Abstract:A Weyl semimetal is a crystal which hosts Weyl fermions as emergent quasiparticles and admits a topological classification that protects Fermi arc surface states on the boundary of a bulk sample. This unusual electronic structure has deep analogies with particle physics and leads to unique topological properties. We report the experimental discovery of the first Weyl semimetal, TaAs. Using photoemission spectroscopy, we directly observe Fermi arcs on the surface, as well as the Weyl fermion cones and Weyl nodes in the bulk of TaAs single crystals.We find that Fermi arcs terminate on the Weyl nodes, consistent with their topological character.Our work opens the field for the experimental study of Weyl fermions in physics and materials science.NoteAdded: This experimental discovery (Science 2015) is based on our earlier 2014 theoretical discovery/prediction reported at [Huang et al., Nature Commun. 6:7373 (2015)

Weyl fermions are massless chiral fermions that play an important role in quantum field theory but have never been observed as fundamental particles. A Weyl semimetal is an unusual crystal that hosts Weyl fermions as quasiparticle excitations and features Fermi arcs on its surface. Such a semimetal not only provides a condensed matter realization of the anomalies in quantum field theories but also demonstrates the topological classification beyond the gapped topological insulators. Here, we identify a topological Weyl semimetal state in the transition metal monopnictide materials class. Our first-principles calculations on TaAs reveal its bulk Weyl fermion cones and surface Fermi arcs. Our results show that in the TaAs-type materials the Weyl semimetal state does not depend on fine-tuning of chemical composition or magnetic order, which opens the door for the experimental realization of Weyl semimetals and Fermi arc surface states in real materials.

Recent discovery of spin-polarized single-Dirac-cone insulators, whose variants can host magnetism and superconductivity, has generated widespread research activity in condensed-matter and materials-physics communities. Some of the most interesting topological phenomena, however, require topological insulators to be placed in multiply connected, highly constrained geometries with magnets and superconductors, all of which thus require a large number of functional variants with materials design flexibility as well as electronic, magnetic and superconducting tunability. Given the optimum materials, topological properties open up new vistas in spintronics, quantum computing and fundamental physics. We have extended the search for topological insulators from the binary Bi-based series to the ternary thermoelectric Heusler compounds. Here we show that, although a large majority of the well-known Heuslers such as TiNiSn and LuNiBi are rather topologically trivial, the distorted LnPtSb-type (such as LnPtBi or LnPdBi, Ln = f(n) lanthanides) compounds belonging to the half-Heusler subclass harbour Z(2) = -1 topological insulator parent states, where Z(2) is the band purity product index. Our results suggest that half-Heuslers provide a new platform for deriving a host of topologically exotic compounds and their nanoscale or thin-film device versions through the inherent flexibility of their lattice parameter, spin-orbit strength and magnetic moment tunability paving the way for the realization of multifunctional topological devices.

Weyl semimetals are conductors whose low-energy bulk excitations are Weyl fermions, whereas their surfaces possess metallic Fermi arc surface states. These Fermi arc surface states are protected by a topological invariant associated with the bulk electronic wavefunctions of the material.Recently, it has been shown that the TaAs and NbAs classes of materials harbor such a state of topological matter. We review the basic phenomena and experimental history of the discovery of the first Weyl semimetals, starting with the observation of topological Fermi arcs and Weyl nodes in TaAs and NbAs by angle and spin-resolved surface and bulk sensitive photoemission spectroscopy and continuing through magnetotransport measurements reporting the Adler-Bell-Jackiw chiral anomaly. We hope that this article provides a useful introduction to the theory of Weyl semimetals, a summary of recent experimental discoveries, and a guideline to future directions.

In topological crystalline insulators (TCIs), topology and crystal symmetry intertwine to create surface states with distinct characteristics. The breaking of crystal symmetry in TCIs is predicted to impart mass to the massless Dirac fermions. Here, we report high-resolution scanning tunneling microscopy studies of a TCI, Pb(1-x)Sn(x)Se that reveal the coexistence of zero-mass Dirac fermions protected by crystal symmetry with massive Dirac fermions consistent with crystal symmetry breaking. In addition, we show two distinct regimes of the Fermi surface topology separated by a Van-Hove singularity at the Lifshitz transition point. Our work paves the way for engineering the Dirac band gap and realizing interaction-driven topological quantum phenomena in TCIs.

Physicists have discovered a novel topological semimetal, the Weyl semimetal, whose surface features a nonclosed Fermi surface while the low energy quasiparticles in the bulk emerge as Weyl fermions. Here they share a brief review of the development and present perspectives on the next step forward.Weyl semimetals are semimetals or metals whose quasiparticle excitation is the Weyl fermion, a particle that played a crucial role in quantum field theory but has not been observed as a fundamental particle in vacuum 1-24 . Weyl fermions have definite chiralities, either left-handed or right handed. In a Weyl semimetal, the chirality can be understood as a topologically protected chiral charge. Weyl nodes of opposite chirality are separated in momentum space and are connected only through the crystal boundary by an exotic nonclosed surface state, the Fermi arcs. Remarkably, Weyl fermions are robust while carrying currents, giving rise to exceptionally high mobilities. Their spins are locked to their momentum directions due to their character of momentum space magnetic monopole configuration.The presence of parallel electrical and magnetic fields can break the apparent conservation of the chiral charge due to the chiral anomaly, making a Weyl metal, unlike ordinary nonmagnetic metals, more conductive with an increasing magnetic field. These new topological phenomena beyond topological insulators make new physics accessible and suggest potential applications, despite the early stage of the research .In this Commentary, we will review key experimental progress and present an outlook for future directions of the field. Through this article, we hope to expound our perspectives on the key results and the experimental approaches currently used to access the novel physics as well as their limitations. Moreover, while most of the current experiments are still focusing on the discovery of new Weyl materials and demonstration of novel Weyl physics such as the chiral anomaly, it is becoming clear that a crucial step forward is to develop schemes for achieving quantum controls of the novel Weyl physics by electrical and optical means. We discuss some theoretical proposals along these lines highlighting the experimental techniques and matching materials conditions that are necessary for realizing these research directions. 2 MATERIAL SEARCHAlthough the theory of Weyl semimetal has been around for a long time in various forms 1-4 , its discovery had to wait until recent developments. This is because finding experimental realization requires appropriate materials simulation and characterizations. Historically, the first two material predictions, the pyrochlore iridates R 2 Ir 2 O 7 4 and the magnetically doped superlattice 6 , were both on time-reversal breaking (magnetic) materials.Perhaps influenced by the first works, for a long while, the community continued to focus on time-reversal breaking Weyl semimetals materials candidates 7,10 . These candidates were extensively studied by many experimental groups. Unfortunately, these ef...

Owing to the unusual geometry of kagome lattices-lattices made of corner-sharing triangles-their electrons are useful for studying the physics of frustrated, correlated and topological quantum electronic states. In the presence of strong spin-orbit coupling, the magnetic and electronic structures of kagome lattices are further entangled, which can lead to hitherto unknown spin-orbit phenomena. Here we use a combination of vector-magnetic-field capability and scanning tunnelling microscopy to elucidate the spin-orbit nature of the kagome ferromagnet FeSn and explore the associated exotic correlated phenomena. We discover that a many-body electronic state from the kagome lattice couples strongly to the vector field with three-dimensional anisotropy, exhibiting a magnetization-driven giant nematic (two-fold-symmetric) energy shift. Probing the fermionic quasi-particle interference reveals consistent spontaneous nematicity-a clear indication of electron correlation-and vector magnetization is capable of altering this state, thus controlling the many-body electronic symmetry. These spin-driven giant electronic responses go well beyond Zeeman physics and point to the realization of an underlying correlated magnetic topological phase. The tunability of this kagome magnet reveals a strong interplay between an externally applied field, electronic excitations and nematicity, providing new ways of controlling spin-orbit properties and exploring emergent phenomena in topological or quantum materials.

By combining transport and photoemission measurements on (Bi(1-x)In(x))(2)Se(3) thin films, we report that this system transforms from a topologically nontrivial metal into a topologically trivial band insulator through three quantum phase transitions. At x ≈ 3%-7%, there is a transition from a topologically nontrivial metal to a trivial metal. At x ≈ 15%, the metal becomes a variable-range-hopping insulator. Finally, above x ≈ 25%, the system becomes a true band insulator with its resistance immeasurably large even at room temperature. This material provides a new venue to investigate topologically tunable physics and devices with seamless gating or tunneling insulators.