Search

Topological magnetic materials are expected to show multiple transport responses because of their unusual bulk electronic topology in momentum space and their topological spin texture in real space. However, such multiple topological properties-hosting materials are rare in nature. In this work, we unambiguously reveal the emergence of magnetic skyrmions in Mn 5 Ge 3 single crystal through detailed electrical transport and Lorentz transmission electron microscopy (L-TEM) combined with ab initio calculations. We demonstrate that Mn 5 Ge 3 is a topological ferromagnet with multiple nodal rings in its electronic structure. Importantly, L-TEM experiments further reveal that the magnetic skyrmions appear in the (001) plane when an appropriate magnetic field is applied along the [001] direction. Skyrmion-induced topological Hall resistivity as large as ∼972 nΩ cm is also observed over a wide temperature-magnetic field region. These prove Mn 5 Ge 3 as a rare magnetic topological nodal-ring semimetal with great significance to explore novel topological multifunctionality, which facilitate the development of spintronics.

Theories have revealed the universality of the band tilting effect in topological Weyl semimetals (WSMs) and its implications for the material physical properties. However, the experimental identification of tilted Weyl bands remains less explored. Here, we report the magneto-optical evidence of the tilting effect in WSM niobium phosphide. Specifically, we observe Landau level transitions with rich features that are well reproduced within a model of coupled tilted Weyl points. Our analysis indicates that the tilting effect relaxes the selection rules and leads to transitions that would otherwise be forbidden in the non-tilt case. Additionally, we observe unconventional interband transitions with flat and negative magnetic field dispersions, highlighting the importance of coupling between Weyl points. Our results not only emphasize the significance of the tilting effect in the optical responses of WSMs but also demonstrate magneto-optics as an effective tool for probing the tilting effect in electronic band structures.

Non-Hermitian physics, studying systems described by non-Hermitian Hamiltonians, reveals unique phenomena not present in Hermitian systems. Unlike Hermitian systems, non-Hermitian systems have complex eigenvalues, making their effects less directly observable. Recently, significant efforts have been devoted to incorporating the non-Hermitian effects into condensed matter physics. However, progress is hindered by the absence of a viable experimental approach. Here, the discovery of the surface-selectively spontaneous reconstructed Weyl semimetal NdAlSi provides a feasible experimental platform for studying non-Hermitian physics. Utilizing angle-resolved photoemission spectroscopy (ARPES) measurements, surface-projected density functional theory (DFT) calculations, and scanning tunneling microscopy (STM) measurements, it is demonstrated that surface reconstruction in NdAlSi alters surface Fermi arc (SFA) connectivity and generates new isolated non-topological SFAs (NTSFAs) by introducing non-Hermitian terms. The surface-selective spontaneous reconstructed Weyl semimetal NdAlSi can be viewed as a Hermitian bulk - non-Hermitian boundary system. The isolated non-topological SFAs on the reconstructed surface act as a loss mechanism and open boundary condition (OBC) for the topological electrons and bulk states, serving as non-Hermitian boundary states. This discovery provides a good experimental platform for exploring new physical phenomena and potential applications based on boundary non-Hermitian effects, extending beyond purely mathematical concepts. Furthermore, it provides important enlightenment for constructing topological photonic crystals with surface reconstruction and studying their topological properties., (© 2025 The Author(s). Advanced Materials published by Wiley‐VCH GmbH.)

Weyl semimetals are a novel class of topological materials with unique electronic structures and distinct properties. HfRhGe stands out as a noncentrosymmetric Weyl semimetal with unconventional superconducting characteristics. Using muon-spin rotation and relaxation (µSR) spectroscopy and thermodynamic measurements, a fully gapped superconducting state is identified in HfRhGe that breaks time-reversal symmetry at the superconducting transition. This breaking can trigger a topological phase transition, as time-reversal symmetry protects the normal-state Weyl topology characterized by comprehensive first-principles calculations. Ginzburg-Landau analysis suggests an unconventional loop supercurrent superconducting state realized in HfRhGe. The presence of multiple Weyl points near the Fermi level and surface Fermi arcs dispersing across the Fermi level further support HfRhGe as a promising platform for topological superconductivity. Additionally, its noncentrosymmetric nature with time-reversal symmetry breaking superconducting state suggests that it can exhibit an intrinsic superconducting diode effect, offering novel optical and transport properties, with potential applications in dissipationless quantum electronics., (© 2024 Wiley‐VCH GmbH.)

Pyrochlore materials are known for their exotic magnetic and topological phases arising from complex interactions among electron correlations, band topology, and geometric frustration. Interfaces between different pyrochlore crystals characterized by complex many-body ground states hold immense potential for novel interfacial phenomena due to the strong interactions between these phases. However, the realization of such interfaces has been severely hindered by limitations in material synthesis methods. In this study, we discover a robust synthesis method that produces the previously unexplored epitaxial pyrochlore interface between spin ice Dy 2 Ti 2 O 7 and Weyl semimetal Eu 2 Ir 2 O 7 . The method relies on an ultrahigh supersaturation regime during deposition aided by directional IR-laser-driven thermal gradients, transforming amorphous covalent networks into nearly perfectly ordered, atomically sharp interfaces with a chemically ideal arrangement of ions. The novel pyrochlore interface enables the study of interactions between relativistic Weyl fermions and spin ice magnetic monopoles, opening a path to designing diverse pyrochlore interfaces.

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.

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]

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.

A large number of hitherto unexplored elastic, thermophysical, acoustic, and optoelectronic properties of a double Weyl semimetal SrSi2 have been investigated in this study. Density functional theory (DFT) based methodology has been employed. Analyses of computed elastic parameters reveal that SrSi2 is a mechanically stable, ductile, moderately machinable, and relatively soft material. The compound is predicted to be dynamically stable and possesses significant metallic bonding. Study of thermophysical properties, namely, Debye temperature, Grüneisen parameter, acoustic parameters, melting temperature, heat capacity, thermal expansion coefficient, and dominant phonon mode is also indicative of soft nature of SrSi2. The electronic band structure calculations without and with spin–orbit coupling disclose semimetallic character with clear Weyl nodes close to the Fermi level. The electronic dispersion is anisotropic as evidenced by nearly flat and linear regions within the Brillouin zone. Optical parameters at different photon energies are investigated. SrSi2 shows excellent nonselective reflection spectrum across an extended range of energy encompassing the visible range implying that the compound under study has significant potential to be used as an efficient solar energy reflector. SrSi2 absorbs ultraviolet light quite efficiently. The compound also possesses high refractive index in the low energy. All these optical features can be useful in optoelectronic device applications.

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]

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]

First-principles calculation has become one of the most reliable approaches in predicting structural, electronic, and magnetic properties for material applications. Alloys in Heusler structures have also attracted much attention recently since they can be easily synthesized and provide interesting properties for future spintronic applications. In this work, we investigate a series of Fe-based Heusler compounds Fe3−xYxZ (x = 0, 1, 2, 3; Y= Cr, Mn, Co; Z= Al, Ga, Si) with L21- and XA-type structures using first-principles calculations based on density functional theory. According to formation energy calculations and mechanical property analysis, most of the studied Heusler compounds are thermodynamically stable and could be synthesized experimentally. The Co substitution leads Fe3−xCoxZ to a ferromagnetic ground state like Fe3Z with a strong magnetization ranging from 4 to 6 μB/f. u. While replacing Fe with Cr or Mn, the exchange coupling between Cr (Mn) and its neighboring atoms generally tend to be anti-parallel. Among the antiferromagnetic compounds, Mn3Al and Mn3Ga are antiferromagnetic half metal while Mn3Si is ferrimagnetic half metal. These rarely found type of half metals with low magnetic moment and high spin polarization at the Fermi level are important for low energy consumption spintronic applications. The estimated Curie temperatures for Mn3Al, and Mn3Si and Co2FeSi (XA) are in good agreement with previously theoretical values, while for Fe3Al and Fe3Si, they are in good agreement with previous experimental results. The good consistency in Curie temperature demonstrates high reliability of our predictions based on first-principles calculations. As for the topological property aspect, we predict Fe2CrAl and Fe2MnAl as the 3-dimensional Weyl semimetal. Furthermore, Fe2CrSi is predicted to be the magnetic nodal-line semimetal. Interestingly, our mechanical property analysis demonstrates that Co3Si and Fe2CoSi (L21) exhibit ultraelastic metal behavior, which is of high potential in advanced mechanical industry. This work suggests that Heusler compounds are excellent candidates for future spintronics as well as for high-performance ultraelastic metals.

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]

The YbCoC2 compound, which crystallizes in a base-centered orthorhombic unit cell in the Amm2 space group CeNiC2 structure, is unique among Yb-based compounds due to the highest magnetic ordering temperature of TN=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 ΔSm(T)—have segments with positive and negative magnetocaloric effects for ΔH≤6 T. For ΔH=9 T, the magnetocaloric effect becomes positive, with a maximum ΔSm(T) value of 4.1 J (kg K)−1 at TN and a refrigerant capacity value of 56.6 J kg−1.

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.)

The emerging all-van der Waals (vdW) magnetic heterostructure provides a new platform to control the magnetization by the electric field beyond the traditional spintronics devices. One promising strategy is using unconventional spin-orbit torque (SOT) exerted by the out-of-plane polarized spin current to enable deterministic magnetization switching and enhance the switching efficiency. However, in all-vdW heterostructures, large unconventional SOT remains elusive and the robustness of the field-free switching against external magnetic field has not been examined, which hinders further applications. Here, the study demonstrates the field-free switching in an all-vdW heterostructure combining a type-II Weyl semimetal TaIrTe 4 and above-room-temperature ferromagnet Fe 3 GaTe 2 . The fully field-free switching can be achieved at 2.56 × 10 10 A m -2 at 300 K and a large SOT effective field efficiency of the out-of-plane polarized spin current generated by TaIrTe 4 is determined to be 0.37. Moreover, it is found that the switching polarity cannot be changed until the external in-plane magnetic field reaches 252 mT, indicating a robust switching against the magnetic field. The numerical simulation suggests the large unconventional SOT reduces the switching current density and enhances the robustness of the switching. The work shows that all-vdW heterostructures are promising candidates for future highly efficient and stable SOT-based devices., (© 2024 The Author(s). Advanced Materials published by Wiley‐VCH GmbH.)

The Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction is an essential long-range magnetic interaction between magnetic impurities or magnetic layered structures, the magnitude of which oscillates with the distance $\mathrm{R}$ between them. We investigated the RKKY interaction between two magnetic impurities in both time-reversal and inversion symmetry-broken Weyl semimetals (WSMs) where the energy dispersion is tilted in momentum space and the momentum of the conduction electrons is locked with the pseudo-spin. Two important features are revealed: firstly, at the small tilt limit, the RKKY coupling varies quadratically with the tilt parameter and strikingly, at large separation distances $\mathrm{R}$ , decays as $1/\mathrm{R}$ compared to the conventional $1/\mathrm{R}^{3}$ dependence exhibited by WSMs with non-tilted dispersion. The slower decay by two orders (i.e. $1/\mathrm{R}$ as opposed to $1/\mathrm{R}^{3}$ ) of the RKKY coupling is significant for maintaining a long-range RKKY coupling. Secondly, the RKKY coupling exhibits an anisotropy with respect to the angle between the tilt direction ( w ) and the separation direction R , unlike the case of non-tilted WSMs, which exhibits isotropic RKKY coupling. Consequently, the RKKY coupling in tilted WSMs alternately favors ferromagnetic and anti-ferromagnetic orders and vice-versa with the change of the angle. Our results are derived analytically and verified by numerical calculations based on realistic parameter values.

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]

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]

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]

Nonreciprocal thermal radiation (NTR) holds great promise in revolutionizing energy harvesting and conversion process, has become a hot topic of various research fields ranging from renewable energy, molecular sensing, to thermal circuitry and camouflage. Despite intensive effort into enhance the nonreciprocal response, static manipulation of NTR has limited the potential application, dynamic control of thermal radiation is highly demanded. Here, by using a multilayered stack comprised of vanadium dioxide (VO 2) thin film, germanium (Ge) spacer layer, Weyl semimetal (WSM) layer terminated by molybdenum (Mo) substrate, we propose a thermally tunable nonreciprocal emitter. The effects of the incident angle (θ i), azimuthal angle (ϕ i), and the thickness of each layer for the VO 2 /Ge/WSM stack on the characteristics of thermal radiation have been discussed thoroughly. Dominant peaks and dips of nonreciprocity have been obtained with an incident angle of θ i = 60 ° and azimuthal angle of ϕ i = 0 °. A switching effect between the near-unity nonreciprocity ("ON") and zero nonreciprocity ("OFF") states are induced when the temperature increases from 20 °C to 68 °C. It is shown that the magnitude and sign of the nonreciprocity can be managed simultaneously for transverse electric (s -) and transverse magnetic (p -) polarizations. The proposed structure shows good near-unity nonreciprocity stability in a wide range of θ i and ϕ i. This structurally lithography-free nonreciprocal thermal emitter not only provides the ability to tune nonreciprocal radiative properties, but also leads to the possible development of power scavenging and conversion devices. [Display omitted] • A strong nonreciprocal emitter with three layers VO 2 /Ge/Weyl semimetal is designed. • Tunable manipulation of magnitude and sign of spectrally selective nonreciprocity can be realized. • This structure is simple and highly favorable for large-scale production. [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.

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.

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

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]

We demonstrate that substituting Bi for Sm in the pyrochlore Sm$&#95;2$Ir$&#95;2$O$&#95;7$ induces an anomalous lattice contraction, with $\Delta a \sim -0.012$~\AA~observed at 10\% Bi substitution, where 'a' denotes the lattice constant. Beyond 10\% Bi substitution, the lattice expands according to Vegard's law. Within this anomalous substitution range, the resistivity shows a 1/T behavior up to 2\% Bi-substitution, while near 10\% substitution a -lnT dependence is observed. These resistivity behaviors suggest the possibility of a Weyl phase up to 2\% Bi substitution, which transforms to a semimetallic quadratic band touching (QBT) topological phase near 10\%. For the intermediate composition (Sm$&#95;{0.95}$Bi$&#95;{0.05}$)$&#95;2$Ir$&#95;2$O$&#95;7$, the resistivity scales as $\rm 1/T^{1/4}$, possibly due to its proximity to a proposed quantum critical point at the Weyl-QBT phase boundary [Phys. Rev. X 4, 041027 (2014)]. The samples were characterized using synchrotron powder X-ray diffraction, X-ray near-edge fine structure (XANES), and Extended X-ray absorption fine structure (EXAFS) probes. Additionally, magnetic susceptibility and heat capacity measurements were conducted to provide further support., (© 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)

In this study, we examine the topological character of spectral singularities by using transverse magnetic (TM) mode configuration in a Topological Weyl Semimetal (TWSM). TM mode configuration restrains the effect of Kerr/Faraday rotations and therefore does not allow an extra degree of freedom to occur. We find out that surface currents arise due to topological terms on the surface of TWSM slab where no Fermi arcs are localized. We also investigate the contribution of the Θ-term, which is the origin of axions in topological materials, and especially the b -term, to the topological properties. As a result of our study, we clearly reveal the topological character of b -term for the first time and we demonstrate the Weyl degeneracy situation in an obvious manner. Our system produces circular currents in the plane of propagation, maintaining a cyclotron shape motion. The presence of b -term causes the induced current to be topologically protected. Our findings verify that topological properties of TWSM containing two opposite chirality Weyl fermions are robust against external influences. With the findings of our study, the appropriate conditions for the construction of a topological laser and the values that the system parameters can take have been demonstrated., (Creative Commons Attribution license.)

The interaction between topology and magnetism can lead to novel topological materials including Chern insulators, axion insulators, and Dirac and Weyl semimetals. In this work, a family of van der Waals layered materials using MnTe and Sb 2 Te 3 or Bi 2 Te 3 superlattices as building blocks are systematically examined in a search for antiferromagnetic Weyl semimetals, preferably with a simple node structure. The approach is based on controlling the strength of the exchange interaction as a function of layer composition to induce the phase transition between the topological and the normal insulators. Our calculations, utilizing a combination of first-principles density functional theory and tight-binding analyses based on maximally localized Wannier functions, clearly indicate a promising candidate for a type-I magnetic Weyl semimetal. This centrosymmetric material, Mn 10 Sb 8 Te 22 (or (MnTe) m (Sb 2 Te 3 ) n with m  = 10 and n  = 4), shows ferromagnetic intralayer and antiferromagnetic interlayer interactions in the antiferromagnetic ground state. The obtained electronic bandstructure also exhibits a single pair of Weyl points in the spin-split bands consistent with a Weyl semimetal. The presence of Weyl nodes is further verified with Berry curvature, Wannier charge center, and surface state (i.e. Fermi arc) calculations. Other combinations of the MnSbTe-family materials are found to be antiferromagnetic topological or normal insulators on either side of the Mn:Sb ratio, respectively, illustrating the topological phase transition as anticipated. A similar investigation in the homologous (MnTe) m (Bi 2 Te 3 ) n system produces mostly nontrivial antiferromagnetic insulators due to the strong spin-orbit coupling. When realized, the antiferromagnetic Weyl semimetals in the simplest form (i.e. a single pair of Weyl nodes) are expected to provide a promising candidate for low-power spintronic applications., (Creative Commons Attribution license.)

Considering the low-energy model of tilted Weyl semimetal, we study the electronic transmission through a periodically driven quantum well, oriented in the transverse direction with respect to the tilt. We adopt the formalism of Floquet scattering theory and investigate the emergence of Fano resonances as an outcome of matching between the Floquet sidebands and quasi-bound states. The Fano resonance energy changes linearly with the tilt strength suggesting the fact that tilt-mediated part of quasi-bound states energies depends on the above factor. Given a value of momentum parallel (perpendicular) to the tilt, we find that the energy gap between two Fano resonances, appearing for two adjacent values of transverse (collinear) momentum with respect to the tilt direction, is insensitive (sensitive) to the change in the tilt strength. Such a coupled (decoupled) behavior of tilt strength and the collinear (transverse) momentum can be understood from the tilt-mediated and normal parts of the quasi-bound state energies inside the potential well. We vary the other tilt parameters and chirality of the Weyl points to conclusively verify the exact form of the tilt-mediated part of the quasi-bound state energy that is the same as the tilt term in the static dispersion. The tilt orientation can significantly alter the transport in terms of evolution of Fano resoance energy with tilt momentum. We analytically find the explicit form of the bound state energy that further supports all our numerical findings. Our work paves the way to probe the tilt-mediated part of quasi-bound state energy to understand the complex interplay between the tilt and Fano resonance., (© 2024 IOP Publishing Ltd.)

The tungsten ditelluride WTe2 was suggested to belong to the Weyl semimetal family. We studied 125Te spin-lattice relaxation and NMR spectra in a WTe2 single crystal within a large range from 28 K up to room temperature. Measurements were carried out on a Bruker Avance 500 NMR pulse spectrometer for the crystalline c axis directed in parallel and perpendicular to the magnetic field. Relaxation proved to be single-exponential. The relaxation time varied depending on the sample position in magnetic field and frequency offset. The relaxation rate increased about linearly with temperature below 70 K, however the dependence became nearly quadratic at higher temperatures. The relaxation rate within the total temperature range was fitted using a theoretical model developed in Ref. [41] for Weyl semimetals and assuming the decrease of the chemical potential with increasing temperature. The results obtained for 125Te spin-lattice relaxation evidence in favor of the topological nontriviality of the WTe2 semimetal. The 125Te NMR spectra agreed with the occurrence of nonequivalent tellurium sites and varied insignificantly with temperature.

Td-WTe2 is a topological Weyl semimetal. WTe2 in the orthorhombic structure is stable at room temperature. Elastic, electronic, bonding, and optoelectronic properties of WTe2 have been investigated in detail in this work using the density functional theory. Elastic behaviour together with anisotropy indices of WTe2 have been investigated for the first time. Bonding nature among the constituent atoms and electric field polarization dependent optical constants are also explored theoretically for the first time. WTe2 is elastically anisotropic; optical anisotropy on the other hand is low. The electronic band structure reveals quasi-linear dispersions along certain direction in the Brillouin zone with semi-metallic features. The Fermi level is located at a pseudogap separating bonding and anti-bonding density of states. The electronic effective mass tensor is predicted to be highly direction dependent. The energy dispersion is significantly weaker in the c-direction. The bonding in WTe2 is an admixture of covalent and metallic bonds. Optoelectronic properties show strongly reflecting character over a wide band of photon energies. The compound is a strong absorber of ultraviolet radiation. The Debye temperature has been calculated from the elastic constants. We have compared all the calculated physical properties of WTe2 with those of isostructural MoTe2 Weyl semimetal studied in a previous work. The properties of WTe2 and MoTe2 have been compared and contrasted. The calculated parameters of WTe2 have also been compared with those already available in the literature. Good agreements have been found.

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.

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]