with optimal parameters. • The sign of nonreciprocity can be converted by changing the layer thickness and incidence angle. • This structure is highly favorable for standard mask-based nano-fabrication. [ABSTRACT FROM AUTHOR]

A mid-infrared (mid-IR) chiral absorber based on multilayer films of silicon (Si)/Weyl semimetal (WSM)/Si is proposed and numerically studied. The proposal exhibits high quality factor circular dichroism (CD) with different absorptance properties for right-hand circular polarization (RCP) and left-hand circular polarization (LCP) light. The absorption characteristics have been explored with the generalized 4 × 4 transfer matrix algorithm, and the effects of structural parameters on the circularly polarized (CP) absorptance are systematically investigated. The circular polarization efficiency (η) and CD are defined as crucial parameters for quantifying the chirality response. The results prove that the chiral absorber exhibits extraordinary efficiencies of 1 and 0 for LCP and RCP light at a wavelength of 11.5 μm, respectively. Thus a maximum CD in absorption of unity with a resonant quality factor (Q-factor) of 295 can be obtained. The performance of dual-band chiral responses can be engineered via the thickness of each layer. Moreover, it is identified that the design results in angle-independent performance, which is essential trait for an efficient chiral absorber. [Display omitted] • A chiral absorber with dual-band circular dichroism (CD) effect in the mid-infrared region is proposed. • The CD effect and circular polarization efficiency can be flexibly adjusted by varying the layer thickness. • A maximum CD in absorption of unity with a resonant quality factor of 295 has been obtained. [ABSTRACT FROM AUTHOR]

In this work, magneto-optical properties of a dichroic cholesteric liquid-crystal layer are theoretically investigated at large values of the magneto-optical parameter. Features of all solutions of the dispersion equation are studied in detail. Peculiarities of the reflection, transmission, absorption spectra and the influence of dielectric boundaries on them are investigated. Specific properties of the localization of light and magnetically induced transparency in dichroic cholesteric liquid crystals are considered. The study of the light localization features showed that the presence of an external magnetic field, as well as the presence of dielectric boundaries, led to the appearance of oscillations in the dependence of the intensity of the layer-confined energy on the coordinate of the axis directed along the cholesteric axis. A strong influence of the refractive index of isotropic half-spaces adjacent to a dichroic cholesteric liquid crystal layer on the optics of the layer under consideration is shown. In particular, magnetically induced transparency and diffraction transmission appear only at certain intervals of the refractive index of isotropic half-spaces.

Although various nonreciprocal thermal emitters have been suggested to break the balance between absorption and emission, the majority can only achieve nonreciprocal effect at one certain angle, and the angular range in which these structures exhibit nonreciprocal effect is heavily restricted. In this work, the scheme to realize extremely wide-angle nonreciprocal thermal radiation effect is proposed and investigated. It is achieved by placing a dielectric grating atop a Weyl semimetal film backed with a metal mirror. The results show that the emitter exhibits strong nonreciprocal radiation at the wavelength of 15.98 μm when the incident angle is 30°. What's more, the strong nonreciprocal radiation can be achieved in a wide angular range. Such behavior results from the guided mode resonance, which is revealed by investigating the distribution of electromagnetic field at the resonant wavelength and is also confirmed by the dispersion relation of guided mode. This work provides a new approach to the design of novel nonreciprocal thermal emitters.

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]

The combination of antiferromagnetism and topological properties in Mn 3 X (X = Sn,Ge,Ga) offers a unique platform to explore novel spin-dependent phenomena and develop innovative spintronic devices. Here, we have systematically investigated the phase transition of Mn 3 Ga thin films on SiO 2 (001)/Si substrates under various growth parameters such as seeding layer structure, annealing conditions, and film thickness. The relatively thick Mn 3 Ga films grown with Ru seeding exhibit a variety of polycrystalline hexagonal phases, including (002), and (201). The addition of a Ta layer to the conventional Ru seeding layer promotes the formation of nearly single-crystal antiferromagnetic (AF) Mn 3 Ga(002) phase from the relatively thin Mn 3 Ga films after annealing at 773 K. The investigation of the growth mechanism of Mn 3 Ga polycrystalline thin films provides a reference strategy for exploring Mn-based AF spintronic devices., (© 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)

The non-trivial magnetic and electronic phases occurring in topological magnets are often entangled, thus leading to a variety of exotic physical properties. Recently, the BaAl 4 -type compounds have been extensively investigated to elucidate the topological features appearing in their real- and momentum spaces. In particular, the topological Hall effect and the spin textures, typical of the centrosymmetric Eu(Al,Ga) 4 family, have stimulated extensive experimental and theoretical research. In this topical review, we discuss the latest findings on the Eu(Al,Ga) 4 topological antiferromagnets and related materials, arising from a wide range of experimental techniques. We show that Eu(Al,Ga) 4 represents a suitable platform to explore the interplay between lattice-, charge-, and spin degrees of freedom, and associated emergent phenomena. Finally, we address some key questions open to future investigation., (© 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)