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The scientific explanation of utilizing the positron and Weyl fermion in semiconductors is presented. In view of the slow e + beam-generation development for imaging technology alongside the Weyl fermion which carries charge like an electron, but has no mass, thus moves much faster, injecting semiconductor devices is addressed. The information gained from this prediction has allowed the broadening of its implementation to semiconductor technology with electronic excitation using sources other than e - . Developing the positron microbeam and Weyl fermions can be described with the concept of type I positron beam source is an alternative source of electron beam, thus harnessing the generation of γ-ray radiations inside the semiconductor heterostructures with indicating e + and e − interaction with materials are different and type II Weyl fermions. Thus, the properties of positrons and Weyl fermion are considered suitable for carrier transport in optoelectronics. Perspectives of the development of alternative beam source for super-transport are provided. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) higher-order topological fermionic phases coexisting with intriguing topological phonon states have recently become a focal point of condensed matter research. However, the coexistence of the second-order topological phase and valley Weyl phonon features in 2D materials remains unexplored. In this Letter, we present a two-dimensional Li-intercalated graphene material family that serves as an ideal platform for demonstrating the coexistence of topological electron and phonon features. Our study, which utilizes first-principles calculations, investigates the structural, electronic, and topological properties of several Li-intercalated graphene materials. The higher-order topological phases are protected by C6 rotation and inversion symmetries in Li-C6 and Li-C6-Li layers, respectively, as confirmed by their calculated topological invariants ( χ 6 , Z 4 ). Topologically protected corner modes are noticed within the gapped bulk and edge states (in the armchair edge) in the nanoflake geometry of the Li-C6 compound. Notably, the phonon spectra of the Li-C6 and Li-C6-Li materials exhibit Weyl phonon nodes in the Brillouin zone where the phonon bands touch at the valley-high symmetry point. The presence of localized Berry curvature and robust topological phonon edge states further confirms the existence of Weyl phonon nodes in these materials. Our first-principles study predicts potential candidates for hosting the coexisting electronic and phononic features and highlights the technological aspects of Li-intercalated graphene materials. [ABSTRACT FROM AUTHOR]

Weyl fermions of spin 1 2 minimally coupled to Einstein's gravity in 4 dimensions cannot be produced purely gravitationally in an expanding Universe at tree level. Surprisingly, as we showed in a recent letter [1], this changes at gravitational 1-loop when cosmic perturbations, like a gravitational wave background, are present. Such a background introduces a new scale, thereby breaking the fermions' conformal invariance. This leads to a non-vanishing gravitational self-energy for Weyl fermions at 1-loop and induces their production. In this paper, we present an extended study of this new mechanism, explicitly computing this effect using the in-in formalism. We work in an expanding Universe in the radiation-dominated era as a fixed background. Gravitational wave-induced fermion production has rich phenomenological consequences. Notably, if Weyl fermions eventually acquire mass, and assuming realistic — and potentially detectable — gravitational wave backgrounds, the mechanism can explain the abundance of dark matter in the Universe. More generally, gravitational-wave induced freeze-in is a new purely gravitational mechanism for generating other feebly interacting fermions, e.g. right-handed neutrinos. We show that this loop level effect can dominate over the conventional — tree-level — gravitational production of superheavy fermions in a sizable part of the parameter space (https://github.com/koppj/GW-freeze-in/). [ABSTRACT FROM AUTHOR]

Using an effective low-energy k · p model on the frontier p x , y orbitals, we establish a general phase diagram of spin-valley locked band inversion by introducing a mechanical strain field into nonmagnetic honeycomb motifs with robust spin–orbit coupling and intrinsically broken inversion symmetry. Using first-principles calculations, we realize such multiple topological phase transitions in a strained InTe monolayer within experimental reach with the Weyl semimetal as the nontrivial boundary state at two critical strains. The massless Weyl fermions endow the spin and valley Hall effects with ultrafast and dissipationless transport over a broad low-energy window. The valley selective circular dichroism can be regulated by strain-induced band inversion. A crossover between the topologically trivial and nontrivial regimes with sizable bandgaps makes InTe suitable for room-temperature (RT) topological strain-effect transistors. Our work not only demonstrates a fundamental mechanism for exploring tunable topological states and valley physics but also provides a potential platform for realizing many exotic phenomena and RT quantum devices. [ABSTRACT FROM AUTHOR]

Topological semimetals represent a novel class of quantum materials displaying non‐trivial topological states that host Dirac/Weyl fermions. The intersection of Dirac/Weyl points gives rise to essential properties in a wide range of innovative transport phenomena, including extreme magnetoresistance, high mobilities, weak antilocalization, electron hydrodynamics, and various electro‐optical phenomena. In this study, the electronic, transport, phonon scattering, and interrelationships are explored in single crystals of the topological semimetal HfAs2. It reveals a weak antilocalization effect at low temperatures with high carrier density, which is attributed to perfectly compensated topological bulk and surface states. The angle‐resolved photoemission spectroscopy (ARPES) results show anisotropic Fermi surfaces and surface states indicative of the topological semimetal, further confirmed by first‐principle density functional theory (DFT) calculations. Moreover, the lattice dynamics in HfAs2 are investigated both with the Raman scattering and density functional theory. The phonon dispersion, density of states, lattice thermal conductivity, and the phonon lifetimes are computed to support the experimental findings. The softening of phonons, the broadening of Raman modes, and the reduction of phonon lifetimes with temperature suggest the enhancement of phonon anharmonicity in this new topological material, which is crucial for boosting the thermoelectric performance of topological semimetals. [ABSTRACT FROM AUTHOR]

Near-surface layers of the mercury chalcogenide (HgSe and n-HgTe) single crystals with the electron concentration in the range of 7 × 1014–4.2 × 1018 сm−3 were studied using the laser pump-probe thermoreflectance (TR) technique combined with the Fabry–Perot (FP) interferometer in the wide temperature interval of 10–300= K. The TR–FP measurements were carried out toward samples with a freshly etched reflective surface. As a result, a minimum in the time dependence of the TR–FP signal and sign inversion of the TR–FP signal were revealed both for HgSe and n-HgTe within the specified temperature range. Noticeably, both anomalies were observed for n-HgTe at substantially lower temperatures than that for HgSe. The proposed qualitative interpretation of the observed anomalies is based on the hypothesis of two types of Weyl nodes located in the Brillouin zone of mercury chalcogenides involving specific for the Weyl semimetals energy barriers, separating the bulk Weyl fermion helical states from the topologically protected Fermi-arc surface states. [ABSTRACT FROM AUTHOR]

We report the observation of in-plane anisotropic magnetoresistance (AMR) and planar Hall effect in our recently discovered kagome antiferromagnetic off-stoichiometric single crystal of Mn3Ga. We found that the in-plane AMR is dominated by a sixfold symmetry at low temperature due to the kagome lattice magnetocrystalline anisotropy. However, an unusual fourfold symmetry is also revealed by the angular-dependent AMR measurements, which originates from the little distortion of the crystal accompanying the slight ferromagnetic transition. Moreover, we also found a clear planar Hall effect signal in off-stoichiometric single crystal of Mn3Ga, which may be related to the chiral anomaly, one of the signatures of the magnetic Weyl fermions. [ABSTRACT FROM AUTHOR]

Based on the demand for an improvement in various corpuscle types of current injection, the objective of this technique is to provide a new concept of carrier generators for optoelectronic pump and injection devices. This investigation is conducted to improve current injection by using a particle other than the electron. The idea was conceived from condensed matter physics for a technique to implement positron for carrier transport in semiconductors with the source based on localized emissions. A radioactive source such as 22Na is incident on a tungsten vane moderator, thus having positive electrons flowing and tunneling as well as a laser-driven high-quality positron into semiconductor-based devices. In addition, tantalum arsenide (TaAs) hosting Weyl particles has been discovered to hold significant potential for cutting-edge technological uses. Injection of different carriers and their behavior in semiconductors will lead to the emergence of solid state optoelectronics with different carrier injections that possesses a high energy (100–500 keV) and the possibility of maximum energy that is approximately several tens of megaelectron volts. Significantly, these various carrier sources have a larger range of operational settings and output characteristics due to their various underlying emission principles, thus obtaining a greater kinetic energy for a positron. The transformation to Weyl fermions carries electric charge via a device far more quickly than ordinary electrons, therefore unlocking the potential of new materials with unusual transport properties. [ABSTRACT FROM AUTHOR]

In this study, we work on understanding the behavior of relativistic fermions within the framework of a (2+1)-dimensional gravitational wave environment. Our main objective is to uncover an analytical solution for the massless Dirac equation in this particular scenario. Initially, we derive a set of coupled equations that describe the quantum dynamics of the system under consideration. Subsequently, we embark on finding an analytical solution for the resulting wave equation governing Weyl fermions by utilizing the generators associated with the s ℓ 2 algebra. This approach enables us to determine the relativistic frequency modes of the system. We then delve into examining how the parameters of the gravitational wave spacetime influence the real oscillation modes of Weyl fermions. Our findings highlight the sensitivity of Weyl fermions to the specific gravitational wave spacetime background being considered. [ABSTRACT FROM AUTHOR]

Light-rare-earth-based RAlSi family serves as an exceptional platform for elucidating the unique properties of Weyl systems, featuring both types of Weyl fermions, with and without magnetism. In this investigation, we conducted a comparative analysis of the resistivity and magnetoresistance (MR) behaviors of single-crystal RAlSi (R = La, Ce, and Pr). A hump appears in the temperature-dependent resistivity curve of CeAlSi, attributed to the crystal-electric-field effect and showing insensitivity to the magnetic field. In nonmagnetic LaAlSi, a quasilinear MR response above the crossover field is linked to mobility fluctuations originating from slight inhomogeneity. Upon heating CeAlSi from 2 K to its critical temperature for ferromagnetic (FM) ordering (TC), a field-induced contraction in the twin Fermi surfaces was observed, likely contributing to negative MR peaks at TC. The robustness of Weyl nodes in FM RAlSi is demonstrated by the inherent broken space inversion symmetry, allowing them to change positions via violation of time-reversal symmetry. Our findings suggest the potential for magnetic modulation of the electronic structure in FM RAlSi, leading not only to shifts in Weyl nodes but also alterations in the Fermi surface. [ABSTRACT FROM AUTHOR]

The chiral anomaly - a hallmark of chiral spin-1/2 Weyl fermions - is an imbalance between left- and right-moving particles that underpins phenomena such as particle decay and negative longitudinal magnetoresistance in Weyl semimetals. The discovery that chiral crystals can host higher-spin generalizations of Weyl quasiparticles without high-energy counterparts, known as multifold fermions, raises the fundamental question of whether the chiral anomaly is a more general phenomenon. Answering this question requires materials with chiral quasiparticles within a sizable energy window around the Fermi level that are unaffected by extrinsic effects such as current jetting. Here, we report the chiral anomaly of multifold fermions in CoSi, which features multifold bands within ~0.85 eV of the Fermi level. By excluding current jetting through the squeezing test, we measure an intrinsic, longitudinal negative magnetoresistance. We develop a semiclassical theory to show that the negative magnetoresistance originates in the chiral anomaly, despite a sizable and detrimental orbital magnetic moment contribution. A concomitant non-linear Hall effect supports the multifold-fermion origin of the magnetotransport. Our work confirms the chiral anomaly of higher-spin generalizations of Weyl fermions, currently inaccessible outside solid-state platforms. Multifold fermions promise a solid-state platform for accessing and studying the effects of the chiral anomaly beyond Weyl fermions. Here, the authors identify multifold fermions in magnetotransport in the chiral semimetal CoSi. [ABSTRACT FROM AUTHOR]

A two-dimensional (2D) Weyl semimetal, akin to a spinful variant of graphene, represents a topological matter characterized by Weyl fermion-like quasiparticles in low dimensions. The spinful linear band structure in two dimensions gives rise to distinctive topological properties, accompanied by the emergence of Fermi string edge states. We report the experimental realization of a 2D Weyl semimetal, bismuthene monolayer grown on SnS(Se) substrates. Using spin and angle-resolved photoemission and scanning tunneling spectroscopies, we directly observe spin-polarized Weyl cones, Weyl nodes, and Fermi strings, providing consistent evidence of their inherent topological characteristics. Our work opens the door for the experimental study of Weyl fermions in low-dimensional materials. 2D Weyl semimetals are spin-polarized analogues of graphene that promise access to various topological properties of matter. Here, the authors evidence spin-polarized Weyl cones, Weyl nodes, and Fermi strings in monolayer bismuthene. [ABSTRACT FROM AUTHOR]

The emergence of various topological semimetal states presents a novel opportunity for enhancing the efficiency of thermoelectric transport. This study introduces a recipe to improve the thermoelectric (TE) performance in topological materials containing single‐pair Weyl points (SP WPs) fermions and phonons. The recipe focuses on two key factors contributing to the enhancement of TE performance: the increase in the density of states to achieve a high power factor, and the introduction of additional phonon scattering to reduce the lattice thermal conductivity. The proposed recipe is confirmed in a half‐metallic SP WPs material BaNiIO6 through first‐principles methods. An enhanced density of states arises near the energy of the SP WPs in BaNiIO6, leading to a peak power factor connected to the complex Fermi surface due to the degeneracy of Weyl pockets in energy. Furthermore, it is shown that the SP WPs phonons in BaNiIO6 possess a high scattering rate and can likely contribute to the low lattice thermal conductivity, especially when two crossing points in SP WPs do not degenerate in frequency. The new recipe can be used for discovering high‐performance thermoelectric materials in the future by utilizing the transport advantages of degenerate‐in‐energy SP WPs fermions and non‐degenerate‐in‐frequency SP WPs phonons. [ABSTRACT FROM AUTHOR]

This work involves augmenting the Δ (5 4) flavor symmetry model by incorporating two Standard Model Higgs particles into the Inverse Seesaw mechanism. The mass matrices are discussed numerically in the framework of Δ (5 4) flavor for Majorana neutrinos. We introduced Vector-like fermions and Weyl fermions, which are gauge singlets in the Standard Model and produces Majorana mass terms. We restrict the undesirable terms in our Lagrangian by using additional symmetry. Due to these additional terms, the mass matrices deviate from the tribimaximal neutrino mixing pattern, resulting in the production of a nonzero reactor angle θ 1 3 . We found that the atmospheric oscillation parameter ( θ 2 3 ) occupies the upper octant under the normal hierarchy situation. We also study the CP violation ( δ CP ), Jarlskog invariant parameter (J) and Neutrinoless double-beta decay parameter ( m e e ) in the parameter space of the normal hierarchy model to see whether they concur with the most recent neutrino observations. [ABSTRACT FROM AUTHOR]

In this work we propose a novel device for controlling the flow of information using Weyl fermions. Based on a previous work by our group, we show that it is possible to fully control the flow of Weyl fermions on several different channels by applying an electric field perpendicular to the direction of motion of the particles on each channel. In this way, we can transmit information as logical bits, depending on the existence or not of a Weyl current on each channel. We also show that the response time of this device is exceptionally low, less than 1 ps, for typical values of its parameters, allowing for the control of the flow of information at extremely high rates of the order of 100 Petabits per second. Alternatively, this device could also operate as an electric field sensor. In addition, we demonstrate that Weyl fermions can be efficiently guided through the proposed device using appropriate magnetic fields. Finally, we discuss some particularly interesting remarks regarding the electromagnetic interactions of high-energy particles. [ABSTRACT FROM AUTHOR]

In the standard lattice domain-wall fermion formulation, one needs two flat domain-walls where both of the left- and right-handed massless modes appear. In this work we investigate a single domain-wall system with a nontrivial curved background. Specifically we consider a massive fermion on a 3D square lattice, whose domain-wall is a 2D sphere. In the free theory, we find that a single Weyl fermion is localized at the wall and it feels gravity through the induced spin connection. With a topologically nontrivial U (1) link gauge field, however, we find a zero mode with the opposite chirality localized at the center where the gauge field is singular. In the latter case, the low-energy effective theory is not chiral but vectorlike. We discuss how to circumvent this obstacle in formulating lattice chiral gauge theory in the single domain-wall fermion system. [ABSTRACT FROM AUTHOR]

The thermal chiral anomaly is a new mechanism for thermal transport that occurs in Weyl semimetals (WSMs). It is attributed to the generation and annihilation of energy at Weyl points of opposite chirality. The effect was observed in the Bi1−xSbx alloy system, at x = 11% and 15%, which are topological insulators at zero field and driven into an ideal WSM phase by an external field. Given that the experimental uncertainty on x is of the order of 1%, any systematic study of the effect over a wider range of x requires precise knowledge of the transition composition xc at which the electronic bands at the L-point in these alloys have Dirac-like dispersions. At x > xc, the L-point bands are inverted and become topologically non-trivial. In the presence of a magnetic field along the trigonal direction, these alloys become WSMs. This paper describes how the temperature dependence of the frequency of the Shubnikov–de Haas oscillations F(x,T) at temperatures of the order of the cyclotron energy can be used to find xc and characterize the topology of the electronic Fermi surface. Semimetallic Bi1−xSbx alloys with topologically trivial bands have dF(x,T)/dT ≥ 0; those with Dirac/Weyl fermions display dF(x,T)/dT < 0. [ABSTRACT FROM AUTHOR]

We theoretically investigate the magnetotransport properties in magnetic junctions based on tilted Weyl semimetals and concentrate on the manifestations of the tilted effect in the differential conductance. Since the projection of the Fermi surface normal to the current direction can be enlarged by the tilted effect, the differential conductance is enhanced by increasing the tilting parameter. Moreover, due to the helical textures of Weyl fermions, the enhancement of differential conductance is robust against interfacial imperfections. [ABSTRACT FROM AUTHOR]

We present a new non-perturbative 't Hooft anomaly afflicting a quantum field theory with symmetry group G = U(1) × ℤ2 in four dimensions. We use the Adams spectral sequence to compute that the bordism group (BG), which classifies anomalies that remain when perturbative anomalies cancel, is ℤ4. By constructing a mapping torus and evaluating the Atiyah-Patodi-Singer η-invariant, we show that the mod 4 anomaly is generated by a pair of Weyl fermions that are vector-like under U(1), but with only one component charged under ℤ2. We construct a simple microscopic field theory that realises the anomaly, before investigating its impact in the hydrodynamic limit. We find that the anomaly dictates transport phenomena in the U(1) current and energy-momentum tensor akin to the chiral vortical and magnetic effects (even though the perturbative anomalies here vanish), but with the conductivities being fractionally quantised in units of a quarter, reflecting the mod 4 nature of the bordism group. Along the way, we compute the (relevant) bordism groups (Bℤ2 × BU(1)) and (BU(1)) in all degrees d = 0 through 5. [ABSTRACT FROM AUTHOR]

The discovery of topological states of matter has led to a revolution in condensed-matter science. While a non-trivial band topology in a material is often associated with intriguing transport properties, much less attention has been given to the impact on spin dynamics and non-equilibrium magnetization states. Here, we provide evidence that a chiral asymmetric magnon dispersion in the two-dimensional Weyl magnet Fe/W(110) is related to the presence of Weyl fermions close to the Fermi energy and surface Fermi arcs. We find that the large anomalous Hall conductivity and the Dzyaloshinskii–Moriya interaction are attributed to the non-trivial band topology in the composite momentum-magnetization space. Our results show the direct impact of Weyl fermions on both the charge and spin dynamics in a two-dimensional magnet. Unveiling these principles can promote innovative technologies in magnonics by utilizing topological materials, where magnons and non-trivial topological electronic states can be manipulated through magnetization. [ABSTRACT FROM AUTHOR]

In this research, the diagrams and divided power, together with their Capelli's identities, were employed to investigate " the Complex of Lascoux " in the situation of partition (7,5,3). [ABSTRACT FROM AUTHOR]

Topological semimetals host interesting new types of low-energy quasiparticles such as type-I and type-II Dirac and Weyl fermions. Type-III topological semimetals can emerge exactly at the border between type-I and II, characterized by a line-like Fermi surface and a flat energy dispersion near the topological band crossing. Here, we theoretically predict that 1T-HfTe2 and 1T-ZrTe2 transition metal dichalcogenides are type-I and type-II DSMs, respectively. By alloying the two materials, a new HfxZr1−xTe2 alloy with type-III Dirac cone emerges at x = 0.2, in combination with 1% in-plane compressive strain. By imaging the electronic energy bands with in situ angle-resolved photoemission spectroscopy of this random alloy with the desired composition, grown by molecular beam epitaxy on InAs(111) substrates, we provide experimental evidence that the tοp of type-III Dirac cone lies at—or very close to—the Fermi level. [ABSTRACT FROM AUTHOR]

We study 4-dimensional SU(N) gauge theory with one adjoint Weyl fermion and fundamental matter — either bosonic or fermionic. Symmetries, their 't Hooft anomalies, and the Vafa-Witten-Weingarten theorems strongly constrain the possible bulk phases. The first part of the paper is dedicated to a single fundamental fermion or boson. As long as the adjoint Weyl fermion is massless, this theory always possesses a ℤ 2 N χ chiral symmetry, which breaks spontaneously, supporting N vacua and domain walls between them for a generic mass of the matter fields. We argue, however, that the domain walls generically undergo a phase transition, and we establish the corresponding 3d gauge theories on the walls. The rest of the paper is dedicated to studying the multi-flavor fundamental matter. Here, the phases crucially depend on the ratio of the number of colors and the number of fundamental flavors. We also discuss the limiting scenarios of heavy adjoint and fundamentals, which align neatly with our current understanding of QCD and N = 1 super Yang-Mills theory. [ABSTRACT FROM AUTHOR]

We compute the trace, diffeomorphism and Lorentz anomalies of a free Weyl fermion in a gravitational background field by path integral methods. This is achieved by regularising the variation of the determinant of the Weyl operator building on earlier work by Leutwyler. The trace anomaly is found to be one half of the one of a Dirac fermion. Most importantly we establish that the potential parity-odd curvature term R R ~ , corresponding to the Pontryagin density, vanishes. This is to the contrary of some recent findings in the literature which gave rise to a controversy. We verify, that the regularisation does not lead to (spurious) anomalies in the Lorentz and diffeomorphism symmetries. We argue that in d = 2 (mod 4) P- and CP-odd terms cannot appear and that for d = 4 (mod 4) they are absent at least at leading order. [ABSTRACT FROM AUTHOR]

Relativistic Weyl fermion quasiparticles in Weyl semimetal bring the electron's chirality degree of freedom into the electrical transport and give rise to exotic phenomena. A topological phase transition from a topological trivial phase to a topological non-trivial phase offers a route to control electronic devices through its topological properties. Here, we report the Weyl semimetal phase in hydrothermally grown two-dimensional Tellurium (2D Te) induced by high hydrostatic pressure (up to 2.47 GPa). The unique chiral crystal structure gives rise to chiral fermions with different topological chiral charges (C = − 1 , + 1 , a n d − 2 ). The highly tunable chemical potential in 2D Te provides comprehensive information for understanding the pressure-dependent electron band structure. The pressure-induced insulator-to-metal transition, two-carrier transport, and the non-trivial π Berry phase shift in quantum oscillations are observed in the 2D Te Weyl semimetal phase. Our work demonstrates the pressure-induced bandgap closing in the inversion asymmetric narrow bandgap semiconductor 2D Te. Driving a quantum material from trivial to non-trivial topological phase can be engineered, for instance, by an applied external field but understanding the physics of the transition can be complex. Here, the authors report a pressure-induced topological phase transition from a semiconductor to a Weyl semimetal phase in 2D Te, and investigate the underlying dynamics using a range of magneto-transport techniques. [ABSTRACT FROM AUTHOR]

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. A Weyl semimetal formally requires either broken time reversal symmetry or inversion symmetry. One class of Weyl semimetals-the crystal family of NdAlSi-exhibits both. Here, Li et al perform angle-resolved photoemission spectroscopy measurements on NdAlSi, and observe the formation of an additional Weyl fermion as the material becomes ferrimagnetic. [ABSTRACT FROM AUTHOR]

We analyze the parity-odd correlators ⟨ J J O ⟩ odd , ⟨ J J T ⟩ odd , ⟨ T T O ⟩ odd and ⟨ T T T ⟩ odd in momentum space, constrained by conformal Ward identities, extending our former investigation of the parity-odd chiral anomaly vertex. We investigate how the presence of parity-odd trace anomalies affect such correlators. Motivations for this study come from holography, early universe cosmology and from a recent debate on the chiral trace anomaly of a Weyl fermion. In the current CFT analysis, O can be either a scalar or a pseudoscalar operator and it can be identified with the trace of the stress–energy tensor. We find that the ⟨ J J O ⟩ odd and ⟨ T T O ⟩ odd can be different from zero in a CFT. This occurs when the conformal dimension of the scalar operator is Δ 3 = 4 , as in the case of O = T μ μ. Moreover, if we assume the existence of parity-odd trace anomalies, the conformal ⟨ J J T ⟩ odd and ⟨ T T T ⟩ odd are nonzero. In particular, in the case of ⟨ J J T ⟩ odd the transverse–traceless component is constrained to vanish, and the correlator is determined only by the trace part with the anomaly pole. [ABSTRACT FROM AUTHOR]

The ferromagnetic Weyl semimetals, such as Co3Sn2S2, feature pairs of Weyl points characterized by the opposite chiralities. We model this type of semimetals by the inversion symmetry protected and the time reversal symmetry broken Bloch Hamiltonian. It involves terms representing the tunnelling effect, exchange field corresponding to the ferromagnetic order, chirality index of Weyl points with related energy parameters, and the angle formed by the spin magnetic moments and the axis perpendicular to the system-plane. While for the in-plane spin moment order the Weyl nodes are absent at some points of the first Brillouin zone, the bands of opposite chirality non-linearly cross each other with band inversion at Weyl points for the spin moment order along the perpendicular axis. The absence of linearity implies that the system is unable to host massless Weyl fermions. We also show that, in the absence of the exchange field, the incidence of the circularly polarized radiation leads to the emergence of a novel state with broken time reversal symmetry. [ABSTRACT FROM AUTHOR]

The interplay between magnetism and electronic band topology enriches topological phases and has promising applications. However, the role of topology in magnetic fluctuations has been elusive. Here, we report evidence for topology stabilized magnetism above the magnetic transition temperature in magnetic Weyl semimetal candidate CeAlGe. Electrical transport, thermal transport, resonant elastic X-ray scattering, and dilatometry consistently indicate the presence of locally correlated magnetism within a narrow temperature window well above the thermodynamic magnetic transition temperature. The wavevector of this short-range order is consistent with the nesting condition of topological Weyl nodes, suggesting that it arises from the interaction between magnetic fluctuations and the emergent Weyl fermions. Effective field theory shows that this topology stabilized order is wavevector dependent and can be stabilized when the interband Weyl fermion scattering is dominant. Our work highlights the role of electronic band topology in stabilizing magnetic order even in the classically disordered regime. Recently, rich condensed matter physics has emerged from the interplay between band topology and magnetic order. Here, the authors characterize the magnetic Weyl semimetal CeAlGe and find evidence for the role of Weyl fermions in stabilizing the magnetic order above the local transition temperature. [ABSTRACT FROM AUTHOR]

The Gd3Ru4Al12 structure type compounds, where the rare-earth magnetic ions form a breathing kagome lattice present a promising material landscape for exploring the various magnetic frustration-driven exotic states of matter. Here, we highlight the various magnetic, thermodynamic, and transport properties of several of the Gd3Ru4Al12 structure type magnets and provide intuitive insights into their rich electronic and magnetic ground states. The realization of key properties such as spin trimerization and skyrmion textures accompanied by a large topological (geometrical) Hall effect (THE) in some of these compounds is currently at the heart of several research endeavors searching for efficient data storage and spintronic devices. Features such as helical ordering and anomalous Hall effect (AHE) arising from the formation of Berry curvature by the Weyl fermions present an open window to tuning the electron spins for several practical applications. Therefore, these compounds are projected as promising candidates for investigating several other topological phases of matter accessible through the interplay of the degree of frustration and crystal field symmetry of the rare-earth ions. [ABSTRACT FROM AUTHOR]

Based on the demand for an improvement in various corpuscle types of current injection, the objective of this technique is to provide a new concept of carrier generators for optoelectronic pump and injection devices. This investigation is conducted to improve current injection by using a particle other than the electron. The idea was conceived from condensed matter physics for a technique to implement positron for carrier transport in semiconductors with the source based on localized emissions. A radioactive source such as 22Na is incident on a tungsten vane moderator, thus having positive electrons flowing and tunneling as well as a laser-driven high-quality positron into semiconductor-based devices. In addition, tantalum arsenide (TaAs) hosting Weyl particles has been discovered to hold significant potential for cutting-edge technological uses. Injection of different carriers and their behavior in semiconductors will lead to the emergence of solid state optoelectronics with different carrier injections that possesses a high energy (100–500 keV) and the possibility of maximum energy that is approximately several tens of megaelectron volts. Significantly, these various carrier sources have a larger range of operational settings and output characteristics due to their various underlying emission principles, thus obtaining a greater kinetic energy for a positron. The transformation to Weyl fermions carries electric charge via a device far more quickly than ordinary electrons, therefore unlocking the potential of new materials with unusual transport properties. [ABSTRACT FROM AUTHOR]

The insulator-metal transition in Mott insulators, known as the Mott transition, is usually accompanied with various novel quantum phenomena, such as unconventional superconductivity, non-Fermi liquid behavior and colossal magnetoresistance. Here, based on high-pressure electrical transport and XRD measurements, and first-principles calculations, we find that a unique pressure-induced Mott transition from an antiferromagnetic Mott insulator to a ferromagnetic Weyl metal in the iron oxychalcogenide La2O3Fe2Se2 occurs around 37 GPa without structural phase transition. Our theoretical calculations reveal that such an insulator-metal transition is mainly due to the enlarged bandwidth and diminishing of electron correlation at high pressure, fitting well with the experimental data. Moreover, the high-pressure ferromagnetic Weyl metallic phase possesses attractive electronic band structures with six pairs of Weyl points close to the Fermi level, and its topological property can be easily manipulated by the magnetic field. The emergence of Weyl fermions in La2O3Fe2Se2 at high pressure may bridge the gap between nontrivial band topology and Mott insulating states. Our findings not only realize ferromagnetic Weyl fermions associated with the Mott transition, but also suggest pressure as an effective controlling parameter to tune the emergent phenomena in correlated electron systems. A Mott transition is a metal-insulator transition driven by electronic correlations, and the Mott insulating state is typically associated with unconventional electronic phases. Here the authors report a pressure-induced transition from a Mott insulator to a ferromagnetic Weyl metal in an iron oxychalcogenide. [ABSTRACT FROM AUTHOR]

The spacetime light cone is central to the definition of causality in the theory of relativity. Recently, links between relativistic and condensed matter physics have been uncovered, where relativistic particles can emerge as quasiparticles in the energy-momentum space of matter. Here, we unveil an energy-momentum analogue of the spacetime light cone by mapping time to energy, space to momentum, and the light cone to the Weyl cone. We show that two Weyl quasiparticles can only interact to open a global energy gap if they lie in each other's energy-momentum dispersion cones–analogous to two events that can only have a causal connection if they lie in each other's light cones. Moreover, we demonstrate that the causality of surface chiral modes in quantum matter is entangled with the causality of bulk Weyl fermions. Furthermore, we identify a unique quantum horizon region and an associated 'thick horizon' in the emergent causal structure. Causality, the relationship between cause and effect, is a central concept in Einstein's theory of relativity. Here, authors show that a causality analogue in energy-momentum space plays an important role in describing quasiparticle interactions in quantum matter. [ABSTRACT FROM AUTHOR]

The magnetic Weyl semimetallic state can lead to intriguing magnetotransport, such as chiral anomaly and the layered quantum Hall effect. Mn3X (X = Sn, Ge) is a noncollinear antiferromagnetic semimetal where a Weyl semimetallic state is stabilized by time-reversal symmetry breaking. Compared to the well-studied Mn3Sn, the Weyl fermion-induced magnetotransport in Mn3Ge has been merely studied. Here, we report an in-depth study on the magnetotransport in a microfabricated Mn3Ge single crystal from room temperature to 10 K. We reveal an anomalous anisotropic magnetoresistance with fourfold symmetry and a positive high-field longitudinal magnetoresistance below the critical temperature (160–170 K). The possible origin is the temperature-induced tilting of the Weyl nodes. Our study helps to understand the magnetotransport properties in the Weyl fermion system. [ABSTRACT FROM AUTHOR]

Non-collinear antiferromagnetic Weyl semimetals, combining the advantages of a zero stray field and ultrafast spin dynamics, as well as a large anomalous Hall effect and the chiral anomaly of Weyl fermions, have attracted extensive interest. However, the all-electrical control of such systems at room temperature, a crucial step toward practical application, has not been reported. Here, using a small writing current density of around 5 × 106 A·cm–2, we realize the all-electrical current-induced deterministic switching of the non-collinear antiferromagnet Mn3Sn, with a strong readout signal at room temperature in the Si/SiO2/Mn3Sn/AlOx structure, and without external magnetic field or injected spin current. Our simulations reveal that the switching originates from the current-induced intrinsic non-collinear spin-orbit torques in Mn3Sn itself. Our findings pave the way for the development of topological antiferromagnetic spintronics. [ABSTRACT FROM AUTHOR]

Transition metal monopnictides represent a new class of topological semimetals with low-energy excitations, namely, Weyl fermions. We report optical properties across a wide spectral energy range for TaAs, TaP, NbAs, and NbP, calculated within density functional theory. Spectra are found to be somewhat independent of the anion and the light polarization. Their features are explained in terms of the upper s , p , d , and f electrons. Characteristic absorption features are related to the frequency dependence of the Fresnel reflectivity. While the lower part of the energy loss spectra is dominated by plasmonic features, the high-energy structures are explained by interband transitions. [ABSTRACT FROM AUTHOR]

Magnetic Weyl semimetals (WSM) have recently attracted much attention due to their potential in realizing strong anomalous Hall effects. Yet, how to design such systems remains unclear. Based on first-principles calculations, we show here that the ferromagnetic half-metallic compound In 2 CoSe 4 has several pairs of Weyl points and is hence a good candidate for magnetic WSM. These Weyl points would approach the Fermi level gradually as the Hubbard U increases, and finally disappear after a critical value U c . The range of the Hubbard U that can realize the magnetic WSM state can be expanded by pressure, manifesting the practical utility of the present prediction. Moreover, by generating two surface terminations at Co or In atom after cleaving the compound at the Co-Se bonds, the nontrivial Fermi arcs connecting one pair of Weyl points with opposite chirality are discovered in surface states. Furthermore, it is possible to observe the nontrivial surface state experimentally, e.g., angle-resolved photoemission spectroscopy (ARPES) measurements. As such, the present findings imply strongly a new magnetic WSM which may host a large anomalous Hall conductivity.&#xD., (Creative Commons Attribution license.)

Weyl semimetals show unique physical properties exemplified by the colossal anomalous Hall effect, arising from exotic quasiparticles called Weyl fermions emerging around the Weyl nodes. Manipulating these topologically protected Weyl nodes is anticipated to play a leading role towards the on-demand control of quantum properties in Weyl semimetals. We demonstrate non-volatile chirality switching in a ferromagnetic Weyl semimetal Co3Sn2S2 via all-optical magnetization reversal. When excited by circularly polarized mid-infrared light pulses, the sign reversal of the anomalous Hall conductivity stemming from the Berry curvature is observed, manifesting the switching of the chirality of the Weyl nodes accompanying with the magnetization reversal. Magneto-optical imaging measurements reveal that the mechanism of the magnetization/chirality switching is attributed to the helicity-dependent deterministic magnetization associated with the magnetic circular dichroism. Weyl semimetals exhibit a unique feature known as Weyl nodes, which give rise to non-trivial topological features such as an anomalous Hall effect, and there are many efforts to try and control such properties. Here, the authors report light-induced chirality switching in a ferromagnetic Weyl semimetal Co3Sn2S2 using circularly polarized mid-infrared light pulse excitation. [ABSTRACT FROM AUTHOR]

Weyl semimetals show unique physical properties exemplified by the colossal anomalous Hall effect, arising from exotic quasiparticles called Weyl fermions emerging around the Weyl nodes. Manipulating these topologically protected Weyl nodes is anticipated to play a leading role towards the on-demand control of quantum properties in Weyl semimetals. We demonstrate non-volatile chirality switching in a ferromagnetic Weyl semimetal Co3Sn2S2 via all-optical magnetization reversal. When excited by circularly polarized mid-infrared light pulses, the sign reversal of the anomalous Hall conductivity stemming from the Berry curvature is observed, manifesting the switching of the chirality of the Weyl nodes accompanying with the magnetization reversal. Magneto-optical imaging measurements reveal that the mechanism of the magnetization/chirality switching is attributed to the helicity-dependent deterministic magnetization associated with the magnetic circular dichroism. Weyl semimetals exhibit a unique feature known as Weyl nodes, which give rise to non-trivial topological features such as an anomalous Hall effect, and there are many efforts to try and control such properties. Here, the authors report light-induced chirality switching in a ferromagnetic Weyl semimetal Co3Sn2S2 using circularly polarized mid-infrared light pulse excitation. [ABSTRACT FROM AUTHOR]

Chiral crystals, characterized by rotation, screw rotation, and translation symmetries are abundant in nature. The existence of Kramers–Weyl fermions (KWFs) at the time‐reversal‐invariant momenta in nonmagnetic chiral crystals (NCCs) has attracted intense attention due to their unique physical properties beyond conventional Weyl fermions. Although the spin texture, one of the most fundamental physical quantities, is found to be dramatically different for different KWFs in different NCCs, a unified understanding of this puzzling phenomenon is still lacking. In this article, combining k · p theory and first‐principles calculations, k‐linear Hamiltonians for KWFs are constructed and consequently a complete classification for the spin textures of KWFs in all the NCCs is made, which are confirmed by a series of material example calculations. Interestingly, it is found that the spin textures are reversed in NCCs with opposite chirality, leading to reversed Fermi arc surface states. Importantly, this study unveils that the nonlinear optical responses around the KWFs in NCCs can be largely determined by their spin texture classification. This study not only provides a unified understanding of the spin textures of KWFs in all the NCCs, but also suggests a principle to design novel KWF‐related spintronics and nonlinear optics. [ABSTRACT FROM AUTHOR]

Recently, multifold fermions characterized by band crossings with multifold degeneracy and Fermi surfaces with higher Chern numbers have been discovered experimentally in AlPt (Schroter et al. in Nat Phys 15:759–765, 2019) and XSi(X = Rh,Co) (Sanchez et al. in Nature 567:500–505, 2019; Rao et al. in Nature 567:496–499, 2019; Takane et al. in Phys Rev Lett 122:076402, 2019). In this work, we largely expand the family of multifold fermion materials by pointing out that several well-studied noncentrosymmetric superconductors are indeed multifold fermion metals. Importantly, their normal state topological properties, which have been ignored in previous studies, play an important role in the superconducting properties. Taking Li 2 Pd 3 B and Li 2 Pt 3 B as examples, we found a large number of unconventional degenerate points, such as double spin-1, spin-3/2, Weyl and double Weyl topological band crossing points near the Fermi energy, which result in finite Chern numbers on Fermi surfaces. Long Fermi arc states in Li 2 Pd 3 B , originating from the nontrivial band topology were found. Importantly, it has been shown experimentally that Li 2 Pd 3 B and Li 2 Pt 3 B are fully gapped and gapless superconductors, respectively. By analyzing the possible pairing symmetries, we suggest that Li 2 Pd 3 B is a conventional s-wave superconductor or DIII class topological superconductor with Majorana surface states. Li 2 Pt 3 B , being gapless, is likely to be a nodal topological superconductor with dispersionless surface Majorana modes. We further identified that several noncentrosymmetric superconductors, such as Mo 3 Al 2 C , PdBiSe, Y 2 C 3 and La 2 C 3 , are multifold fermion superconductors. This work calls for a revisit for the study of noncentrosymmtric superconductors which provide platforms for investigating the interplay between superconductivity and topologically nontrivial Fermi surfaces. [ABSTRACT FROM AUTHOR]

The symmetries that protect massless Dirac fermions from a gap opening may become ineffective if the Dirac equation is discretized in space and time, either because of scattering between multiple Dirac cones in the Brillouin zone (fermion doubling) or because of singularities at zone boundaries. Here an implementation of Dirac fermions on a space‐time lattice that removes both obstructions is introduced. The quasi‐energy band structure has a tangent dispersion with a single Dirac cone that cannot be gapped without breaking both time‐reversal and chiral symmetries. It is shown that this topological protection is absent in the familiar single‐cone discretization with a linear sawtooth dispersion, as a consequence of the fact that there the time‐evolution operator is discontinuous at Brillouin zone boundaries. [ABSTRACT FROM AUTHOR]

The anomalous Hall effect (AHE), typically observed in ferromagnetic (FM) metals with broken time-reversal symmetry, depends on electronic and magnetic properties. In Co3Sn2-xInxS2, a giant AHE has been attributed to Berry curvature associated with the FM Weyl semimetal phase, yet recent studies report complicated magnetism. We use neutron scattering to determine the spin dynamics and structures as a function of x and provide a microscopic understanding of the AHE and magnetism interplay. Spin gap and stiffness indicate a contribution from Weyl fermions consistent with the AHE. The magnetic structure evolves from c-axis ferromagnetism at x = 0 to a canted antiferromagnetic (AFM) structure with reduced c-axis moment and in-plane AFM order at x = 0.12 and further reduced c-axis FM moment at x = 0.3 . Since noncollinear spins can induce non-zero Berry curvature in real space acting as a fictitious magnetic field, our results revealed another AHE contribution, establishing the impact of magnetism on transport. [ABSTRACT FROM AUTHOR]

Interplay of magnetism and electronic band topology in unconventional magnets enables the creation and fine control of novel electronic phenomena. In this work, we use scanning tunneling microscopy and spectroscopy to study thin films of a prototypical kagome magnet Fe3Sn2. Our experiments reveal an unusually large number of densely-spaced spectroscopic features straddling the Fermi level. These are consistent with signatures of low-energy Weyl fermions and associated topological Fermi arc surface states predicted by theory. By measuring their response as a function of magnetic field, we discover a pronounced evolution in energy tied to the magnetization direction. Electron scattering and interference imaging further demonstrates the tunable nature of a subset of related electronic states. Our experiments provide a direct visualization of how in-situ spin reorientation drives changes in the electronic density of states of the Weyl fermion band structure. Combined with previous reports of massive Dirac fermions, flat bands, and electronic nematicity, our work establishes Fe3Sn2 as an interesting platform that harbors an extraordinarily wide array of topological and correlated electron phenomena. [ABSTRACT FROM AUTHOR]

The interplay of topology with magnetism in Weyl semimetals recently arose to a vanguard topic, because of novel physical scenarios with anomalous transport properties. Here, we address the charge dynamics of the noncentrosymmetric and ferromagnetic (TC ~ 15 K) PrAlGe material and discover that it harbours electronic correlations, which are reflected in a sizeable reduction of the Fermi velocity with respect to the bare band value at low temperatures (T). At T < TC, the optical response registers a band reconstruction, which additionally causes a reshuffling of spectral weight, pertinent to the electronic environment of the type-I Weyl fermions and tracing the remarkable anomalous Hall conductivity (AHC). With the support of first-principles calculations, we provide evidence for the intimate relationship between a topological resonance of the absorption spectrum and the progressively enhanced occupation of non-trivial states with large Berry curvatures, a requirement for AHC. [ABSTRACT FROM AUTHOR]

Using a generalized Madelung transformation, we derive the hydrodynamic representation of the Dirac equation in arbitrary curved space-times coupled to an electromagnetic field. We obtain Dirac–Euler equations for fermions involving a continuity equation and a first integral of the Bernoulli equation. Comparing between the Dirac and Klein–Gordon equations we obtain the balance equation for fermion particles. We also use the correspondence between fermions and bosons to derive the hydrodynamic representation of the Weyl equation which is a chiral form of the Dirac equation. [ABSTRACT FROM AUTHOR]

Symmetry imposed restrictions to the Hamiltonian are systematized and applied to all of 80 clusters of single/double ordinary/gray groups (320 groups in total), to single out linear (in all directions) band crossings and corresponding effective Hamiltonians in high-symmetry Brillouin zone points of layered materials. The resulting dispersion types are isotropic or anisotropic forms of: single cone (with double degenerate crossing point and non-degenerate branches, or four-fold degenerate crossing point with double degenerate conical branches), poppy-flower (four-fold degenerate crossing point with two pairs of non-degenerate mutually rotated conical branches), and fortune teller (with nodal lines). Further, we describe the nontrivial patterns of dispersions’ behavior in high symmetry points when symmetry is varied within a cluster. Namely, Clebschâ€"Gordan series of the products of spin representation with the integer ones are relevant when spinâ€"orbit coupling is included, and clarify observed scenarios (gap closing, gap opening, cone preserving, cone splitting etc). Analogously, analysis of behavior of dispersions in transition from ordinary to gray group enlightens the role of time reversal symmetry. The results refine and expand data existing in literature, and interesting or even unexpected cases are singled out in discussion. [ABSTRACT FROM AUTHOR]

Multifold chiral fermions with high‐fold degeneracy beyond the twofold Weyl fermions have recently been reported in the transition metal silicide CoSi (space group P213). Here, by alloying parent compound CoSi, an alloy engineering approach to realize different topological phases in CoSi1−xAx (A = Ge, Sn) is demonstrated. By using first‐principles calculations and symmetry analysis, it is shown that the gradual decrease of crystal symmetry due to alloying makes Weyl‐type low‐energy excitation phases appear in CoSi1−xAx. For the alloy structures considered, surface states (SS) and Fermi arcs (FAs) are investigated, and the dynamic stability of the alloy structures is also confirmed by the phonon spectrum. These findings provide a feasible method for customizing the fermion behavior of multifold fermions semimetal and expect future experimental verification. [ABSTRACT FROM AUTHOR]

Topological semimetals are a frontier of quantum materials. In multiband electronic systems, topological band crossings can form closed curves, known as nodal lines. In the presence of spin–orbit coupling and/or symmetry-breaking operations, topological nodal lines can break into Dirac/Weyl nodes and give rise to interesting transport properties, such as the chiral anomaly and giant anomalous Hall effect. Recently, the time-reversal symmetry-breaking induced Weyl fermions are observed in a kagome-metal Co3Sn2S2, triggering interests in nodal-line excitations in multiband kagome systems. Here, using first-principles calculations and symmetry-based indicator theories, we find six endless nodal lines along the stacking direction of kagome layers and two nodal rings in the kagome plane in nonmagnetic Ni3In2S2. The linear dipsersive electronic structure, confirmed by angle-resolved photoemission spectroscopy, induces large magnetoresistance up to 2000% at 9 T. Our results establish a diverse topological landscape of multiband kagome metals. [ABSTRACT FROM AUTHOR]