Weyl Semimetals Research Articles

High quality factor and unity circular dichroism enabled by dielectric-Weyl semimetal-dielectric stack

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.

Magnetothermopower of Nodal-Line Semimetals

The search for materials with large thermopower is of great practical interest. Dirac and Weyl semimetals have recently proven to exhibit superior thermoelectric properties, particularly when subjected to a quantizing magnetic field. Here, we consider whether a similar enhancement arises in nodal-line semimetals, for which the conduction and valence band meet at a line or ring in momentum space. We compute the Seebeck and Nernst coefficients for arbitrary temperature and magnetic field and we find a wealth of different scaling regimes. Most strikingly, when a sufficiently strong magnetic field is applied along the direction of a straight nodal line or in the plane of a nodal ring, the large degeneracy of states leads to a large linear-in-B thermopower that is temperature independent even at low temperatures. Our results suggest that nodal-line semimetals may offer significant opportunity for efficient low-temperature thermoelectrics. Published by the American Physical Society 2024

Extended Berry Curvature Tail in Ferromagnetic Weyl Semimetals NiMnSb and PtMnSb.

Heusler compounds belong to a large family of materials and exhibit numerous physical phenomena with promising applications, particularly ferromagnetic Weyl semimetals for their use in spintronics and memory devices. Here, anomalous Hall transport is reported in the room-temperature ferromagnets NiMnSb (half-metal with a Curie temperature (TC) of 660 K) and PtMnSb (pseudo half-metal with a TC of 560 K). They exhibit 4 µB/f.u. magnetic moments and non-trivial topological states. Moreover, NiMnSb and PtMnSb are the first half-Heusler ferromagnets to be reported as Weyl semimetals, and they exhibit anomalous Hall conductivity (AHC) due to the extended tail of the Berry curvature in these systems. The experimentally measured AHC values at 2 K are 1.8×102Ω-1cm-1 for NiMnSb and 2.2×103Ω-1 cm-1 for PtMnSb. The comparatively large value between them can be explained in terms of the spin-orbit coupling strength. The combined approach of using ab initio calculations and a simple model shows that the Weyl nodes located far from the Fermi energy act as the driving mechanism for the intrinsic AHC. This contribution of topological features at higher energies can be generalized.

Thermally tunable broadband circular dichroism with Weyl semimetal/vanadium dioxide planar structure

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 (VO2) 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 VO2, which may facilitate the design of more efficient chiral optical devices.

Antisymmetric planar Hall effect in rutile oxide films induced by the Lorentz force

The conventional Hall effect is linearly proportional to the field component or magnetization component perpendicular to a film. Despite the increasing theoretical proposals on the Hall effect to the in-plane field or magnetization in various special systems induced by the Berry curvature, such an unconventional Hall effect has only been experimentally reported in Weyl semimetals and in a heterodimensional superlattice. Here, we report an unambiguous experimental observation of the antisymmetric planar Hall effect (APHE) with respect to the in-plane magnetic field in centrosymmetric rutile RuO2 and IrO2 single-crystal films. The measured Hall resistivity is found to be linearly proportional to the component of the applied in-plane magnetic field along a particular crystal axis and to be independent of the current direction or temperature. Both the experimental observations and theoretical calculations confirm that the APHE in rutile oxide films is induced by the Lorentz force. Our findings can be generalized to ferromagnetic materials for the discovery of anomalous Hall effects and quantum anomalous Hall effects induced by in-plane magnetization. In addition to significantly expanding knowledge of the Hall effect, this work opens the door to explore new members in the Hall effect family.

Reconfigurable Chiral Spintronic THz Emitters

AbstractCollective spin arrangements manifest diverse spin textures, encompassing ferromagnetism, antiferromagnetism, and chiral vortices. However, mapping these spin textures in ultrathin magnetic multilayers has remained elusive. A reconfigurable chiral spintronic terahertz emission method is introduced through investigations on a model system of synthetic antiferromagnet and the spin information of individual ferromagnet (FM) layers is extracted. Upon femtosecond photoexcitation of the synthetic antiferromagnet (FM1/Ru/FM2), the ferromagnets generate a pair of linearly polarized ultrafast spin currents, which, after relaxation at the FM/Ru interface, emit corresponding THz pulses. The Ruderman–Kittel–Kasuya–Yosida interactions between the two FMs give rise to magnetic‐field‐controlled spin textures causing a spin relaxation imbalance at the interfaces and induce a phase shift between the orthogonal components of the emitted terahertz fields, consequently reconfiguring the polarization of the emitted terahertz field from linear to circular. This approach offers a novel means of investigating electronic and magnetic states in ultrathin spintronic heterostructures, low‐dimensional quantum materials, topological insulators, and Weyl semimetals. Furthermore, it enables the exploration of spin textures, including skyrmions, merons, and solitons.

Non-reciprocal optical bistability of sandwiched structure containing magnetic Weyl semimetals

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.

Precise Fermi level engineering in a topological Weyl semimetal via fast ion implantation

The precise controllability of the Fermi level is a critical aspect of quantum materials. For topological Weyl semimetals, there is a pressing need to fine-tune the Fermi level to the Weyl nodes and unlock exotic electronic and optoelectronic effects associated with the divergent Berry curvature. However, in contrast to two-dimensional materials, where the Fermi level can be controlled through various techniques, the situation for bulk crystals beyond laborious chemical doping poses significant challenges. Here, we report the milli-electron-volt (meV) level ultra-fine-tuning of the Fermi level of bulk topological Weyl semimetal tantalum phosphide using accelerator-based high-energy hydrogen implantation and theory-driven planning. By calculating the desired carrier density and controlling the accelerator profiles, the Fermi level can be experimentally fine-tuned from 5 meV below, to 3.8 meV below, to 3.2 meV above the Weyl nodes. High-resolution transmission electron microscopy reveals the crystalline structure is largely maintained under irradiation, while electrical transport indicates that Weyl nodes are preserved and carrier mobility is also largely retained. Our work demonstrates the viability of this generic approach to tune the Fermi level in semimetal systems and could serve to achieve property fine-tuning for other bulk quantum materials with ultrahigh precision.

Thermoelectric effects in ballistic junctions of 2D-Weyl semimetals

Two dimensional Weyl semimetals are a variant of topological materials in two dimensions, which are realized in three atomic layer materials. These materials possess isotropic or anisotropic dispersions around Weyl nodes and present some common physical properties with their 3D counterparts. We study the thermoelectric effects in a ballistic junction of 2D-Weyl semimetal with isotropic or anisotropic dispersions around the Weyl points. We demonstrate that these two types of 2D-Weyl semimetals exhibit different thermoelectric responses at low chemical potentials of the leads. The Weyl semimetal layer’s physical characteristics can significantly affect these junctions’ thermoelectric properties. Moreover, significant Seebeck coefficient and thermoelectric figure of merit of these junctions are only observed at the low chemical potential of the leads. In particular, we unveil that thermoelectric effects in the ballistic junction of 2D-Weyl semimetals are not as efficient as their 3D counterparts.

Weak and strong chirality-anomaly-manipulations in a superconducting Weyl semimetal sandwich structure

We investigate the quantum interference of the electron–hole conversions from the two interfaces in a Weyl semimetal (WSM)-based hybrid structure, in which a superconducting WSM is sandwiched in between two normal ones. The quantum interference is characterized by the chirality-anomaly-manipulation (CAM). It is found that only low energy is in favor for s-wave BCS pairing states. The Andreev reflection (AR) chirality blockade can be tuned by the stagger angle α for the relative orientation of paired Weyl points, accompanied by an AR bipolar chirality diode. Thus, a strong CAM is indicated for the electron–hole conversion. However, the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) pairing states have no energy preference, with the weak and strong CAMs being near and far away from the zero energy, respectively. More interestingly, a perfect AR with the normal reflection suppressed thoroughly can be obtained at any α as a result of the FFLO paring with the same chirality. In addition, the conductance or noise power, which incorporates the contributions of the two paired Weyl nodes, not only, in turn, embodies the respective features of their contributions but also can be experimentally measured to discern between the BCS and FFLO paring states.

Terahertz and Infrared Plasmon Polaritons in PtTe2 Type-II Dirac Topological Semimetal.

Surface plasmon polaritons (SPPs) are electromagnetic excitations existing at the interface between a metal and a dielectric. SPPs provide a promising path in nanophotonic devicesforlight manipulation at the micro and nanoscale with applications in optoelectronics, biomedicine, and energy harvesting. Recently, SPPs are extended to unconventional materials like graphene, transparent oxides, superconductors, and topological systems characterized by linearly dispersive electronic bands. In this respect, 3D Dirac and Weyl semimetals offer a promising frontier for infrared (IR) and terahertz (THz) radiation tuning by topologically-protected SPPs.In this work, the THz-IR optical response of platinum ditelluride (PtTe2) type-II Dirac topological semimetal films grown on Si substrates is investigated. SPPs generated on microscale ribbon arrays of PtTe2are detected in the far-field limit,finding an excellent agreement among measurements, theoretical models, and electromagnetic simulation data.The far-field measurements are further supported by near-field IR data which indicate a strong electric field enhancement due to the SPP excitation near the ribbon edges. The present findings indicate that the PtTe2ribbon array appears an idealactive layout for geometrically tunable SPPs thus inspiring a new fashion of optically tunable materials in the technologically demanding THz and IR spectrum.

Absence of localization in Weyl semimetals

Absence of localization in Weyl semimetals

Light-Field-Driven Non-Ohmic Current Generation by an Intense THz Pulse in a Weyl Semimetal

Light-Field-Driven Non-Ohmic Current Generation by an Intense THz Pulse in a Weyl Semimetal

High-harmonic generation in semi-Dirac and Weyl semimetals with broken time-reversal symmetry: Exploration of the merging of Weyl nodes

High-harmonic generation in semi-Dirac and Weyl semimetals with broken time-reversal symmetry: Exploration of the merging of Weyl nodes

Photoinduced Floquet higher-order Weyl semimetal in C6 symmetric Dirac semimetals

Abstract Topological Dirac semimetals are a parent state from which other exotic topological phases of matter, such as Weyl semimetals and topological insulators, can emerge. In this study, we investigate a Dirac semimetal possessing sixfold rotational symmetry and hosting higher-order topological hinge Fermi arc states, which is irradiated by circularly polarized light. Our findings reveal that circularly polarized light splits each Dirac node into a pair of Weyl nodes due to the breaking of time-reversal symmetry, resulting in the realization of the Weyl semimetal phase. This Weyl semimetal phase exhibits rich boundary states, including two-dimensional surface Fermi arc states and hinge Fermi arc states confined to six hinges. Furthermore, by adjusting the incident direction of the circularly polarized light, we can control the degree of tilt of the resulting Weyl cones, enabling the realization of different types of Weyl semimetals.

Dichroism and broadband nonreciprocal transmission in Weyl semimetals

Dichroism and broadband nonreciprocal transmission in Weyl semimetals

Bifunctional dual-channel polarization converter based on Weyl semimetal multilayer structure

In this paper, a multilayer structure (MS) of Weyl semimetals (WSMs) is utilized as the foundation for a polarization converter (PCT) having two different functions in two frequency channels. An analysis of the transmission spectrum of this layer structure at varying incident angles and polarization directions is also included. We define the polarization deflection angle ϕ and discuss the structure’s potential as a PCT with respect to the ϕ = 45° scenario. The incident angles were chosen as 0°, 5°, 10°, 15° and 20° for the analysis, and the results reveal that the incident linear polarization waves in the interval of 78.87 ∼ 81.90 THz can be effectively transformed into circular polarization ones for transmission case. While, for electromagnetic waves with frequencies ranging from 67.44 ∼ 72.11 THz, the direction of vibration for the electric field deflects by 45°. This work also provides positive insights into the design of polarization-related isolators, angle selectors, and other optical devices.

Circular dichroism in nonlinear topological Weyl semimetals

In recent years, the field of topological photonics has emerged as a promising area of research due to its potential for the development of new photonic devices with unique properties. Topological Weyl semimetals (TWSs), which are characterized by the presence of Weyl points in their electronic band structure, are one such example of a material with interesting topological properties. In this study, Kerr and Faraday rotations were used to determine the nonlinear characteristics of TWSs. We focused on surfaces where no Fermi arcs are involved, so that Maxwell’s equations would contain some peculiar topological terms. In Weyl semimetals with a specific topology, the distance between Weyl nodes aligned along the z-direction functions as a magnet. This results in a significant polar Kerr/Faraday rotation effect that is proportional to the separation distance, when light is directed onto the surface of the TWS that lacks Fermi arc states. Conversely, when the light is directed onto a surface with Fermi arc states, the Voigt effect is quadratically proportional to the separation distance. We considered electromagnetic wave propagation in a nonlinear Kerr-type medium. We have derived and solved the linear and nonlinear Helmholtz equations for TWSs using the tanh method. Our findings reveal that wave solutions could have some potentially significant implications for the design and optimization of photonic devices based on TWSs.

High-entropy engineering of the crystal and electronic structures in a Dirac material

Dirac and Weyl semimetals are a central topic of contemporary condensed matter physics, and the discovery of new compounds with Dirac/Weyl electronic states is crucial to the advancement of topological materials and quantum technologies. Here we show a widely applicable strategy that uses high configuration entropy to engineer relativistic electronic states. We take the AMnSb2 (A = Ba, Sr, Ca, Eu, and Yb) Dirac material family as an example and demonstrate that mixing of Ba, Sr, Ca, Eu and Yb at the A site generates the compound (Ba0.38Sr0.14Ca0.16Eu0.16Yb0.16)MnSb2 (denoted as A5MnSb2), giving access to a polar structure with a space group that is not present in any of the parent compounds. A5MnSb2 is an entropy-stabilized phase that preserves its linear band dispersion despite considerable lattice disorder. Although both A5MnSb2 and AMnSb2 have quasi-two-dimensional crystal structures, the two-dimensional Dirac states in the pristine AMnSb2 evolve into a highly anisotropic quasi-three-dimensional Dirac state triggered by local structure distortions in the high-entropy phase, which is revealed by Shubnikov–de Haas oscillations measurements.

Connecting the avoided quantum critical point to the magic-angle transition in three-dimensional Weyl semimetals

Connecting the avoided quantum critical point to the magic-angle transition in three-dimensional Weyl semimetals