Ultrahigh mobility and giant magnetoresistance in the Dirac semimetal Cd3As2

Ratio of 4:1 between ZnGeAs2and MnAs phases in a single composite and its impact on the structure-driven magnetoresistance

A strong influence of the lattice degree of freedom on magnetoresistance (MR) under high pressure underlies the conception of 'structure-driven' magnetoresistance (SDMR). In most magnetic or topological materials, the suppression of MR with increasing pressure is a general trend, while for some magnetic composites the MR enhances and even shows unusual behavior as a consequence of structural transition. Here we investigated the SDMR in the composite material based on the ZnGeAs2semiconductor matrix and MnAs magnetic inclusions in a phase ratio of 4:1. At ambient pressure, its magnetic and transport properties are governed by MnAs inclusions, i.e. it shows a Curie temperatureTC≈ 320 K and metallic-like conductivity. Under high pressure, the low-field room temperature MR undergoes multiple changes in the pressure range up to 7.2 GPa. The structural transition in the ZnGeAs2matrix has been found at ∼6 GPa, slightly lower than in the pure ZnGeAs2(6.2 GPa). The huge SDMR as high as 85% at 6.8 GPa and 2.5 kOe, which contains both positive and negative MR components, is accompanied by a pressure-induced metallic-like-to-semiconductor-like transition and the enhanced ferromagnetic order of MnAs inclusions. This observation offers a competing mechanism between the robust extrinsic ferromagnetism and high-pressure electronic properties of ZnGeAs2.

Recent progress on topological semimetal IrO2: electronic structures, synthesis, and transport properties

5dtransition metal oxides, such as iridates, have attracted significant interest in condensed matter physics throughout the past decade owing to their fascinating physical properties that arise from intrinsically strong spin-orbit coupling (SOC) and its interplay with other interactions of comparable energy scales. Among the rich family of iridates, iridium dioxide (IrO2), a simple binary compound long known as a promising catalyst for water splitting, has recently been demonstrated to possess novel topological states and exotic transport properties. The strong SOC and the nonsymmorphic symmetry that IrO2possesses introduce symmetry-protected Dirac nodal lines (DNLs) within its band structure as well as a large spin Hall effect in the transport. Here, we review recent advances pertaining to the study of this unique SOC oxide, with an emphasis on the understanding of the topological electronic structures, syntheses of high crystalline quality nanostructures, and experimental measurements of its fundamental transport properties. In particular, the theoretical origin of the presence of the fourfold degenerate DNLs in band structure and its implications in the angle-resolved photoemission spectroscopy measurement and in the spin Hall effect are discussed. We further introduce a variety of synthesis techniques to achieve IrO2nanostructures, such as epitaxial thin films and single crystalline nanowires, with the goal of understanding the roles that each key parameter plays in the growth process. Finally, we review the electrical, spin, and thermal transport studies. The transport properties under variable temperatures and magnetic fields reveal themselves to be uniquely sensitive and modifiable by strain, dimensionality (bulk, thin film, nanowire), quantum confinement, film texture, and disorder. The sensitivity, stemming from the competing energy scales of SOC, disorder, and other interactions, enables the creation of a variety of intriguing quantum states of matter.

A dual functional tunable terahertz metamaterial absorber based on vanadium dioxide

A dual-functional switchable metamaterial absorber (MMA) based on vanadium dioxide (VO2), which achieves flexible switching between broadband absorption and four-band absorption by adjusting the VO2 conductivity, was proposed. The device has a broadband absorption function when VO2 is in the metal phase, and the conductivity is 3 × 105 S m-1. Numerical simulation shows that the absorption rate of the device reaches over 90% in the frequency range of 3.36-6.98 THz. The absorber exhibits polarization insensitivity and wide-angle absorption to transverse electric (TE) and transverse magnetic (TM) waves. When VO2 is in the insulator phase, and the conductivity is 3 × 102 S m-1, the device switches to a narrowband absorber with a band-efficient absorption function. Numerical simulation shows that the device has an absorption rate of 99.7% at 2.39 THz, 98.3% at 2.83 THz, 95.6% at 3.84 THz, and 96.1% at 4.61 THz. It can be used as a sensor with high sensitivity. In addition, to verify the absorption mechanism of the absorber, we introduced impedance matching theory to analyze the device. Finally, the influence of structural parameters on the performance of resonators was investigated. Through the joint action of multi-layer structures, the proposed MMA concentrates broadband and narrowband absorption functions on one device, achieving flexible switching between tasks without changing the structure. The switchable metamaterial absorber designed through simple tuning methods has broad application prospects in stealth technology and thermal emitters. It provides a wide range of ideas for the design of terahertz functional devices.

Symmetry-selective quasiparticle scattering and electric field tunability of the ZrSiS surface electronic structure

Three-dimensional Dirac semimetals with square-net non-symmorphic symmetry, such as ternary ZrXY (X = Si, Ge; Y = S, Se, Te) compounds, have attracted significant attention owing to the presence of topological nodal lines, loops, or networks in their bulk. Orbital symmetry plays a profound role in such materials as the different branches of the nodal dispersion can be distinguished by their distinct orbital symmetry eigenvalues. The presence of different eigenvalues suggests that scattering between states of different orbital symmetry may be strongly suppressed. Indeed, in ZrSiS, there has been no clear experimental evidence of quasiparticle scattering reported between states of different symmetry eigenvalues at small wave vectorq⃗.Here we show, using quasiparticle interference, that atomic step-edges in the ZrSiS surface facilitate quasiparticle scattering between states of different symmetry eigenvalues. This symmetry eigenvalue mixing quasiparticle scattering is the first to be reported for ZrSiS and contrasts quasiparticle scattering with no mixing of symmetry eigenvalues, where the latter occurs with scatterers preserving the glide mirror symmetry of the crystal lattice, e.g. native point defects in ZrSiS. Finally, we show that the electronic structure of the ZrSiS surface, including its unique floating band surface state, can be tuned by a vertical electric field locally applied by the tip of a scanning tunneling microscope (STM), enabling control of a spin-orbit induced avoided crossing near the Fermi level by as much as 300%.

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.

Magnetoresistive-coupled transistor using the Weyl semimetal NbP

Semiconductor transistors operate by modulating the charge carrier concentration of a channel material through an electric field coupled by a capacitor. This mechanism is constrained by the fundamental transport physics and material properties of such devices-attenuation of the electric field, and limited mobility and charge carrier density in semiconductor channels. In this work, we demonstrate a new type of transistor that operates through a different mechanism. The channel material is a Weyl semimetal, NbP, whose resistivity is modulated via a magnetic field generated by an integrated superconductor. Due to the exceptionally large electron mobility of this material, which reaches over 1,000,000 cm2/Vs, and the strong magnetoresistive coupling, the transistor can generate significant transconductance amplification at nanowatt levels of power. This type of device can enable new low-power amplifiers, suitable for qubit readout operation in quantum computers.

Anomalous Hall Transport by Optically Injected Isospin Degree of Freedom in Dirac Semimetal Thin Film

Chirality of massless fermions emerging in condensed matter is a key to understand their characteristic behavior as well as to exploit their functionality. However, the chiral nature of massless fermions in Dirac semimetals has remained elusive, due to equivalent occupation of carriers with the opposite chirality in thermal equilibrium. Here, we show that the isospin degree of freedom, which labels the chirality of massless carriers from a crystallographic point of view, can be injected by circularly polarized light. Terahertz Faraday rotation spectroscopy successfully detects the anomalous Hall conductivity by a light-induced isospin polarization in a three-dimensional Dirac semimetal, Cd3As2. Spectral analysis of the Hall conductivity reveals a long scattering time and a long decay time, which are characteristic of the isospin. The long-lived, robust, and reversible character of the isospin promises a potential application of Dirac semimetals in future information technology.

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.

Evidence of unconventional superconductivity on the surface of the nodal semimetal CaAg1-xPdxP

Surface states of topological materials provide extreme electronic states for unconventional superconducting states. CaAg1-xPdxP is an ideal candidate for a nodal-line Dirac semimetal with drumhead surface states and no additional bulk bands. Here, we report that CaAg1-xPdxP has surface states that exhibit unconventional superconductivity (SC) around 1.5 K. Extremely sharp magnetoresistance, tuned by surface-sensitive gating, determines the surface origin of the ultrahigh-mobility "electrons." The Pd-doping elevates the Fermi level towards the surface states, and as a result, the critical temperature (Tc) is increased up to 1.7 K from 1.2 K for undoped CaAgP. Furthermore, a soft point-contact study at the surface of Pd-doped CaAgP proved the emergence of unconventional SC on the surface. We observed the bell-shaped conductance spectra, a hallmark of the unconventional SC. Ultrahigh mobility carriers derived from the surface flat bands generate a new class of unconventional SC.

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.

Colossal negative magnetoresistance in field-induced Weyl semimetal of magnetic half-Heusler compound

The discovery of topological insulators and semimetals triggered enormous interest in exploring emergent electromagnetic responses in solids. Particular attention has been focused on ternary half-Heusler compounds, whose electronic structure bears analogy to the topological zinc-blende compounds while also including magnetic rare-earth ions coupled to conduction electrons. However, most of the research in this system has been in band-inverted zero-gap semiconductors such as GdPtBi, which still does not fully exhaust the large potential of this material class. Here, we report a less-studied member of half-Heusler compounds, HoAuSn, which we show is a trivial semimetal or narrow-gap semiconductor at zero magnetic field but undergoes a field-induced transition to a Weyl semimetal, with a negative magnetoresistance exceeding four orders of magnitude at low temperatures. The combined study of Shubnikov-de Haas oscillations and first-principles calculation suggests that the exchange field from Ho 4f moments reconstructs the band structure to induce Weyl points which play a key role in the strong suppression of large-angle carrier scattering. Our findings demonstrate the unique mechanism of colossal negative magnetoresistance and provide pathways towards realizing topological electronic states in a large class of magnetic half-Heusler compounds.

Broad and colossal edge supercurrent in Dirac semimetal Cd3As2 Josephson junctions

Edge supercurrent has attracted great interest recently due to its crucial role in achieving and manipulating topological superconducting states. Proximity-induced superconductivity has been realized in quantum Hall and quantum spin Hall edge states, as well as in higher-order topological hinge states. Non-Hermitian skin effect, the aggregation of non-Bloch eigenstates at open boundaries, promises an abnormal edge channel. Here we report the observation of broad edge supercurrent in Dirac semimetal Cd3As2-based Josephson junctions. The as-grown Cd3As2 nanoplates are electron-doped by intrinsic defects, which enhance the non-Hermitian perturbations. The superconducting quantum interference indicates edge supercurrent with a width of ~1.6 μm and a magnitude of ~1 μA at 10 mK. The wide and large edge supercurrent is inaccessible for a conventional edge system and suggests the presence of non-Hermitian skin effect. A supercurrent nonlocality is also observed. The interplay between band topology and non-Hermiticity is beneficial for exploiting exotic topological matter.

Anisotropic magnetoresistance and electronic features of the candidate topological compound praseodymium monobismuthide

PrBi, a sister member of the rare-earth monopnictide family, is an excellent candidate for studying extreme magnetoresistance and nontrivial topological electronic states. In this study, we perform angular magnetoresistance measurements as well as bulk and surface band structure calculations on this compound. PrBi's magnetoresistance is revealed to be significantly angle-dependent and shows a fourfold symmetry as always observed in the nonmagnetic isostructural counterparts, including LaSb, LaBi, and LuBi. Its angular magnetoresistance can be reproduced well using the semiclassical two-band model. The deduced parameters suggest that PrBi hosts an elongated electron pocket with a mobility anisotropy of ∼3.13 and is slightly uncompensated in its carrier concentration. Our bulk and surface band structure calculations confirm the anisotropic electronic features. Moreover, we reveal that a nodal-line-shaped surface state appears at the X̄ point, and is associated with the quadratic dispersion along the -X̄ direction, and the linear type-I Dirac dispersion along the X̄-M̄ direction. Owing to the type-I Dirac dispersion feature, PrBi could serve as a promising material platform for studying many unexpected physical properties, such as the highly anisotropic transport and valley polarization of electrons.

Weyl nodal ring states and Landau quantization with very large magnetoresistance in square-net magnet EuGa4

Magnetic topological semimetals allow for an effective control of the topological electronic states by tuning the spin configuration. Among them, Weyl nodal line semimetals are thought to have the greatest tunability, yet they are the least studied experimentally due to the scarcity of material candidates. Here, using a combination of angle-resolved photoemission spectroscopy and quantum oscillation measurements, together with density functional theory calculations, we identify the square-net compound EuGa4 as a magnetic Weyl nodal ring semimetal, in which the line nodes form closed rings near the Fermi level. The Weyl nodal ring states show distinct Landau quantization with clear spin splitting upon application of a magnetic field. At 2 K in a field of 14 T, the transverse magnetoresistance of EuGa4 exceeds 200,000%, which is more than two orders of magnitude larger than that of other known magnetic topological semimetals. Our theoretical model suggests that the non-saturating magnetoresistance up to 40 T arises as a consequence of the nodal ring state.

High-performance dual-tunable terahertz absorber based on strontium titanate and bulk Dirac semimetal for temperature sensing and switching function

In this study, a perfect metamaterial absorber based on strontium titanate and bulk Dirac semimetals is proposed. When the temperature of strontium titanate was 300K, the dual-band absorptions were 99.74% and 99.99% at 1.227 and 1.552 THz, respectively. The sensitivities based on a transverse magnetic (TM) wave were 0.95 and 1.22 GHz/K; the sensitivity based on a transverse electric (TE) wave was 0.76 GHz/K. The TE and TM waves were modulated by inserting a bulk Dirac semimetal between the concave and convex devices. The modulation depth of the TE wave was 97.9% at 1.1 THz; the extinction ratio was 16.9 dB. The modulation depth of the TE wave at 1.435 THz was 95.9%; the extinction ratio was 13.89 dB. The TM wave modulation depth at 1.552 THz was 95.9%; the extinction ratio was 13.98 dB. Irrespective of a TE or TM wave, the terahertz absorber has good switching and temperature-sensing performance based on strontium titanate and bulk Dirac semimetals as well as broad application prospects in temperature sensing and switching devices.

Robust negative longitudinal magnetoresistance and spin-orbit torque in sputtered Pt3Sn and Pt3SnxFe1-x topological semimetal

Contrary to topological insulators, topological semimetals possess a nontrivial chiral anomaly that leads to negative magnetoresistance and are hosts to both conductive bulk states and topological surface states with intriguing transport properties for spintronics. Here, we fabricate highly-ordered metallic Pt₃Sn and Pt₃Sn_xFe_1-x thin films via sputtering technology. Systematic angular dependence (both in-plane and out-of-plane) study of magnetoresistance presents surprisingly robust quadratic and linear negative longitudinal magnetoresistance features for Pt₃Sn and Pt₃Sn_xFe_1-x, respectively. We attribute the anomalous negative longitudinal magnetoresistance to the type-II Dirac semimetal phase (pristine Pt₃Sn) and/or the formation of tunable Weyl semimetal phases through symmetry breaking processes, such as magnetic-atom doping, as confirmed by first-principles calculations. Furthermore, Pt₃Sn and Pt₃Sn_xFe_1-x show the promising performance for facilitating the development of advanced spin-orbit torque devices. These results extend our understanding of chiral anomaly of topological semimetals and can pave the way for exploring novel topological materials for spintronic devices.

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.

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.

Disorder-Induced Magnetotransport Anomalies in Amorphous and Textured Co1-xSix Semimetal Thin Films

In recent times the chiral semimetal cobalt monosilicide (CoSi) has emerged as a prototypical, nearly ideal topological conductor hosting giant, topologically protected Fermi arcs. Exotic topological quantum properties have already been identified in CoSi bulk single crystals. However, CoSi is also known for being prone to intrinsic disorder and inhomogeneities, which, despite topological protection, risk jeopardizing its topological transport features. Alternatively, topology may be stabilized by disorder, suggesting the tantalizing possibility of an amorphous variant of a topological metal, yet to be discovered. In this respect, understanding how microstructure and stoichiometry affect magnetotransport properties is of pivotal importance, particularly in case of low-dimensional CoSi thin films and devices. Here we comprehensively investigate the magnetotransport and magnetic properties of ≈25 nm Co_1-xSi_x thin films grown on a MgO substrate with controlled film microstructure (amorphous vs textured) and chemical composition (0.40 < x < 0.60). The resistivity of Co_1-xSi_x thin films is nearly insensitive to the film microstructure and displays a progressive evolution from metallic-like (dρ_xx/dT > 0) to semiconducting-like (dρ_xx/dT < 0) regimes of conduction upon increasing the silicon content. A variety of anomalies in the magnetotransport properties, comprising for instance signatures consistent with quantum localization and electron-electron interactions, anomalous Hall and Kondo effects, and the occurrence of magnetic exchange interactions, are attributable to the prominent influence of intrinsic structural and chemical disorder. Our systematic survey brings to attention the complexity and the challenges involved in the prospective exploitation of the topological chiral semimetal CoSi in nanoscale thin films and devices.

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.

CoTe2 : A Quantum Critical Dirac Metal with Strong Spin Fluctuations

Quantum critical points separating weak ferromagnetic and paramagnetic phases trigger many novel phenomena. Dynamical spin fluctuations not only suppress the long-range order, but can also lead to unusual transport and even superconductivity. Combining quantum criticality with topological electronic properties presents a rare and unique opportunity. Here, by means of ab initio calculations and magnetic, thermal, and transport measurements, it is shown that the orthorhombic CoTe₂ is close to ferromagnetism, which appears suppressed by spin fluctuations. Calculations and transport measurements reveal nodal Dirac lines, making it a rare combination of proximity to quantum criticality and Dirac topology.

Dominant Two-Dimensional Electron-Phonon Interactions in the Bulk Dirac Semimetal Na3Bi

Bulk Dirac semimetals (DSMs) exhibit unconventional transport properties and phase transitions due to their peculiar low-energy band structure, yet the electronic interactions governing nonequilibrium phenomena in DSMs are not fully understood. Here we show that electron-phonon (e-ph) interactions in a prototypical bulk DSM, Na₃Bi, are predominantly two-dimensional (2D). Our first-principles calculations reveal a 2D optical phonon with strong e-ph interactions associated with in-plane vibrations of Na atoms. We show that this 2D mode governs e-ph scattering and charge transport in Na₃Bi and induces a dynamical phase transition to a Weyl semimetal. Our work advances the quantitative analysis of electron interactions in Na₃Bi and reveals a dominant low-dimensional interaction in a bulk Dirac semimetal.

Correlation-driven organic 3D topological insulator with relativistic fermions

Exploring new topological phenomena and functionalities induced by strong electron correlation has been a central issue in modern condensed-matter physics. One example is a topological insulator (TI) state and its functionality driven by the Coulomb repulsion rather than a spin-orbit coupling. Here, we report a 'correlation-driven' TI state realized in an organic zero-gap system α-(BETS)₂I₃. The topological surface state and chiral anomaly are observed in temperature and field dependences of resistance, indicating a three-dimensional TI state at low temperatures. Moreover, we observe a topological phase switching between the TI state and non-equilibrium Dirac semimetal state by a dc current, which is a unique functionality of a correlation-driven TI state. Our findings demonstrate that correlation-driven TIs are promising candidates not only for practical electronic devices but also as a field for discovering new topological phenomena and phases.

Giant magnetoresistance of Dirac plasma in high-mobility graphene

The most recognizable feature of graphene's electronic spectrum is its Dirac point, around which interesting phenomena tend to cluster. At low temperatures, the intrinsic behaviour in this regime is often obscured by charge inhomogeneity1,2 but thermal excitations can overcome the disorder at elevated temperatures and create an electron-hole plasma of Dirac fermions. The Dirac plasma has been found to exhibit unusual properties, including quantum-critical scattering3-5 and hydrodynamic flow6-8. However, little is known about the plasma's behaviour in magnetic fields. Here we report magnetotransport in this quantum-critical regime. In low fields, the plasma exhibits giant parabolic magnetoresistivity reaching more than 100 per cent in a magnetic field of 0.1 tesla at room temperature. This is orders-of-magnitude higher than magnetoresistivity found in any other system at such temperatures. We show that this behaviour is unique to monolayer graphene, being underpinned by its massless spectrum and ultrahigh mobility, despite frequent (Planckian limit) scattering3-5,9-14. With the onset of Landau quantization in a magnetic field of a few tesla, where the electron-hole plasma resides entirely on the zeroth Landau level, giant linear magnetoresistivity emerges. It is nearly independent of temperature and can be suppressed by proximity screening15, indicating a many-body origin. Clear parallels with magnetotransport in strange metals12-14 and so-called quantum linear magnetoresistance predicted for Weyl metals16 offer an interesting opportunity to further explore relevant physics using this well defined quantum-critical two-dimensional system.

Tunable broadband absorber based on a layered resonant structure with a Dirac semimetal

With the development of science and technology, intermediate infrared technology has gained more and more attention in recent years. In the research described in this paper, a tunable broadband absorber based on a Dirac semimetal with a layered resonant structure was designed, which could achieve high absorption (more than 0.9) of about 8.7 THz in the frequency range of 18-28 THz. It was confirmed that the high absorption of the absorber comes from the strong resonance absorption between the layers, and the resonance of the localised surface plasmon. The absorber has a gold substrate, which is composed of three layers of Dirac semimetal and three layers of optical crystal plates. In addition, the resonance frequency of the absorber can be changed by adjusting the Fermi energy of the Dirac semimetal. The absorber also shows excellent characteristics such as tunability, absorption stability at different polarisation waves and incident angles, and has a high application value for use in radar countermeasures, biotechnology and other fields.

First Principles Computation of New Topological B2X2Zn (X = Ir, Rh, Co) Compounds

Recent attempts at searching for new materials have revealed a large class of materials that show topological behaviors with unusual physical properties and potential applications leading to enthralling discoveries both theoretically and experimentally. We computationally predict new three-dimensional topological compounds of space group 139(I/4mmm). After conducting a full volume optimization process by allowing the rearrangement of atomic positions and lattice parameters, the first-principles calculation with a generalized gradient approximation is utilized to identify multiple Dirac-type crossings around X and P symmetric points near Fermi energy. Importantly, the band inversion at point P is recognized. Further, we investigate the compound for topological crystalline insulating behavior by conducting surface state calculation and by investigating gapping behavior by increasing lattice parameters. Additionally, we perform formation energy, elastic properties, and phonon modes calculations to verify the structural, mechanical, and dynamical stability of the compounds. Therefore, we suggest compounds for further investigation and experimental realization.

Positive longitudinal magnetoconductivity induced by chiral magnetic effect in mercury selenide

A negative longitudinal magnetoresistance without any sign of saturation was found in a non-centrosymmetric Weyl semimetal (WSM) candidate mercury selenide in an electron concentration range of 5.5 × 10¹⁵-1.7 × 10¹⁷cm^-3and a temperature range of 0.33-150 K. The magnitude of the effect varies with a sample from≈10% up to≈30% in a magnetic field of 12 T atT= 150 K. Moreover, the positive contribution to magnetoconductivity has a characteristic quadratic dependence on the magnetic field, increasing with a charged center concentration atT= 150 K. The most likely explanation for the discovered longitudinal magnetoconductivity feature lies in the chiral magnetic effect, which is inherent to WSMs. The role of the Dyakonov-Perel mechanism in inter-nodal spin relaxation is discussed in regard to HgSe.

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.

Crystal growth and electronic properties of LaSbSe

The ZrSiS-type materials have gained intensive attentions. The magnetic version of the ZrSiS-type materials, LnSbTe (Ln = Lanthanide), offers great opportunities to explore new quantum states owing to the interplay between magnetism and electronic band topology. Here, we report the growth and characterization of the non-magnetic LaSbSe of this material family. We found the metallic transport, low magnetoresistance and non-compensated charge carriers with relatively low carrier density in LaSbSe. The specific heat measurement has revealed distinct Sommerfeld coefficient and Debye temperature in comparison to LaSbTe. Such addition of a new LnSbSe selenide compound could provide the alternative material choices in addition to LnSbTe telluride 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.

Electronic and transport properties of semimetal ZrBeSi crystal: a first-principles study

In recent years, semimetals have aroused people's research interest. Here, we systematically study phonon and electronic transport properties of the ZrBeSi with semimetal character by using the first-principles calculations together with the Boltzmann transport theory. Calculated lattice thermal conductivities of the ZrBeSi alongaandcaxes are 31.3 W (m · K)^-1and 56.0 W (m · K)^-1at room temperature, respectively, which are larger than the most semiconductors and semimetals. By comparing with other semimetals, we find that the larger lattice thermal conductivity of ZrBeSi is due to its smaller Grüneisen parameter, which indicates the weaker phonon scattering. Main contributions to the lattice thermal conductivities alongaandcaxis come from the acoustic branches, and conversely, the contributions of optical branches are very small. In addition, we calculate the Seebeck coefficient and the electron thermal conductivity of ZrBeSi based on the relaxation time approximation. The electronic transport properties of ZrBeSi exhibit strong anisotropy in bothaandbdirections. Calculated electronic thermal conductivities of pristine ZrBeSi alongaandcaxes are 8.8 W (m · K)^-1and 9.7 W (m · K)^-1at room temperature, respectively. Furthermore, we also obtain the figure of meritZTon the basis of phonon and electron transport. The obtainedZTalongcaxis reaches a maximum of 0.11 at 900 K, demonstrating that ZrBeSi has a generalZT, but it has good heat conduction ability. Our research will help to understand the transport properties of semimetals and expand the application of semimetals to heat conduction devices. At the same time, it also provides some reference for the future experimental work.

Shubnikov-de Haas oscillations and nontrivial topological states in Weyl semimetal candidate SmAlSi

The RAlX (R = Light rare earth; X = Ge, Si) compounds, as a family of magnetic Weyl semimetal, have recently attracted growing attention due to the tunability of Weyl nodes and its interactions with diverse magnetism by rare-earth atoms. Here, we report the magnetotransport evidence and electronic structure calculations on nontrivial band topology of SmAlSi, a new member of this family. At low temperatures, SmAlSi exhibits large non-saturated magnetoresistance (MR) (as large as ∼5500% at 2 K and 48 T) and distinct Shubnikov-de Haas (SdH) oscillations. The field dependent MRs at 2 K deviate from the semiclassical (μ₀H)²variation but follow the power-law relation MR∝(μ₀H)^mwith a crossover fromm∼ 1.52 at low fields (μ₀H< 15 T) tom∼ 1 under high fields (μ₀H> 18 T), which is attributed to the existence of Weyl points and electron-hole compensated characteristics with high mobility. From the analysis of SdH oscillations, two fundamental frequencies originating from the Fermi surface pockets with non-trivialπBerry phases and small cyclotron mass can be identified, this feature is supported by the calculated electronic band structures with two Weyl pockets near the Fermi level. Our study establishes SmAlSi as a paradigm for researching the novel topological states of RAlX family.

A two-dimensional topological nodal-line material MgN4 with extremely large magnetoresistance

Using first-principles calculations, we predict a stable two-dimensional atomically thin material MgN₄. This material has a perfect intrinsic electron-hole compensation characteristic with high carrier mobility, making it a promising candidate material with extremely large magnetoresistance. As the magnetic field increases, the magnetoresistance of the monolayer MgN₄ will show a quadratic dependence on the strength of the magnetic field without saturation. Furthermore, nontrivial topological properties are also found in this material. In the absence of spin-orbit coupling, the monolayer MgN₄ belongs to a topological nodal-line material, in which the band crossings form a closed saddle-shape nodal-ring near the Fermi level in the Brillouin zone. Once the spin-orbit coupling is considered, a small local energy gap is opened along the nodal ring, resulting in a topological insulator defined on a curved Fermi surface with ₂ = 1. The combination of two-dimensional single-atomic-layer thickness, an extremely large magnetoresistance effect, and topological non-trivial properties in the monolayer MgN₄ makes it an excellent platform for designing novel multi-functional devices.

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.

Shubnikov-de Haas and de Haas-van Alphen oscillations in Czochralski grown CoSi single crystal

Anisotropic transport, Shubnikov-de Haas (SdH), and de Haas-van Alphen (dHvA) quantum oscillations studies are reported on a high-quality CoSi single crystal grown by the Czochralski method. Temperature-dependent resistivities indicate the dominating electron-electron scattering. Magnetoresistance (MR) at 2 K reaches 610% forI ∥ [111]andB ∥ [011-], whereas it is 500% forI ∥ [011-] andB ∥ [111]. A negative slope in field-dependent Hall resistivity suggests electrons are the majority carriers. The carrier concentration extracted from Hall conductivity indicates no electron-hole compensation. In 3D CoSi, the electron transport lifetime is found to be approximately in the same order as the quantum lifetime, whereas in 2Delectron gas the long-range scattering drives the transport life much larger than the quantum lifetime. From MR and Hall SdH oscillations, the effective masses and Dingle temperatures have been calculated. The dHvA oscillation reveals three frequencies at 18 T (γ), 558 T (α) and 663 T (β), whereas, SdH oscillation results in only two frequenciesαandβ. Theγfrequency observed in dHvA oscillation is a tiny hole pocket at the Γ point.

Photodetection and Infrared Imaging Based on Cd3As2 Epitaxial Vertical Heterostructures

Due to the nontrivial electronic structure, Cd₃As₂ is predicted to possess various transport properties and outstanding photoresponses. Photodetectors based on topological materials are mostly made up of nanoplates, yet monolithic in situ heteroepitaxial Cd₃As₂ photodetectors are rarely reported to date owing to the crystal mismatch between Cd₃As₂ and semiconductors. Here, we demonstrate Cd₃As₂/Zn_xCd_1-xTe/GaSb vertical heteroepitaxial photodetectors via molecule beam epitaxy. By constructing dual-Schottky junctions, these photodetectors show high responsivity and external quantum efficiency in a broadband spectrum. Based on the strong and fast photoresponse, we achieved visible light to near-infrared imaging using a one-pixel imaging system with a galvo. Our results illustrate that the integration of three-dimensional Dirac semimetal Cd₃As₂ with semiconductors has potential applications in broadband photodetection and infrared cameras.

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.

Controlling and visualizing Dirac physics in topological semimetal heterostructures

A bulk crystal of cadmium arsenide is a three-dimensional Dirac semimetal, but, in a thin film, it can behave like a three-dimensional topological insulator. This tunability provides unique opportunities to manipulate and explore a topological insulator phase. However, an obstacle to engineering such tunability is the subtlety of transport-based discriminants for topological phases. In this work, the quantum capacitance of cadmium arsenide-based heterostructures provides two direct experimental signatures of three-dimensional topological insulator physics: an insulating three-dimensional bulk and a Landau level at zero energy that does not disperse in a magnetic field. We proceed to join our ability to see these fingerprints of the topological surface states with flexibility afforded by our epitaxial heterostructures to demonstrate a route toward controlling the energy of the Dirac nodes on each surface. These results point to new avenues for engineering topological insulators based on cadmium arsenide.

Low threshold optical bistability based on topological edge state in photonic crystal heterostructure with Dirac semimetal

The special band structure of three-dimensional Dirac semimetal (3D DSM) makes it show strong nonlinear optical characteristics in the terahertz region, which provides a new way to develop terahertz nonlinear devices with low threshold. In this paper, we theoretically study the optical bistability (OB) of transmitted light in a multilayer structure with 3D DSM embedded in two one-dimensional photonic crystals (1D PhC). The topological edge state (TES) excited by the 1D PhC heterostructure significantly enhances the local electric field near the nonlinear 3D DSM, which provides a positive condition for the realization of low threshold OB. Through parameter optimization, we obtain a threshold electric field with an incident electric field of 10⁶ V/m levels. Furthermore, the influences of the Fermi energy and thickness of 3D DSM and the angle of the incident light on the hysteretic behavior as well as the threshold of OB are clarified. 3D DSM-based optical devices with intrinsic OB provide a building block for future integrated optical and all-optical networks.

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.

Benchmarking Noise and Dephasing in Emerging Electrical Materials for Quantum Technologies

As quantum technologies develop, a specific class of electrically conducting materials is rapidly gaining interest because they not only form the core quantum-enabled elements in superconducting qubits, semiconductor nanostructures, or sensing devices, but also the peripheral circuitry. The phase coherence of the electronic wave function in these emerging materials will be crucial when incorporated in the quantum architecture. The loss of phase memory, or dephasing, occurs when a quantum system interacts with the fluctuations in the local electromagnetic environment, which manifests in "noise" in the electrical conductivity. Hence, characterizing these materials and devices therefrom, for quantum applications, requires evaluation of both dephasing and noise, although there are very few materials where these properties are investigated simultaneously. Here, the available data on magnetotransport and low-frequency fluctuations in electrical conductivity are reviewed to benchmark the dephasing and noise. The focus is on new materials that are of direct interest to quantum technologies. The physical processes causing dephasing and noise in these systems are elaborated, the impact of both intrinsic and extrinsic parameters from materials synthesis and devices realization are evaluated, and it is hoped that a clearer pathway to design and characterize both material and devices for quantum applications is thus provided.

A Review: The Functional Materials-Assisted Terahertz Metamaterial Absorbers and Polarization Converters

When metamaterial structures meet functional materials, what will happen? The recent rise of the combination of metamaterial structures and functional materials opens new opportunities for dynamic manipulation of terahertz wave. The optical responses of functional materials are greatly improved based on the highly-localized structures in metamaterials, and the properties of metamaterials can in turn be manipulated in a wide dynamic range based on the external stimulation. In the topical review, we summarize the recent progress of the functional materials-based metamaterial structures for flexible control of the terahertz absorption and polarization conversion. The reviewed devices include but are not limited to terahertz metamaterial absorbers with different characteristics, polarization converters, wave plates, and so on. We review the dynamical tunable metamaterial structures based on the combination with functional materials such as graphene, vanadium dioxide (VO2) and Dirac semimetal (DSM) under various external stimulation. The faced challenges and future prospects of the related researches will also be discussed in the end.

Synthesis of Weyl Semi-Metal Co3Sn2S2 by Hydrothermal Method and Its Physical Properties

In the field of condensed matter physics, as new quantum materials, topological semimetals have a special topological energy band structure and nontrivial band crossings in the energy band, which will have many excellent topological properties, such as internal insulation of topological insulators and the presence of conduction electrons on the surface; this makes topological semimetals exhibit wider application prospects in electronic devices. So far, the experimental synthesis of topological semimetals was performed using physical methods to synthesize bulk single crystals, which is not conducive to the commercial application of micro and small devices. Weyl semimetal Co3Sn2S2 with shandite structure was successfully synthesized experimentally by a green and environmentally friendly hydrothermal method. Adjusting its reaction temperature, molar atomic ratio of elements and annealing temperature, and other experimental conditions, we analyze the crystal structure and physical properties of Co3Sn2S2, with the nanocrystal size being about 200 nm. We found that the Co3Sn2S2 synthesized by the hydrothermal method has a Curie temperature at 100 K to undergo ferromagnetic transition.

Large Transverse and Longitudinal Magneto-Thermoelectric Effect in Polycrystalline Nodal-Line Semimetal Mg3 Bi2

Topological semimetals provide new opportunities for exploring novel thermoelectric phenomena, owing to their exotic and nontrivial electronic structure topology around the Fermi surface. Herein, the discovery of large transverse and longitudinal magneto-thermoelectric (MTE) effects in Mg₃ Bi₂ is reported and predicted to be a type-II nodal-line semimetal in the absence of spin-orbit coupling (SOC). The maximum transverse power factor is 2182 μW m^-1 K^-2 at 13.5 K and 6 Tesla. The longitudinal power factor reaches up to 3043 μW m^-1 K^-2 , which is 20 times higher than that in a zero-strength magnetic field and is also comparable to state-of-the-art MTE materials. By compensating the Mg loss in Mg-rich conditions for tuning the carrier concentration close to intrinsic state, the sample fabricated in this study exhibits a large linear non-saturating magnetoresistance of 940% under a field of 14 Tesla. Using density functional calculations, the authors attribute the underlying mechanism to the parent linear-dispersed nodal-line electronic structure without SOC and the anisotropic Fermi surface shape with SOC, highlighting the essential role of high carrier mobility and open electron orbits in the moment space. This work offers a new avenue toward highly efficient MTE materials through defect engineering in polycrystalline topological semimetals.

Unusual magnetotransport in twisted bilayer graphene

When two sheets of graphene are twisted to the magic angle of 1.1^∘, the resulting flat moiré bands can host exotic correlated electronic states such as superconductivity and ferromagnetism. Here, we show transport properties of a twisted bilayer graphene device at 1.38^∘, far enough above the magic angle that we do not expect exotic correlated states. Instead, we see several unusual behaviors in the device’s resistivity upon tuning both charge carrier density and perpendicular magnetic field. We can reproduce these behaviors with a surprisingly simple model based on Hofstadter’s butterfly. These results shed light on the underlying properties of twisted bilayer graphene.

We present transport measurements of bilayer graphene with a 1.38^∘ interlayer twist. As with other devices with twist angles substantially larger than the magic angle of 1.1^∘, we do not observe correlated insulating states or band reorganization. However, we do observe several highly unusual behaviors in magnetotransport. For a large range of densities around half filling of the moiré bands, magnetoresistance is large and quadratic. Over these same densities, the magnetoresistance minima corresponding to gaps between Landau levels split and bend as a function of density and field. We reproduce the same splitting and bending behavior in a simple tight-binding model of Hofstadter’s butterfly on a triangular lattice with anisotropic hopping terms. These features appear to be a generic class of experimental manifestations of Hofstadter’s butterfly and may provide insight into the emergent states of twisted bilayer graphene.

Ultrafast photothermoelectric effect in Dirac semimetallic Cd3As2 revealed by terahertz emission

The thermoelectric effects of topological semimetals have attracted tremendous research interest because many topological semimetals are excellent thermoelectric materials and thermoelectricity serves as one of their most important potential applications. In this work, we reveal the transient photothermoelectric response of Dirac semimetallic Cd₃As₂, namely the photo-Seebeck effect and photo-Nernst effect, by studying the terahertz (THz) emission from the transient photocurrent induced by these effects. Our excitation polarization and power dependence confirm that the observed THz emission is due to photothermoelectric effect instead of other nonlinear optical effect. Furthermore, when a weak magnetic field (~0.4 T) is applied, the response clearly indicates an order of magnitude enhancement on transient photothermoelectric current generation compared to the photo-Seebeck effect. Such enhancement supports an ambipolar transport nature of the photo-Nernst current generation in Cd₃As₂. These results highlight the enhancement of thermoelectric performance can be achieved in topological Dirac semimetals based on the Nernst effect, and our transient studies pave the way for thermoelectric devices applicable for high field circumstance when nonequilibrium state matters. The large THz emission due to highly efficient photothermoelectric conversion is comparable to conventional semiconductors through optical rectification and photo-Dember effect.

Many topological semimetals are excellent thermoelectric materials, but previous studies were limited to steady-state properties. Here, the authors observe a transient thermoelectric response in Cd₃As₂ by detecting the resulting THz emission, with an enhanced response when a small magnetic field is applied.