Weyl Nodal-line Semimetals Research Articles

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.

Half-Metals and Weyl Nodal Line Semimetals in Two-Dimensional Phthalocyanine-Based Metal Organic Frameworks with Ni–Fe Dual Metals

Two-dimensional organic ferromagnetic half-metals and Weyl semimetals are desirable for nanoscale spintronic and electronic devices. However, two-dimensional half-metals and Weyl semimetals are sensitive to doping and strain, which hinder their further applications. Here, the electronic structure and magnetic properties of two-dimensional nickel phthalocyanine (NiPc)-based metal–organic frameworks are studied by first-principles calculations. NiPc linked by square-planar Fe–O4 (NiPc–Fe2) exhibits in-plane ferromagnetic half-metal properties with a large band gap and high Curie temperature. NiPc–Ni2 and NiPc–NiFe exhibit in-plane ferromagnetic half-metal properties with tunable band gaps and abundant electronic transitions by applying biaxial strain and charge doping. NiPc–Ni2 and NiPc–NiFe also exhibit Weyl semimetal properties with two closed nodal lines and one nodal line near the Fermi level, which shift toward the Fermi level under charge doping. When spin–orbit coupling is taken into account, nodal lines will open small gaps. These results enrich the two-dimensional magnetic half-metals and semimetals, which have potential applications in spintronic devices.

Spin-triplet superconductivity in Weyl nodal-line semimetals

Topological semimetals are three dimensional materials with symmetry-protected massless bulk excitations. As a special case, Weyl nodal-line semimetals are realized in materials having either no inversion or broken time-reversal symmetry and feature bulk nodal lines. The 111-family, including LaNiSi, LaPtSi and LaPtGe materials (all lacking inversion symmetry), belongs to this class. Here, by combining muon-spin rotation and relaxation with thermodynamic measurements, we find that these materials exhibit a fully-gapped superconducting ground state, while spontaneously breaking time-reversal symmetry at the superconducting transition. Since time-reversal symmetry is essential for protecting the normal-state topology, its breaking upon entering the superconducting state should remarkably result in a topological phase transition. By developing a minimal model for the normal-state band structure and assuming a purely spin-triplet pairing, we show that the superconducting properties across this family can be described accurately. Our results demonstrate that the 111 materials reported here provide an ideal test-bed for investigating the rich interplay between the exotic properties of Weyl nodal-line fermions and unconventional superconductivity.

Topological Quantum Materials from the Viewpoint of Chemistry.

Topology, a mathematical concept, has recently become a popular and truly transdisciplinary topic encompassing condensed matter physics, solid state chemistry, and materials science. Since there is a direct connection between real space, namely atoms, valence electrons, bonds, and orbitals, and reciprocal space, namely bands and Fermi surfaces, via symmetry and topology, classifying topological materials within a single-particle picture is possible. Currently, most materials are classified as trivial insulators, semimetals, and metals or as topological insulators, Dirac and Weyl nodal-line semimetals, and topological metals. The key ingredients for topology are certain symmetries, the inert pair effect of the outer electrons leading to inversion of the conduction and valence bands, and spin–orbit coupling. This review presents the topological concepts related to solids from the viewpoint of a solid-state chemist, summarizes techniques for growing single crystals, and describes basic physical property measurement techniques to characterize topological materials beyond their structure and provide examples of such materials. Finally, a brief outlook on the impact of topology in other areas of chemistry is provided at the end of the article.

Emergence of topological phases by stacking of two-dimensional lattices with nonsymmorphic symmetry

Topological semimetals have a variety of phases, whose Fermi surfaces can be nodal points, nodal lines and nodal loops. Here we construct four classes of 3D minimal models via vertically stacking a 2D nonsymmorphic lattice with and without breaking crystalline symmetries. As a result, four distinct topological phases can be generated in our minimal model, such as Dirac nodal line semimetals, Weyl nodal line semimetals, unconventional Weyl semimetals with topological charge , and weak topological insulators. Unexpectedly, Weyl nodal loops are generated without mirror symmetry protection, where nontrivial ‘drumhead’ surface states emerge within the loops.

Continuously tunable topological pump in high-dimensional cold atomic gases

Controllability of ultracold atomic gases has reached an unprecedented level, allowing for experimental realization of the long-sought-after Thouless pump, which can be interpreted as a dynamical quantum Hall effect. On the other hand, Weyl semimetals and Weyl nodal line semimetals with touching points and rings in band structures have sparked tremendous interest in various fields in the past few years. Here, we show that dynamical Weyl points and dynamical 4D Weyl nodal rings, which are protected by the first Chern number on a parameter surface formed by quasi-momentum and time, emerge in a two-dimensional and three-dimensional system, respectively. We find that the topological pump occurs in these systems but the amount of pumped particles is not quantized and can be continuously tuned by controlling experimental parameters over a wide range. We also propose an experimental scheme to realize the dynamical Weyl points and 4D Weyl nodal rings and to observe their corresponding topological pump in cold atomic gases.

Drumhead surface states and their signatures in quasiparticle scattering interference

We consider a two-orbital tight-binding model defined on a layered three-dimensional hexagonal lattice to investigate the properties of topological nodal lines and their associated drumhead surface states. We examine these surface states in centrosymmetric systems, where the bulk nodal lines are of Dirac type (i.e., four-fold degenerate), as well as in non-centrosymmetric systems with strong Rashba and/or Dresselhaus spin-orbit coupling, where the bulk nodal lines are of Weyl type (i.e., two-fold degenerate). We find that in non-centrosymmetric systems the nodal lines and their corresponding drumhead surface states are fully spin polarized due to spin-orbit coupling. We show that unique signatures of the topologically nontrivial drumhead surface states can be measured by means of quasiparticle scattering interference, which we compute for both Dirac and Weyl nodal line semimetals. At the end, we analyze the possible crystal structures with a symmetry that supports flat surface states which are effectively ringlike.

Boosting the discovery of 3D topological materials: mixing chemistry with physics via a two-step computational screening strategy

Topological materials in crystal solids, including topological insulators (TIs), topological crystalline insulators (TCIs), topological Dirac semimetals (DSMs), topological Weyl semimetals (WSMs), topological Dirac or Weyl nodal line semimetals (NLSMs) and beyond, are mainly featured with topological, protected non-trivial surface states, and their bulk phases are insulators or semimetals with the proper presence of Dirac cones, Weyl nodes or Dirac nodal lines around the Fermi level. The author suggests a two-step computational screening strategy of 3D topological materials by mixing chemistry with physics with the considerations of fully filled bands and band inversion.