A stable three-dimensional topological Dirac semimetal Cd3As2

Unusually High Occupation of Co 3d State in Magnetic Weyl Semimetal Co3Sn2S2

The physical properties of magnetic topological materials are strongly influenced by their nontrivial band topology coupled with the magnetic structure. Co3Sn2S2 is a ferromagnetic kagome Weyl semimetal displaying giant intrinsic anomalous Hall effect which can be further tuned via elemental doping, such as Ni substitution for Co. Despite significant interest, the exact valency of Co and the magnetic order of the Ni dopants remained unclear. Here, we report a study of Ni-doped Co3Sn2S2 single crystals using synchrotron-based X-ray magnetic circular dichroism (XMCD), X-ray photoelectron emission microscopy (XPEEM), and hard/soft X-ray photoemission spectroscopy (XPS) techniques. We confirm the presence of spin-dominated magnetism from Co in the host material, and also the establishment of ferromagnetic order from the Ni dopant. The oxygen-free photoemission spectrum of the Co 2p core levels in the crystal well resembles that of a metallic Co film, indicating a Co0+ valency. Surprisingly, we find the electron filling in the Co 3d state can reach 8.7-9.0 electrons in these single crystals. Our results highlight the importance of element-specific X-ray spectroscopy in understanding the electronic and magnetic properties that are fundamental to a heavily studied Weyl semimetal, which could aid in developing future spintronic applications based on magnetic topological materials.

Light-Induced Ultrafast Glide-Mirror Symmetry Breaking in Black Phosphorus

Symmetry breaking plays an important role in the fields of physics, ranging from particle physics to condensed matter physics. In solid-state materials, phase transitions are deeply linked to the underlying symmetry breakings, resulting in a rich variety of emergent phases. Such symmetry breakings are often induced by controlling the chemical composition and temperature or applying an electric field, strain, etc. In this work, we demonstrate ultrafast glide-mirror symmetry breaking in black phosphorus through Floquet engineering. Upon near-resonance pumping, a light-induced full gap opening is observed at the glide-mirror symmetry protected nodal ring, suggesting light-induced breaking of the glide-mirror symmetry. Moreover, the full gap is observed only in the presence of the light-field and disappears almost instantaneously (≪100 fs) when the light-field is turned off, suggesting the ultrafast manipulation of the symmetry and its Floquet engineering origin. This work not only demonstrates light-matter interaction as an effective way to realize ultrafast symmetry breaking in solid-state materials but also moves forward toward the long-sought Floquet topological phases.

Polymorphism in Type-II Dirac Semimetal WSi2 under Pressure: Structural, Mechanical, and Electronic Insights

The type-II Dirac candidate semimetal WSi2 is a promising candidate for electronic devices, quantum computing, and topological materials research, owing its distinct electronic structure and superior mechanical properties. Here, we synthesized high-quality WSi2 materials and systematically investigated their compressive behavior, and structural and electronic properties under high pressure using in-situ high pressure experiments, complemented by first-principles calculations. The results confirms that WSi2 has the properties of a type-II Dirac semimetal. Our results demonstrate that WSi2 maintains structural stability under high pressure but undergoes an electronic phase transition from a semimetal to a metal around 40 GPa. Additionally, the mechanical hardness softens discontinuously at this pressure. The structural stability of WSi2 under high pressure is attributed to the strong hybridization of Si-3p and W-5d electrons, the rigid crystal lattice, and the adaptable electronic structure. The pressure-induced electronic phase transition and softening are primarily governed by the energy band reconstruction and W-5d orbitals. This study provides valuable insights into the high-pressure behavior of type-II Dirac semimetal, highlighting their potential for advanced applications in electronic devices and topological quantum computing under extreme conditions by elucidating their structural stability and electronic phase transition mechanisms.

Recent advances in understanding and manipulating magnetic and electronic properties of EuM2X2(M= Zn, Cd;X= P, As)

Over the past five years, significant progress has been made in understanding the magnetism and electronic properties of CaAl2Si2-type EuM2X2(M= Zn, Cd;X= P, As) compounds. Prior theoretical work and experimental studies suggested that EuCd2As2had the potential to host rich topological phases, particularly an ideal magnetic Weyl semimetal state when the spins are polarized along thecaxis. However, this perspective is challenged by recent experiments utilizing samples featuring ultra-low carrier densities, as well as meticulous calculations employing various approaches. Nonetheless, the EuM2X2family still exhibit numerous novel properties that remain to be satisfactorily explained, such as the giant nonlinear anomalous Hall effect and the colossal magnetoresistance effect. Moreover, EuM2X2compounds can be transformed from semiconducting antiferromagnets to metallic ferromagnets by introducing a small number of carriers or applying external pressure, and a further increase in the ferromagnetic transition temperature can be achieved by reducing the unit cell volume. These features make the EuM2X2family a fertile platform for studying the interplay between magnetism and charge transport, and an excellent candidate for applications in spintronics. This paper presents a comprehensive review of the magnetic and transport behaviors of EuM2X2compounds with varying carrier densities, as well as the current insights into these characteristics. An outlook for future research opportunities is also provided.

Progressive Control of Rashba State on Topological Dirac Semimetal KZnBi

Rashba states have been actively revisited as a platform for advanced applications such as spintronics and topological quantum computation. Yet, access to the Rashba state is restricted to the specific material sets, and the methodology to control the Rashba state is not established. Here, we report the Rashba states on the (001) surface of KZnBi, a 3D Dirac semimetal. Using angle-resolved photoemission spectroscopy and first-principles calculations, we investigated the evolution of Rashba states under different surface conditions controlled by alkali metal deposition. We observed that restoring surface ordering enables a Rashba state, which is absent in freshly cleaved surfaces. Interestingly, we were able to modify the dispersion of the Rashba state from an ordinary parabolic dispersion to a linearly dispersing Dirac-like state by additional alkali-metal deposition. Our findings provide a methodology for engineering the properties of Rashba states for advanced applications and redefine topological systems as generic hosts of Rashba states.

Excited State Band Mapping and Ultrafast Nonequilibrium Dynamics in Topological Dirac Semimetal 1T-ZrTe2

We performed time- and polarization-resolved extreme ultraviolet momentum microscopy on the topological Dirac semimetal candidate 1T-ZrTe2. Excited state band mapping uncovers the previously inaccessible linear dispersion of the Dirac cone above the Fermi level. We study the orbital texture of bands using linear dichroism in photoelectron angular distributions. These observations provide hints about the topological character of 1T-ZrTe2. Time-, energy-, and momentum-resolved nonequilibrium carrier dynamics reveal that intra- and interband scattering processes play a major role in the relaxation mechanism, leading to multivalley electron-hole accumulation near the Fermi level. We also show that electrons' inverse lifetime has a linear dependence as a function of their excess energy. Our time- and polarization-resolved XUV photoemission results shed light on the excited state electronic structure of 1T-ZrTe2 and provide valuable insights into the relatively unexplored field of quantum-state-resolved ultrafast dynamics in 3D topological Dirac semimetals.

Enhancement of the thermoelectric figure of merit in the Dirac semimetal Cd3As2 by band-structure and -filling control

Topological materials attract a considerable research interest because of their characteristic band structure giving rise to various new phenomena in quantum physics. Besides this, they are tempting from a functional materials point of view: Topological materials bear potential for an enhanced thermoelectric efficiency because they possess the required ingredients, such as intermediate carrier concentrations, large mobilities, heavy elements etc. Against this background, this work reports an enhanced thermoelectric performance of the topological Dirac semimetal Cd3As2 upon alloying the trivial semiconductor Zn3As2. This allows to gain fine-tuned control over both the band filling and the band topology in Cd3-x Zn x As2. As a result, the thermoelectric figure of merit exceeds 0.5 around x = 0.6 and x = 1.2 at elevated temperatures. The former is due to an enhancement of the power factor, while the latter is a consequence of a strong suppression of the thermal conductivity. In addition, in terms of first-principle band structure calculations, the thermopower in this system is theoretically evaluated, which suggests that the topological aspects of the band structure change when traversing x = 1.2 .

High-Order Harmonic Generation in Photoexcited Three-Dimensional Dirac Semimetals

High-order harmonic generation (HHG) in condensed matter is highly important for potential applications in various fields, such as materials characterization, all-optical switches, and coherent light source generation. Linking HHG to the properties or dynamic processes of materials is essential for realizing these applications. Here, a bridge has been built between HHG and the transient properties of materials through the engineering of interband polarization in a photoexcited three-dimensional Dirac semimetal (3D-DSM). It has been found that HHG can be efficiently manipulated by the electronic relaxation dynamics of 3D-DSM on an ultrafast time scale of several hundred femtoseconds. Furthermore, time-resolved HHG (tr-HHG) has been demonstrated to be a powerful spectroscopy method for tracking electron relaxation dynamics, enabling the identification of electron thermalization and electron-phonon coupling processes and the quantitative extraction of electron-phonon coupling strength. This demonstration provides insights into the active control of HHG and measurements of the electron dynamics.

Terahertz tunable three-dimensional photonic jets

Highly localized electromagnetic field distributions near the "shadow-side" surface of certain transparent mesoscale bodies illuminated by light waves are called photonic jets. We demonstrated formation of three-dimensional (3D) tunable photonic jets in terahertz regime (terajets, TJs) by dielectric micro-objects -including spheres, cylinders, and cubes-coated with a bulk Dirac semimetal (BDS) layer, under uniform beam illumination. The optical characteristics of the produced TJs can be modulated dynamically through tuning the BDS layer's index of refraction via changing its Fermi energy. It is demonstrated that the Fermi energy of BDS layer has a significant impact on tuning the optical characteristics of the produced photonic jets for both TE and TM polarizations. A notable polarization dependency of the characteristics of the TJs was also observed. The impact of obliquity of the incident beam was studied as well and it was demonstrated that electromagnetic field distributions corresponding to asymmetric photonic jets can be formed in which the intensity at the focal region is preserved in a wide angular range which could find potential application in scanning devices. It was found that the maximum intensity of the TJ occurs at a non-trivial morphology-dependent source-angle.

Asymmetric Tilt-Induced Quantum Beating of Conductance Oscillation in Magnetically Modulated Dirac Matter Systems

Herein, we investigate the effect of tilt mismatch on the quantum oscillations of spin transport properties in two-dimensional asymmetrically tilted Dirac cone systems. This study involves the examination of conductance oscillation in two distinct junction types: transverse- and longitudinal-tilted Dirac cones (TTDCs and LTDCs). Our findings reveal an unusual quantum oscillation of spin-polarized conductance within the TTDC system, characterized by two distinct anomaly patterns within a single period, labeled as the linear conductance phase and the oscillatory conductance phase. Interestingly, these phases emerge in association with tilt-induced orbital pseudo-magnetization and exchange interaction. Our study also demonstrates that the structure of the LTDC can modify the frequency of spin conductance oscillation, and the asymmetric effect within this structure results in a quantum beating pattern in oscillatory spin conductance. We note that an enhancement in the asymmetric longitudinal tilt velocity ratio within the structure correspondingly amplifies the beating frequency. Our research potentially contributes valuable insights for detecting the asymmetry of tilted Dirac fermions in type-I Dirac semimetal-based spintronics and quantum devices.

Uncooled Broadband Photodetection via Light Trapping in Conformal PtTe2-Silicon Nanopillar Heterostructures

Dirac semimetals have demonstrated significant attraction in the field of optoelectronics due to their unique bandgap structure and high carrier mobility. Combining them with classical semiconductor materials to form heterojunctions enables broadband optoelectronic conversion at room temperature. However, the low light absorption of layered Dirac semimetals substantially limits the device's responsivity in the infrared band. Herein, a three-dimensional (3D) heterostructure, composed of silicon nanopillars (SiNPs) and a conformal PtTe2 film, is proposed and demonstrated to enhance the photoresponsivity for uncooled broadband detection. The light trapping effect in the 3D heterostructure efficiently promotes the interaction between light and PtTe2, while also enhancing the light absorption efficiency of silicon, which enables the enhancement of the device responsivity across a broadband spectrum. Experimentally, the PtTe2-SiNPs heterojunction device demonstrates excellent photoelectric conversion behavior across the visible, near-infrared, and long-wave infrared (LWIR) bands, with its responsivity demonstrating an order-of-magnitude improvement compared to the counterparts with planar silicon heterojunctions. Under 11 μm laser irradiation, the noise equivalent power (NEP) can reach 1.76 nW·Hz-1/2 (@1 kHz). These findings offer a strategic approach to the design and fabrication of high-performance broadband photodetectors based on Dirac semimetals.

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.

Antiferromagnetically ordered topological semimetals in Hubbard model with spin-orbit coupling

We examine the possible existence of Dirac semimetal with magnetic order in a two-dimensional system with a nonsymmorphic symmetry by using the Hartree-Fock mean-field theory within the Hubbard model. We locate the region in the second-neighbor spin-orbit coupling vs Hubbard interaction phase diagram, where such a state is stabilized. The edge states for the ribbons along two orthogonal directions concerning the orientation of in-plane magnetic moments are obtained. Finally, the effect of the in-plane magnetic field, which results in the stabilization of the Weyl semimetallic (WSM) state, and the nature of the edge states corresponding to the WSM state for ribbon geometries are also explored.

Coexistence of giant Goos-Hänchen shift and high reflectance in Dirac semimetal based multilayered structure

Bulk Dirac semimetals (BDSs) possess Fermi energy dependent optical parameters, providing unprecedented opportunities for the study of the controllable Goos-Hänchen (GH) shift. However, the enhancement of GH shifts often comes at the cost of the reflectance in the previous BDS-based structures, which hinders their practical application. In this work, we theoretically present the investigation of the GH shift in a multilayered structure composed of one BDS film and a symmetric one-dimensional photonic crystal (1DPC) with a defect layer. We demonstrate that this well-designed structure supports a large GH shift at the specific working wavelength, whose magnitude can be enhanced up to 3883 times the incident wavelength. In particular, such an enhanced GH shift achieved in this structure is associated with high reflectance (0.94) and these remarkable features can be attributed to the sharp change in the reflective phase and the destructive interference that occurs between the simultaneously excited optical Tamm state (OTS) at the BDS/1DPC interface and the defect state at the 1D defected PC. In addition, we also explore the manipulation of the GH shift by adjusting the Fermi energy of the BDS as well as the geometrical parameter of the multilayered structure. Our results provide a new approach for realizing an enhanced and controllable GH shift in a BDS-based multilayered structure, which endows it with promising prospects for application in optical sensors, optical detectors and beam controllers.

Active multi-focus vortex beam terahertz encoding metasurface based on Dirac semimetals

An electromagnetic wavefront can be flexibly manipulated by discrete phase coding on the coding unit. In this paper, we designed two coding metasurfaces with 1-bit and 3-bit based on active tuning of Dirac semimetals by controlling the Fermi level (E F) with an external polarization voltage. The size and structure of the metasurface remain unchanged with this strategy. Both designs were found to be dynamically tunable. The 1-bit coding metasurface enables beam conversion, single-focus switching, and switching between single-focus and multi-focus. On the other hand, the 3-bit coding metasurface enables the switching between vortex beams and single-beam mirror reflections. These proposed structures have potential applications in terahertz (THz) communications and terahertz-focused imaging, opening up new possibilities for the dynamic modulation of THz waves.

Chiral anomaly and Weyl orbit in three-dimensional Dirac semimetal Cd3As2grown on Si

Preparing Cd3As2, which is a three-dimensional (3D) Dirac semimetal in certain crystal orientation, on Si is highly desirable as such a sample may well be fully compatible with existing Si CMOS technology. However, there is a dearth of such a study regarding Cd3As2films grown on Si showing the chiral anomaly. Here,for the first time, we report the novel preparation and fabrication technique of a Cd3As2(112) film on a Si (111) substrate with a ZnTe (111) buffer layer which explicitly shows the chiral anomaly with a nontrivial Berry's phase ofπ. Despite the Hall carrier density (n3D≈9.42×1017cm-3) of our Cd3As2film, which is almost beyond the limit for the portents of a 3D Dirac semimetal to emerge, we observe large linear magnetoresistance in a perpendicular magnetic field and negative magnetoresistance in a parallel magnetic field. These results clearly demonstrate the chiral magnetic effect and 3D Dirac semimetallic behavior in our silicon-based Cd3As2film. Our tailoring growth of Cd3As2on a conventional substrate such as Si keeps the sample quality, while also achieving a low carrier concentration.

High-Mobility Topological Semimetals as Novel Materials for Huge Magnetoresistance Effect and New Type of Quantum Hall Effect

The quantitative description of electrical and magnetotransport properties of solid-state materials has been a remarkable challenge in materials science over recent decades. Recently, the discovery of a novel class of materials-the topological semimetals-has led to a growing interest in the full understanding of their magnetotransport properties. In this review, the strong interplay among topology, band structure, and carrier mobility in recently discovered high carrier mobility topological semimetals is discussed and their effect on their magnetotransport properties is outlined. Their large magnetoresistance effect, especially in the Hall transverse configuration, and a new version of a three-dimensional quantum Hall effect observed in high-mobility Weyl and Dirac semimetals are reviewed. The possibility of designing novel quantum sensors and devices based on solid-state semimetals is also examined.

Emergence of Weyl fermions by ferrimagnetism in a noncentrosymmetric magnetic Weyl semimetal

Condensed matter physics has often provided a platform for investigating the interplay between particles and fields in cases that have not been observed in high-energy physics. Here, using angle-resolved photoemission spectroscopy, we provide an example of this by visualizing the electronic structure of a noncentrosymmetric magnetic Weyl semimetal candidate NdAlSi in both the paramagnetic and ferrimagnetic states. We observe surface Fermi arcs and bulk Weyl fermion dispersion as well as the emergence of new Weyl fermions in the ferrimagnetic state. Our results establish NdAlSi as a magnetic Weyl semimetal and provide an experimental observation of ferrimagnetic regulation of Weyl fermions in condensed matter.

Colossal magnetoresistance in the multiple wave vector charge density wave regime of an antiferromagnetic Dirac semimetal

Colossal negative magnetoresistance is a well-known phenomenon, notably observed in hole-doped ferromagnetic manganites. It remains a major research topic due to its potential in technological applications. In contrast, topological semimetals show large but positive magnetoresistance, originated from the high-mobility charge carriers. Here, we show that in the highly electron-doped region, the Dirac semimetal CeSbTe demonstrates similar properties as the manganites. CeSb0.11Te1.90 hosts multiple charge density wave modulation vectors and has a complex magnetic phase diagram. We confirm that this compound is an antiferromagnetic Dirac semimetal. Despite having a metallic Fermi surface, the electronic transport properties are semiconductor-like and deviate from known theoretical models. An external magnetic field induces a semiconductor metal-like transition, which results in a colossal negative magnetoresistance. Moreover, signatures of the coupling between the charge density wave and a spin modulation are observed in resistivity. This spin modulation also produces a giant anomalous Hall response.

Drumhead surface states promoted hydrogen evolution reactions in type-II nodal-line topological catalyst Mg3Bi2

An excellent catalyst generally meets three indicators: high electron mobility, high surface density of states and low Gibbs free energy (ΔG) [H. Luo et al. Nat. Rev. Phys., 2022, 4, 611-624]. Recent studies have confirmed that topological materials exhibit more advantages than conventional precious metals with regard to the above-mentioned indicators. Herein, based on DFT calculations and symmetry analysis, we discovered for the first time that the topological surface states of Mg3Bi2 with a Kagome lattice promote hydrogen evolution reactions (HERs). In particular, there exists a snake-like type-II nodal loop (NL), located on kz = 0 plane in Mg3Bi2. Besides, the NL forms a topologically protected drumhead surface state on the (001) surface. It was found that the ΔG (0.176 eV) value of the (001) surface is comparable to that of the precious metal Pt. Then, through hole doping and strain regulation, it was found that the catalytic activity of Mg3Bi2 is closely related to the drumhead surface state formed by NL. With the above-mentioned results, this study not only provides a promising candidate material for hydrogen electrolysis, but also deepens our understanding of the dominant factors of NL semimetals for the catalytic activity.

Actively tunable plasmon-induced transparency in terahertz based on Dirac semimetal metamaterials

We numerically investigate a tunable plasmon-induced transparency based on bulk Dirac semimetal (BDS) metamaterial in the terahertz band. In the unit cell, the prominent transparent peak appears to be due to the interference between the cut wires (CWs) and split-ring resonators (SRRs). An active modulation via near-field coupling is obtained by varying the Fermi level of the BDS. Introducing photoactive silicon, it will be found that once the intensity of the pump light is adjusted, a tunable transparent peak will appear. Furthermore, by shifting the coupling distance between CWs and SRRs, the depth of the transparent peak will change accordingly. Finally, we place the structure in environments with different refractive indices, which will exhibit excellent sensitivity and facilitate the application of biochemical sensors. This simple and easy-to-fabricate metamaterial structure will have excellent potential applications in modulation, filters, and detection.

Photocurrent Induced by a Bicircular Light Drive in Centrosymmetric Systems

A bicircular light (BCL) consists of left and right circularly polarized lights with different frequencies, and draws a roselike pattern with a rotational symmetry determined by the ratio of the two frequencies. Here we show that an application of a BCL to centrosymmetric systems allows a photocurrent generation through introduction of an effective polarity to the system. We derive formulas for the BCL-induced photocurrent from a standard perturbation theory, which is then applied to a simple 1D model and 3D Dirac and Weyl semimetals. A nonperturbative effect with strong light intensity is also discussed with the Floquet technique.

Circular Photogalvanic Current in Ni-Doped Cd3As2 Films Epitaxied on GaAs(111)B Substrate

Magnetic element doped Cd₃As₂ Dirac semimetal has attracted great attention for revealing the novel quantum phenomena and infrared opto-electronic applications. In this work, the circular photogalvanic effect (CPGE) was investigated at various temperatures for the Ni-doped Cd₃As₂ films which were grown on GaAs(111)B substrate by molecular beam epitaxy. The CPGE current generation was found to originate from the structural symmetry breaking induced by the lattice strain and magnetic doping in the Ni-doped Cd₃As₂ films, similar to that in the undoped ones. However, the CPGE current generated in the Ni-doped Cd₃As₂ films was approximately two orders of magnitude smaller than that in the undoped one under the same experimental conditions and exhibited a complex temperature variation. While the CPGE current in the undoped film showed a general increase with rising temperature. The greatly reduced CPGE current generation efficiency and its complex variation with temperature in the Ni-doped Cd₃As₂ films was discussed to result from the efficient capture of photo-generated carriers by the deep-level magnetic impurity bands and enhanced momentum relaxation caused by additional strong impurity scattering when magnetic dopants were introduced.

Sensitivity-Tunable Terahertz Liquid/Gas Biosensor Based on Surface Plasmon Resonance with Dirac Semimetal

In this paper, we study the sensitivity-tunable terahertz (THz) liquid/gas biosensor in a coupling prism-three-dimensional Dirac semimetal (3D DSM) multilayer structure. The high sensitivity of the biosensor originates from the sharp reflected peak caused by surface plasmon resonance (SPR) mode. This structure achieves the tunability of sensitivity due to the fact that the reflectance could be modulated by the Fermi energy of 3D DSM. Besides, it is found that the sensitivity curve depends heavily on the structural parameters of 3D DSM. After parameter optimization, we obtained sensitivity over 100°/RIU for liquid biosensor. We believe this simple structure provides a reference idea for realizing high sensitivity and a tunable biosensor device.

Experimental Realization of a Three-Dimensional Dirac Semimetal Phase with a Tunable Lifshitz Transition in Au_{2}Pb

Three-dimensional Dirac semimetals are an exotic state of matter that continue to attract increasing attention due to the unique properties of their low-energy excitations. Here, by performing angle-resolved photoemission spectroscopy, we investigate the electronic structure of Au_{2}Pb across a wide temperature range. Our experimental studies on the (111)-cleaved surface unambiguously demonstrate that Au_{2}Pb is a three-dimensional Dirac semimetal characterized by the presence of a bulk Dirac cone projected off-center of the bulk Brillouin zone (BZ), in agreement with our theoretical calculations. Unusually, we observe that the bulk Dirac cone is significantly shifted by more than 0.4 eV to higher binding energies with reducing temperature, eventually going through a Lifshitz transition. The pronounced downward shift is qualitatively reproduced by our calculations indicating that an enhanced orbital overlap upon compression of the lattice, which preserves C_{4} rotational symmetry, is the main driving mechanism for the Lifshitz transition. These findings not only broaden the range of currently known materials exhibiting three-dimensional Dirac phases, but also show a viable mechanism by which it could be possible to switch on and off the contribution of the degeneracy point to electron transport without external doping.

Parallel and anti-parallel helical surface states for topological semimetals

Weyl points, carrying a Z-type monopole charge [Formula: see text], have bulk-surface correspondence (BSC) associated with helical surface states (HSSs). When |[Formula: see text]| [Formula: see text], multi-HSSs can appear in a parallel manner. However, when a pair of Weyl points carrying [Formula: see text] [Formula: see text] meet, a Dirac point carrying [Formula: see text] = 0 can be obtained and the BSC vanishes. Nonetheless, a recent study in Zhang et al. (Phys Rev Res 4:033170, 2022) shows that a new BSC can survive for Dirac points when the system has time-reversal ([Formula: see text])-glide ([Formula: see text]) symmetry ([Formula: see text]=TG), i.e., anti-parallel double/quad-HSSs associated with a new [Formula: see text]-type monopole charge [Formula: see text] appear. In this paper, we systematically review and discuss both the parallel and anti-parallel multi-HSSs for Weyl and Dirac points, carrying two different kinds of monopole charges. Two material examples are offered to understand the whole configuration of multi-HSSs. One carries the Z-type monopole charge [Formula: see text], showing both local and global topology for three kinds of Weyl points, and it leads to parallel multi-HSSs. The other carries the [Formula: see text]-type monopole charge [Formula: see text], only showing the global topology for [Formula: see text]-invariant Dirac points, and it is accompanied by anti-parallel multi-HSSs.

Two step I to II type transitions in layered Weyl semi-metals and their impact on superconductivity

Novel "quasi two dimensional" typically layered (semi) metals offer a unique opportunity to control the density and even the topology of the electronic matter. Along with doping and gate voltage, a robust tuning is achieved by application of the hydrostatic pressure. In Weyl semi-metals the tilt of the dispersion relation cones, [Formula: see text] increases with pressure, so that one is able to reach type II ([Formula: see text]starting from the more conventional type I Weyl semi-metals [Formula: see text]. The microscopic theory of such a transition is constructed. It is found that upon increasing pressure the I to II transition occurs in two continuous steps. In the first step the cones of opposite chirality coalesce so that the chiral symmetry is restored, while the second transition to the Fermi surface extending throughout the Brillouin zone occurs at higher pressures. Flattening of the band leads to profound changes in Coulomb screening. Superconductivity observed recently in wide range of pressure and chemical composition in Weyl semi-metals of both types. The phonon theory of pairing including the Coulomb repulsion for a layered material is constructed and applied to recent extensive experiments on [Formula: see text].

3D Dirac semimetal supported thermal tunable terahertz hybrid plasmonic waveguides

By depositing the trapezoidal dielectric stripe on top of the 3D Dirac semimetal (DSM) hybrid plasmonic waveguide, the thermal tunable propagation properties have been systematically investigated in the terahertz regime, taking into account the influences of the structure of the dielectric stripe, temperature and frequency. The results manifest that as the upper side width of the trapezoidal stripe increases, the propagation length and figure of merit (FOM) both decrease. The propagation properties of hybrid modes are closely associated with temperature, in that when the temperature changes in the scope of 3-600 K, the modulation depth of propagation length is more than 96%. Additionally, at the balance point of plasmonic and dielectric modes, the propagation length and FOM manifest strong peaks and indicate an obvious blue shift with the increase of temperature. Furthermore, the propagation properties can be improved significantly with a Si-SiO₂ hybrid dielectric stripe structure, e.g., on the condition that the Si layer width is 5 µm, the maximum value of the propagation length reaches more than 6.46 × 10⁵µm, which is tens of times larger than those pure SiO₂ (4.67 × 10⁴µm) and Si (1.15 × 10⁴µm) stripe. The results are very helpful for the design of novel plasmonic devices, such as cutting-edge modulator, lasers and filters.

Narrowband, Angle-Tunable, Helicity-Dependent Terahertz Emission from Nanowires of the Topological Dirac Semimetal Cd3As2

All-optical control of terahertz pulses is essential for the development of optoelectronic devices for next-generation quantum technologies. Despite substantial research in THz generation methods, polarization control remains difficult. Here, we demonstrate that by exploiting band structure topology, both helicity-dependent and helicity-independent THz emission can be generated from nanowires of the topological Dirac semimetal Cd₃As₂. We show that narrowband THz pulses can be generated at oblique incidence by driving the system with optical (1.55 eV) pulses with circular polarization. Varying the incident angle also provides control of the peak emission frequency, with peak frequencies spanning 0.21-1.40 THz as the angle is tuned from 15 to 45°. We therefore present Cd₃As₂ nanowires as a promising novel material platform for controllable terahertz emission.

Applications of synthetic biology in medical and pharmaceutical fields

Synthetic biology aims to design or assemble existing bioparts or bio-components for useful bioproperties. During the past decades, progresses have been made to build delicate biocircuits, standardized biological building blocks and to develop various genomic/metabolic engineering tools and approaches. Medical and pharmaceutical demands have also pushed the development of synthetic biology, including integration of heterologous pathways into designer cells to efficiently produce medical agents, enhanced yields of natural products in cell growth media to equal or higher than that of the extracts from plants or fungi, constructions of novel genetic circuits for tumor targeting, controllable releases of therapeutic agents in response to specific biomarkers to fight diseases such as diabetes and cancers. Besides, new strategies are developed to treat complex immune diseases, infectious diseases and metabolic disorders that are hard to cure via traditional approaches. In general, synthetic biology brings new capabilities to medical and pharmaceutical researches. This review summarizes the timeline of synthetic biology developments, the past and present of synthetic biology for microbial productions of pharmaceutics, engineered cells equipped with synthetic DNA circuits for diagnosis and therapies, live and auto-assemblied biomaterials for medical treatments, cell-free synthetic biology in medical and pharmaceutical fields, and DNA engineering approaches with potentials for biomedical applications.

Multiple Magnetic Topological Phases in the van der Waals Crystal Mn(Bi,Sb)4Se7

Magnetic topological materials have drawn markedly attention recently due to the strong coupling of their novel topological properties and magnetic configurations. In particular, the MnBi₂Te₄/(Bi₂Te₃)_n family highlights the researches of multiple magnetic topological materials. Via first-principles calculations, we predict that Mn(Bi, Sb)₄Se₇, the close relatives of MnBi₂Te₄/(Bi₂Te₃)_n family, are topological nontrivival in both antiferromagnetic and ferromagnetic configurations. In the antiferromagnetic ground state, Mn(Bi, Sb)₄Se₇ are simultaneously topological insulators and axion insulators. Massless Dirac surface states emerge on the surfaces parallel to the z axis. In ferromagnetic phases, they are axion insulators. Particularly, when the magnetization direction is along the x axis, they are also topological crystalline insulators. Mirror-symmetry-protected gapless surface states exist on the mirror-invariant surfaces. Hence, the behaviors of surface states are strongly dependent on the magnetization directions and surface orientations. Our work provides more opportunities for the study of magnetic topological physics.

Tunable multichannel Fibonacci one-dimensional terahertz photonic crystal filter

This paper proposes a multichannel terahertz optical filter based on a one-dimensional photonic crystal with a third-order Fibonacci structure, including a bulk Dirac semimetal. The tuning of the optical properties of the proposed structure has been theoretically studied as a function of the Dirac semimetals' Fermi energy. Furthermore, the effects of the Fibonacci structure's periodic number and light's incident angle on optical channels were investigated. The results reveal that changes in the Fermi energy and incident angle remarkably affect the frequency and transmission of the optical channels. Additionally, the number of optical channels increases by increasing the periodic number of the Fibonacci structure.

Two-Dimensional Topological Insulator State in Cadmium Arsenide Thin Films

Two-dimensional topological insulators (2D TIs) are a highly desired quantum phase but few materials have demonstrated clear signatures of a 2D TI state. It has been predicted that 2D TIs can be created from thin films of three-dimensional TIs by reducing the film thickness until the surface states hybridize. Here, we employ this technique to report the first observation of a 2D TI state in epitaxial thin films of cadmium arsenide, a prototype Dirac semimetal in bulk form. Using magnetotransport measurements with electrostatic gating, we observe a Landau level spectrum and quantum Hall effect that are in excellent agreement with those of an ideal 2D TI. Specifically, we observe a crossing of the zeroth Landau levels at a critical magnetic field. We show that the film thickness can be used to tune the critical magnetic field. Moreover, a larger change in film thickness causes a transition from a 2D TI to a 2D trivial insulator, just as predicted by theory. The high degree of tunability available in epitaxial cadmium arsenide heterostructures can thus be used to fine-tune the 2D TI, which is essential for future topological devices.

Two -dimensional semimetal AlSb monolayer with multiple nodal-loops and extraordinary transport properties under uniaxial strain

Two-dimensional (2D) nodal-loop semimetal (NLSM) materials have attracted much attention for their high-speed and low-consumption transporting properties as well as their fantastic symmetry protection mechanisms. In this paper, using systematic first-principles calculations, we present an excellent NLSM candidate, a 2D AlSb monolayer, in which the conduction and valence bands cross with each other forming fascinating multiple nodal-loop (NL) states. The NLSM properties of the AlSb monolayer are protected by its glide mirror symmetry, which was confirmed using a symmetry-constrained six-band tight-binding model. The transport properties of the AlSb monolayer under in-plane uniaxial strains are also studied, based on a non-equilibrium Green's function method. It is found that both compressive and tensile strains from -10% to 10% improve the transporting properties of AlSb, and it is interesting to see that flexure configurations are energetically favored when compressive uniaxial strains are applied. Our studies not only provide a novel 2D NLSM candidate with a new symmetry protection mechanism, but also raise the novel possibility for the detection of out-of-plane flexure in 2D semimetal materials.

Terahertz Biosensor Based on Mode Coupling between Defect Mode and Optical Tamm State with Dirac Semimetal

Bulk Dirac semimetal (BDS) has emerged as a "3D graphene" material for the development of optical devices in the past few years. In this study, a BDS-based tunable highly sensitive terahertz (THz) biosensor is proposed by using a Dirac semimetal/Bragg reflector multilayer structure. The high sensitivity of the biosensor originates from the sharp Fano resonance peak caused by coupling the Optical Tamm State (OTS) mode and defect mode. Besides, the sensitivity of the proposed structure is sensitive to the Fermi energy of Dirac semimetal and the refractive index of the sensing medium. The maximum sensitivity of 1022°/RIU is obtained by selecting structural and material parameter appropriately, which has certain competitiveness compared to conventional surface plasmon resonance (SPR) sensors. From the standpoint of the fabrication facility and integration, we judged that the BDS-based layered structure has the potential application in biosensor field.

Metasurface Enabled Photothermoelectric Photoresponse of Semimetal Cd3As2 for Broadband Photodetection

The artificial engineering of photoresponse is crucial for optoelectronic applications, especially for photodetectors. Here, we designed and fabricated a metasurface on a semimetallic Cd₃As₂ nanoplate to improve its thermoelectric photoresponse. The metasurface can enhance light absorption, resulting in a temperature gradient. This temperature gradient can contribute to thermoelectric photoresponse through the photothermoelectric effect. Furthermore, power-dependent measurements showed a linearly dependent photoresponse of the Cd₃As₂ metasurface device, indicating a second-order photocurrent response. Wavelength-dependent measurements showed that the metasurface can efficiently separate photoexcited carriers in the broadband range of 488 nm to 4 μm. The photoresponse near the metasurface boundaries exhibits a responsivity of ∼1 mA/W, which is higher than that near the electrode junctions. Moreover, the designed metasurface device provided an anisotropic polarization-dependent photoresponse rather than the isotropic photoresponse of the original Cd₃As₂ device. This study demonstrates that metasurfaces have excellent potential for artificial controllable photothermoelectric photoresponse of various semimetallic materials.

Chiral Dirac-like fermion in spin-orbit-free antiferromagnetic semimetals

Dirac semimetal is a phase of matter whose elementary excitation is described by the relativistic Dirac equation. In the limit of zero mass, its parity-time symmetry enforces the Dirac fermion in the momentum space, which is composed of two Weyl fermions with opposite chirality, to be non-chiral. Inspired by the flavor symmetry in particle physics, we theoretically propose a massless Dirac-like equation yet linking two Weyl fields with the identical chirality by assuming SU ( 2 ) isospin symmetry, independent of the space-time rotation exchanging the two fields. Dramatically, such symmetry is hidden in certain solid-state spin-1/2 systems with negligible spin-orbit coupling, where the spin degree of freedom is decoupled with the lattice. Therefore, the existence of the corresponding quasiparticle, dubbed as flavor Weyl fermion, cannot be explained by the conventional (magnetic) space group framework. The 4-fold degenerate flavor Weyl fermion manifests linear dispersion and a Chern number of ± 2, leading to a robust network of topologically protected Fermi arcs throughout the Brillouin zone. For material realization, we show that the transition-metal chalcogenide CoNb3S6 with experimentally confirmed collinear antiferromagnetic order is ideal for flavor Weyl semimetal under the approximation of vanishing spin-orbit coupling. Our work reveals a counterpart of the flavor symmetry in magnetic electronic systems, leading to further possibilities of emergent phenomena in quantum materials.

Two-dimensional antiferromagnetic nodal-line semimetal and quantum anomalous Hall state in the van der Waals heterostructure germanene/Mn2S2

Materials with interactions between the topology and magnetism are triggering increasing interest. We constructed a two-dimensional (2D) van der Waals heterostructure germanene/Mn₂S₂, where the germanene is a quantum spin Hall insulator and Mn₂S₂provides antiferromagnetic (AFM) interactions. In this structure, a 2D AFM nodal-line semimetal (NLSM) phase is expected without the spin-orbit coupling (SOC), which is of a high density of states around the Fermi level. The band touching rings originate from the intersection between different spin components ofporbitals of germanene. This result provides a possible 2D realization of NLSMs, which are usually realized in three-dimensional systems. When the SOC is present, a quantum anomalous Hall (QAH) state emerges with the annihilation of the band-touching rings. The nontrivial topology is determined by calculating the Chern number and Wannier charge centers. This provides an alternative platform to realize QAH states. These results could also provide the possibility of further understanding the topological states in NLSM and electronic applications.

Ultrafast Optical Probe of Coherent Acoustic Phonons in Dirac Semimetal Cd3As2 Film Epitaxied on GaAs(111)B Substrate

Coherent longitudinal acoustic phonon (CAP) generation in epitaxial Dirac semimetal Cd₃As₂ films with different thicknesses was investigated by a time-resolved reflectance technique. The short-lived weak CAP oscillations can be observed only in the thicker Cd₃As₂ films, and their central frequency of 0.039 THz has no dependence on sample thickness, but is nearly inversely proportional to the probe wavelength. For the 20 nm thin film, the observed long-lived CAP with a central frequency of 0.049 THz is generated in the GaAs(111)B substrate underneath. A sound velocity of 3800 m/s for the Cd₃As₂ film and 5360 m/s for the GaAs(111)B substrate is thus deduced. In addition, the opposite CAP amplitude and lifetime dependence on temperature further confirms the electronic and thermal stress origination of CAP generated in GaAs(111)B and Cd₃As₂ film, respectively, based on the propagating strain pulse model. The central frequency of CAP is found to be stable with increasing pumping fluence and temperature, which makes Cd₃As₂ a potential material for thermoelectric device applications.

A predicted orthogonal semimetallic carbon with negative thermal expansion and compressibility

Carbon has many allotropes possessing unique properties. In this work, we predicted an orthogonal carbon crystal, named ort-C24, with dynamic, mechanical and thermodynamic stability. Studies indicate that it is a topological semimetal having both nodal rings and nodal lines in its Brillouin zone. Ab initio molecular dynamics simulations reveal that it is a rare material having a negative thermal expansion coefficient along the a axis. It also has negative compressibility along the same axis under hydrostatic pressure. Its b axis can bear an astonishing strain of 115% even if the dynamical stability is considered. Tensioning along different axes can either change it into a metal or alter the nodal ring into nodal lines or only modify the shape of the nodal ring, together with the variation of the number of Dirac cones. Theoretically, temperature has a limited influence on its electronic topological properties while a hydrostatic pressure of 5 GPa can alter it noticeably. The simulated X-ray diffraction peaks indicate the possible existence of ort-C24 in carbon soot. These adjustable electronic topological properties may provide us with an interesting platform for studying such topological semimetals.

Fermi surface of the chiral topological semimetal PtGa

PtGa is a topological semimetal with giant spin-split Fermi arcs. Here, we report on angular-dependent de Haas-van Alphen (dHvA) measurements combined with band-structure calculations to elucidate the details of the bulk Fermi surface of PtGa. The strong spin-orbit coupling leads to eight bands crossing the Fermi energy that form a multitude of Fermi surfaces with closed extremal orbits and results in very rich dHvA spectra. The large number of experimentally observed dHvA frequencies make the assignment to the equally large number of calculated dHvA orbits challenging. Nevertheless, we find consistency between experiment and calculations verifying the topological character with maximal Chern number of the spin-split Fermi surface.

Evidence of decoupling of surface and bulk states in Dirac semimetal Cd3As2

Dirac semimetals have attracted a great deal of current interests due to their potential applications in topological quantum computing, low-energy electronic devices, and single photon detection in the microwave frequency range. Herein are results from analyzing the low magnetic (B) field weak-antilocalization behaviors in a Dirac semimetal Cd₃As₂thin flake device. At high temperatures, the phase coherence lengthl_ϕfirst increases with decreasing temperature (T) and follows a power law dependence ofl_ϕ∝T^-0.4. Below ∼3 K,l_ϕtends to saturate to a value of ∼180 nm. Another fitting parameterα, which is associated with independent transport channels, displays a logarithmic temperature dependence forT > 3 K, but also tends to saturate below ∼3 K. The saturation value, ∼1.45, is very close to 1.5, indicating three independent electron transport channels, which we interpret as due to decoupling of both the top and bottom surfaces as well as the bulk. This result, to our knowledge, provides first evidence that the surfaces and bulk states can become decoupled in electronic transport in Dirac semimetal Cd₃As₂.

3D Dirac semimetals-dielectric elliptical fiber supported tunable terahertz hybrid waveguide

Based on the proposed elliptical dielectric fiber-polyethylene gap-3D Dirac semimetal (DSM) hybrid plasmonic waveguide structure, the tunable propagation characteristics have been systematically investigated in the terahertz region, taking into account the influences of the structural parameters, the modified dielectric fiber, and the 3D DSM Fermi levels. The results show that as the ratio of the elliptical semi-axis along the y-direction a_y and the x-direction a_x (a_y/a_x) increases, the hybrid mode confinement increases. The real part of the effective mode index and propagation length increase with increasing the refractive index of the elliptical fiber. The propagation length and figure of merit of the hybrid modes reach 1.56×10⁴µm and 300, respectively. In addition, by changing the Fermi level of the 3D DSM layer, the propagation properties of the hybrid modes can also be modulated in a wide range, e.g., the modulation depth of the propagation length reaches about 71.53% if the Fermi level varies in the range of 0.03-0.15 eV. The propagation properties of the hybrid modes are enhanced significantly by utilizing the modified three elliptical fiber structures, the real part of the effective mode index, and the propagation length of the modified structure are enhanced simultaneously. The results are very helpful for understanding the tunable mechanism of the 3D DSM devices and aids the design of novel plasmonic devices, e.g., lasers, modulators, and resonators.

Prediction of negative refraction in Dirac semimetal metamaterial

Negative refraction materials are indispensable building blocks in the optoelectric devices for their unique functionalities of controlling the light propagations, such as, superlens and transformation optics. However, material realizations of negative refraction are still limited to the conventional metals, semiconductors as well as magnetic materials. Here, we show that three dimensional Dirac semimetals have the opportunity to enable the negative refraction, which can be achieved through alternatively stacking three dimensional Dirac semimetals and the dielectric layers together. It is found that the effective perpendicular and parallel permittivities in this multilayered stack display the respective negative and positive values over a certain frequency region, which enables its negative group refractive angle and it can be controlled by the Fermi energy of Dirac semimetals. The spectra of transmittance in the multilayered structure for transverse magnetic wave also reveals an incident angle-independent transmittance dip, which originates from the zero value of the real part of the effective perpendicular permittivity. Our findings unveil the essential role of three dimensional Dirac semimetals in producing the negative group refraction responses and promise their applications in the metamaterial-based devices.

Tunable angle-selective optical transparency induced by photonic topological transition in Dirac semimetals-based hyperbolic metamaterials

The tunable angle-selective transparency of hyperbolic metamaterials consisting of various multilayers of Dirac semimetal and dielectric materials are theoretically and numerically studied in the terahertz range. Three stack configurations are considered: alternating, sandwiched, and disordered. It is found that the proposed structures exhibit strong optical angular selectivity induced by photonic topological transition for transverse magnetic waves. Interestingly, the topological transition frequency can be flexibly modulated by changing the Fermi energy, temperature, and the releasing time of the Dirac semimetal, as well as the thickness ratio of the dielectric and semimetal layers. It is also noticed that the angular optical transparency properties are independent of the order of the proposed structure even in alternating/disordered/random configurations if the total thickness ratio of the semimetal to dielectric are the same, which makes the properties particularly easy to realize experimentally. The proposed hyperbolic metamaterial structures present a promising opportunity for wavefront engineering, offering crucial properties for applications in private screens, optical detectors, and light manipulation.

A Class of Magnetic Topological Material Candidates with Hypervalent Bi Chains

The link between crystal and electronic structure is crucial for understanding structure-property relations in solid-state chemistry. In particular, it has been instrumental in understanding topological materials, where electrons behave differently than they would in conventional solids. Herein, we identify 1D Bi chains as a structural motif of interest for topological materials. We focus on Sm₃ZrBi₅, a new quasi-one-dimensional (1D) compound in the Ln₃MPn₅ (Ln = lanthanide; M = metal; Pn = pnictide) family that crystallizes in the P6₃/mcm space group. Density functional theory calculations indicate a complex, topologically nontrivial electronic structure that changes significantly in the presence of spin-orbit coupling. Magnetic measurements show a quasi-1D antiferromagnetic structure with two magnetic transitions at 11.7 and 10.7 K that are invariant to applied field up to 9 T, indicating magnetically frustrated spins. Heat capacity, electrical, and thermoelectric measurements support this claim and suggest complex scattering behavior in Sm₃ZrBi₅. This work highlights 1D chains as an unexplored structural motif for identifying topological materials, as well as the potential for rich physical phenomena in the Ln₃MPn₅ family.

All topological bands of all nonmagnetic stoichiometric materials

Topological quantum chemistry and symmetry-based indicators have facilitated large-scale searches for materials with topological properties at the Fermi energy (E_F). We report the implementation of a publicly accessible catalog of stable and fragile topology in all of the bands both at and away from E_F in the 96,196 processable entries in the Inorganic Crystal Structure Database. Our calculations, which represent the completion of the symmetry-indicated band topology of known nonmagnetic materials, have enabled the discovery of repeat-topological and supertopological materials, including rhombohedral bismuth and Bi₂Mg₃. We find that 52.65% of all materials are topological at E_F, roughly two-thirds of bands across all materials exhibit symmetry-indicated stable topology, and 87.99% of all materials contain at least one stable or fragile topological band.

Crystal Growth and Characterization of ZrSiS-Type Topological Dirac Semimetals

WHM materials (W = Zr/Hf, H = Si/Ge/Sn, M = S/Se/Te) represent a large family of topological semimetals, which have attracted intensive interest since they are considered to be good candidates for studying various topological states. Here, we report the crystal growth, characterization, and electronic properties of HfSiS, ZrGeS, and ZrGeSe. All samples were prepared by a chemical vapor transport method with I2 as a transport agent, and the growth conditions were optimized. X-ray diffraction (XRD) measurements showed that the as-grown crystals crystallized in a PbFCl-type layered structure. They all showed metallic behavior from temperature-dependent resistivity measurements and the carrier densities were estimated to be in the order of 1021 cm−3. A large magnetoresistance of up to 1200% and an obvious Shubnikov–de Hass (SdH) oscillation were observed for HfSiS.