Weyl Nodes Research Articles

Tuning of anomalous magnetotransport properties in half-Heusler topological semimetal GdPtBi.

Half-Heusler compounds from the REPtBi family exemplify Weyl semimetals in which an external magnetic field induces Weyl nodes. These materials exceptionally host topologically non-trivial states near the Fermi level and their manifestation can be clearly seen in the magnetotransport properties. In this study, we tune the Fermi level of the archetypal half-Heusler Weyl semimetal GdPtBi through high-energy electron irradiation, moving it away from the Weyl nodes to investigate the resilience of the contribution of topologically non-trivial states to magnetotransport properties. Remarkably, we observe that the negative longitudinal magnetoresistance, which is a definitive indicator of the chiral magnetic anomaly occurring in topological semimetals, persists even when the Fermi level is shifted by 100 meV from its original position in the pristine sample. Additionally, the anomalous Hall effect shows complex variations as the Fermi level is altered, attributed to the energy-dependent nature of the Berry curvature, which arises from avoided band crossing. Our findings show the robust influence of Weyl nodes on the magneto-transport properties of GdPtBi, irrespective of the Fermi level position, a behaviour likely applicable to many half-Heusler Weyl semimetals.

Tunable photonic spin Hall effect in the Weyl semimetals

Time-reversal symmetry-breaking Weyl semimetals (WSMs) exhibit an interesting optical response in the mid-infrared band, offering what we believe to be a novel approach to study the manipulation of the photonic spin Hall effect (PSHE). This article theoretically investigates the PSHE phenomenon in a prism-coupled structure composed of the WSMs. It is found that the excitation of surface plasmon polaritons (SPPs) and the nonreciprocity inherent in WSMs have a positive effect on enhancing the PSHE phenomenon of the reflected light. Especially, the PSHE can also be manipulated by varying the separation distance and twist angle between the Weyl nodes. Through parameter optimization, we obtained a lateral displacement of 12.14 μm. The effects of the separation distance between Weyl nodes, the twist angle, and the incident light angle on the PSHE were further elucidated. We believe the tunable PSHE with WSMs can provide promising avenues for developing spintronic devices, as well as other photonic applications.

Realization of a Photonic Higher‐Order Double‐Weyl Semimetal

AbstractHigher‐order Weyl semimetal (HOWSM) with coexisting 2D topological surface states and 1D topological hinge states establishes the fundamental connection between the Weyl physics and higher‐order topology. However, existing experimental demonstrations of HOWSM have primarily focused on Weyl nodes with chiral charges of ±1. In this work, based on the space group symmetry, a photonic higher‐order double‐Weyl semimetal (HODWSM) with larger chiral charges is theoretically designed and experimentally realized in a 3D photonic metamaterial. It is demonstrated that this photonic HODWSM exhibits a single pair of quadratic double‐Weyl points with chiral charges of ±2 in the plane, supporting nontrivial higher‐order band topology in the remaining kz planes and achieving unprecedentedly long hinge Fermi arc traversing the entire Brillouin zone. This work extends the research scope of HOWSM and offers an ideal photonic platform for exploring higher‐order Weyl physics.

Mirror Chern Bands and Weyl Nodal Loops in Altermagnets.

The electronic spectra of altermagnets are a fertile ground for nontrivial topology due to the unique interplay between time-reversal and crystalline symmetries. This is reflected in the unconventional Zeeman splitting between bands of opposite spins, which emerges in the absence of spin-orbit coupling (SOC) and displays nodes along high-symmetry directions. Here, we argue that even for a small SOC, the direction of the magnetic moments in the altermagnetic state has a profound impact on the electronic spectrum, enabling novel topological phenomena to appear. By investigating microscopic models for two-dimensional (2D) and three-dimensional (3D) altermagnets, motivated in part by rutile materials, we demonstrate the emergence of hitherto unexplored Dirac crossings between bands of the same spin but opposite sublattices. The direction of the moments determines the fate of these crossings when the SOC is turned on. We focus on the case of out-of-plane moments, which forbid an anomalous Hall effect and thus ensure that no weak magnetization is triggered in the altermagnetic state. In the 2D model, the SOC gaps out the Dirac crossings, resulting in mirror Chern bands that enable the Quantum Spin Hall Effect and undergo a topological transition to trivial bands upon increasing the magnitude of the magnetic moment. On the other hand, in the 3D model the crossings persist even in the presence of SOC, forming Weyl nodal loops protected by mirror symmetry. Finally, we discuss possible ways to control these effects in altermagnetic material candidates.

Strain-induced Kramers–Weyl phase in III–V zinc blende systems

We present theoretical observations on the topological nature of strained III–V semiconductors. By k·p perturbation, it can be shown that the strain-engineered conduction band hosts a Kramers–Weyl node at the Γ point. It is theoretically shown that a curated strain can create and then tune the sign of the topological charge. Furthermore, we outline experimental methods for both the realization and detection of strain-induced topological phase transitions.

Coexistence of topological nodal lines and Weyl nodes in a room-temperature half-metallic ferromagnet Cr3Si2Te6

Topological half-metallic ferromagnets, featured by topologically nontrivial states and fully spin-polarized electronic carriers at the Fermi level, are fertile playgrounds for exploring topo-spintronic applications. However, such exotic compounds are still limited. Here, we systematically study a self-intercalated van der Waals magnet Cr3Si2Te6 by combining first-principles calculations and Monte Carlo simulations. We demonstrate that Cr3Si2Te6 is thermodynamically stable and a half-metallic ferromagnet with a Curie temperature up to 485 K, much above room temperature. Excitingly, we find that topological nodal lines and Weyl nodes coexist in the Cr3Si2Te6 bulk. Correspondingly, the Cr3Si2Te6 bulk exhibits a large anomalous Hall conductivity of 160 and 427 Ω−1 cm−1 when its magnetization is along its magnetic easy and hard axes, respectively. Moreover, its anomalous Hall conductivity can be increased to 374 (882) Ω−1 cm−1 by doping electrons (holes). Finally, we disclose that the 7-layer Cr3Si2Te6 thin film possesses half-metallic ferromagnetism with a high TC of 425 K, strong out-of-plane magnetic anisotropy energy, and large anomalous Hall conductivity. Our findings suggest that Cr3Si2Te6 is a room-temperature topological half-metallic ferromagnet, which could have promising prospects for practical applications in next-generation topo-spintronic and nanoscale electronic devices.

Significant chiral magnetotransport magnified by multiple Weyl nodes

Significant chiral magnetotransport magnified by multiple Weyl nodes

Disorder-induced instability of a Weyl nodal loop semimetal towards a diffusive topological metal with protected multifractal surface states

Disorder-induced instability of a Weyl nodal loop semimetal towards a diffusive topological metal with protected multifractal surface states

High pressure induced superconductivity and chirality-neutral Fermi surface in SrSi2

In this study, we investigate the electronic structure and topological properties of the compound SrSi2 using angle-resolved photoemission spectroscopy (ARPES), electrical transport measurements, and calculations. In contrast to recent theoretical predictions that SrSi2 is a Weyl semimetal with robust Weyl nodes and Fermi arcs, our ARPES measurements on undoped and Ca-doped SrSi2 single crystals show no evidence of the predicted Weyl fermions or Fermi arcs at ambient pressure. Instead, ARPES and transport data show that SrSi2 is a narrow-gap semiconductor at ambient conditions. Our hybrid functional calculations also find a small band gap, in agreement with experiments. However, by applying external pressure, we induce a topological phase transition where the electronic bands overlap and Weyl nodes appear, as confirmed by transport measurements and calculations. Interestingly, we also observe a pressure-induced superconducting transition above 20 GPa. Our results establish SrSi2 as a platform to study the interplay between topological states and superconductivity driven by external pressure. Importantly, our results challenge the previously predicted intrinsic Weyl semimetallic nature of SrSi2 and highlight the need for combined experimental and advanced computational approaches to correctly describe the complex electronic structures in topological materials. Published by the American Physical Society 2024

Topological versus conventional superconductivity in a Weyl semimetal: A microscopic approach

Starting from a microscopic model for the particle-particle interactions in a Weyl semimetal, we analyzed the possibility for conventional as well as monopole Cooper pairing between quasiparticle excitations at the same (intra-nodal) or opposite (inter-nodal) Weyl nodes. We derived a coupled system of self-consistent BCS-like equations, where the angular dependence of the pairings is directly determined from the microscopic interaction symmetries. We studied the competition between conventional and monopole superconducting phases, thus obtaining explicitly the phase diagrams from the microscopic interaction model parameters. We determined the critical temperatures for both phases, and the low temperature critical behavior, including the specific heat, that we suggest as possible experimental probe for topological quantum criticality in Weyl semimetals.

Quantum description of Fermi arcs in Weyl semimetals in a magnetic field

For a Weyl semimetal (WSM) in a magnetic field, a semiclassical description of the Fermi-arc surface state dynamics is usually employed for explaining various unconventional magnetotransport phenomena, e.g., Weyl orbits, the three-dimensional quantum Hall effect, and the high transmission through twisted WSM interfaces. For a half-space geometry, we determine the low-energy quantum eigenstates for a four-band model of a WSM in a magnetic field perpendicular to the surface. The eigenstates correspond to in- and out-going chiral Landau level (LL) states, propagating (anti)parallel to the field direction near different Weyl nodes, which are coupled by evanescent surface-state contributions generated by all other LLs. These replace the Fermi arc in a magnetic field. Computing the phase shift accumulated between in- and out-going chiral LL states, we compare our quantum-mechanical results to semiclassical predictions. We find quantitative agreement between both approaches. Published by the American Physical Society 2024

Exploration of quantum oscillation in antiferromagnetic Weyl semimetal GdSiAl

GdSiAl single crystal has been investigated by means of magnetic and magneto-transport measurements and compared withab-initiodensity functional theory (DFT) calculations. Significant non-saturating magnetoresistance reaching∼18%at 12T and2Kwas observed, alongside the presence of Shubnikov-de Haas oscillations with the fundamental frequencies 22.09T and 77.33T. Shubnikov-de Haas oscillations provide the information about the nontrivialπBerry phase in GdSiAl with the Fermi surface areas of0.00211A˚-2and0.00739A˚-2. Angle-dependent magnetoresistance shows anisotropy withθ, exhibiting a maximum at 180°. The magnetic susceptibility data forH∥candH⊥creveals that the magnetic moments of Gd3+ions orders antiferromagnetically below 32K along with an another transition occurs at∼8K, which is consistent with the heat capacity measurements where a distinctλ-shaped anomaly has been observed near antiferromagnetic ordering temperature 32K. The high value of Debye temperature indicates the contribution of acoustic phonons. Electronic structure calculations suggest the existence of nested Fermi surface pockets characterized by nesting wave vectors that closely align with the observed magnetic ordering wave vector. Furthermore, DFT calculations reveal the presence of Weyl nodes in close proximity to the Fermi surface. Our findings from combined experimental and theoretical techniques indicate GdSiAl to be a potential candidate for an antiferromagnetic topological Weyl semimetal.

Near-field thermal radiation between SiO2/Weyl semimetal multilayers

Near-field thermal radiation between SiO2/Weyl semimetal multilayers

Landau levels in Weyl semimetal under uniaxial strain

Landau levels in Weyl semimetal under uniaxial strain

Transport chirality generated by a tunable tilt of Weyl nodes in a van der Waals topological magnet

Chirality — a characteristic handedness that distinguishes ‘left’ from ‘right’—is a fundamental property of quantum particles under broken symmetry intimately connected to their spins. Chiral fermions have been identified in Weyl semimetals through their unique electrodynamics arising from ‘axial’ charge imbalance between pairs of chiral Weyl nodes—the topologically protected ‘relativistic’ crossings of electronic bands. Chiral magnetotransport phenomena critically depend on the details of electronic band structure. However, the putative emergence of chiral electronic channels through shape altering of Weyl nodes is yet to be revealed. Here, we detect chirality-endowed linear conduction channels promoted by a tilt of Weyl bands in inversion-symmetric Weyl ferromagnet MnSb2Te4. The tuning of Weyl nodes is controlled with ionic hydrogen, which heals the system’s (Mn-Te) bond disorder and lowers the internode scattering. The reshaped Weyl states feature a doubled Curie temperature ≳50 K and a strong angular transport chirality synchronous with a rare field-antisymmetric longitudinal resistance—a low-field tunable ‘chiral switch’ that is rooted in the interplay of Berry curvature, chiral anomaly and a hydrogen-mediated form of Weyl nodes.

Stability of Weyl Node Merging Processes under Symmetry Constraints.

Changes in the number of Weyl nodes in Weyl semimetals occur through merging processes, usually involving a pair of oppositely charged nodes. More complicated processes involving multiple Weyl nodes are also possible, but they typically require fine tuning and are thus less stable. In this Letter, we study how symmetries affect the allowed merging processes and their stability, focusing on the combination of a twofold rotation and time-reversal (C_{2}T) symmetry. We find that, counterintuitively, processes involving a merging of three nodes are more generic than processes involving only two nodes. Our Letter suggests that multi-Weyl merging may be observed in a large variety of quantum materials, and we discuss SrSi_{2} and bilayer graphene as potential candidates.

Robust quantization of circular photogalvanic effect in multiplicative topological semimetals

Nonlinear response signatures are increasingly recognized as useful probes of condensed matter systems, in particular for the characterization of topologically nontrivial states. The circular photogalvanic effect (CPGE) is particularly useful in the study of topological semimetals, as the CPGE tensor quantizes for well-isolated topological degeneracies in strictly linearly dispersing band structures. Here, we study multiplicative Weyl semimetal band structures, and find that the multiplicative structure robustly protects the quantization of the CPGE even in the case of nonlinear dispersion. Computing phase diagrams as a function of Weyl node tilting, we find a variety of quantized values for the CPGE tensor, revealing that the CPGE is also a useful tool in detecting and characterizing the parent topology of multiplicative topological states. Published by the American Physical Society 2024

Selective Control of Electric Charge of Weyl Fermions in Pyrochlore Iridates.

Weyl fermions can exhibit exotic phenomena due to their magnetic charge in momentum space, while Weyl nodes are usually located away from Fermi energy, which forms electron or hole pockets as the electric charges. Previous studies have mostly focused on the magnetic charge, however, the electric charges are rarely explored. Here, the intriguing Hall responses arising from the interplay between magnetic and electric charges of Weyl fermions in pyrochlore iridates are reported. Explicitly, unexpected linear scaling is observed between the anomalous Hall conductivity and Hall carrier density in strained Nd2Ir2O7 thin films, and its slope shows a sign change approaching the Néel temperature of Nd. Theoretical calculations unveil that the cluster magnetic multipoles induce local energy inversion of Weyl nodes, which alters the electric charge of Weyl fermions and accounts for the observed nontrivial Hall responses. Moreover, the correlation between the magnetic and electric charges is further probed by voltage-controlled hydrogenation, which leads to the suppression of the anomalous Hall effect through electron filling. This work not only reveals the essential role of the both magnetic and electric charges of Weyl fermions, but also demonstrates the hydrogenation as an effective tuning knob in exploring correlated topological properties.

An ab-initio study of nodal-arcs, axial strain’s effect on nodal-lines and Weyl nodes and Weyl-contributed Seebeck coefficient in TaAs class of Weyl semimetals

An ab-initio study of nodal-arcs, axial strain’s effect on nodal-lines and Weyl nodes and Weyl-contributed Seebeck coefficient in TaAs class of Weyl semimetals

Controllable Weyl nodes and Fermi arcs from Floquet engineering triple fermions

Controllable Weyl nodes and Fermi arcs from Floquet engineering triple fermions