Three types of fermions play a fundamental role in our understanding of nature: Dirac, Majorana and Weyl. Whereas Dirac fermions have been known for decades, the latter two have not been observed as any fundamental particle in high-energy physics, and have emerged as a much-sought-out treasure in condensed matter physics. A Weyl semimetal is a novel crystal whose low-energy electronic excitations behave as Weyl fermions. It has received worldwide interest and is believed to open the next era of condensed matter physics after graphene and three-dimensional topological insulators. However, experimental research has been held back because Weyl semimetals are extremely rare in nature. Here, we present the experimental discovery of the Weyl semimetal state in an inversion-symmetry-breaking single-crystalline solid, niobium arsenide (NbAs). Utilizing the combination of soft X-ray and ultraviolet photoemission spectroscopy, we systematically study both the surface and bulk electronic structure of NbAs. We experimentally observe both the Weyl cones in the bulk and the Fermi arcs on the surface of this system. Our ARPES data, in agreement with our theoretical band structure calculations, identify the Weyl semimetal state in NbAs, which provides a real platform to test the potential of Weyltronics. W eyl semimetals have received significant attention in recent years because they extend the classification of topological phases beyond insulators, host exotic Fermi arc surface states, demonstrate unusual transport phenomena and provide an emergent condensed matter realization of Weyl fermions, which do not exist as fundamental particles in the standard model 1-21 . Such kind of topologically non-trivial semimetals are believed to open a new era in condensed matter physics. In contrast to topological insulators, where only the surface states are interesting, a Weyl semimetal features unusual band structure in the bulk and on the surface, leading to novel phenomena and potential applications. This opens up unparalleled research opportunities, where both bulk-and surface-sensitive experimental probes can measure the topological nature and detect quantum phenomena. In the bulk, a Weyl semimetal has a band structure with band crossings, Weyl nodes, which are associated with definite chiral charges. Unlike the two-dimensional Dirac points in graphene, the surface-state Dirac point of a threedimensional topological insulator or the three-dimensional Dirac points in the bulk of a Dirac semimetal, the degeneracy associated with a Weyl node does not require any symmetry for its protection, other than the translation symmetry of the crystal lattice. The low-energy quasiparticle excitations of a Weyl semimetal are chiral fermions described by the Weyl equation, well known in highenergy physics, which gives rise to a condensed matter analogue of the chiral anomaly associated with a negative magnetoresistance in transport [16][17][18][19][20][21] . On the surface, the non-trivial topology guarantees the existence of surface states in the f...

Weyl semimetals provide the realization of Weyl fermions in solid-state physics. Among all the physical phenomena that are enabled by Weyl semimetals, the chiral anomaly is the most unusual one. Here, we report signatures of the chiral anomaly in the magneto-transport measurements on the first Weyl semimetal TaAs. We show negative magnetoresistance under parallel electric and magnetic fields, that is, unlike most metals whose resistivity increases under an external magnetic field, we observe that our high mobility TaAs samples become more conductive as a magnetic field is applied along the direction of the current for certain ranges of the field strength. We present systematically detailed data and careful analyses, which allow us to exclude other possible origins of the observed negative magnetoresistance. Our transport data, corroborated by photoemission measurements, first-principles calculations and theoretical analyses, collectively demonstrate signatures of the Weyl fermion chiral anomaly in the magneto-transport of TaAs.

Topological semimetals can support one-dimensional Fermi lines or zero-dimensional Weyl points in momentum space, where the valence and conduction bands touch. While the degeneracy points in Weyl semimetals are robust against any perturbation that preserves translational symmetry, nodal lines require protection by additional crystalline symmetries such as mirror reflection. Here we report, based on a systematic theoretical study and a detailed experimental characterization, the existence of topological nodal-line states in the non-centrosymmetric compound PbTaSe2 with strong spin-orbit coupling. Remarkably, the spin-orbit nodal lines in PbTaSe2 are not only protected by the reflection symmetry but also characterized by an integer topological invariant. Our detailed angle-resolved photoemission measurements, first-principles simulations and theoretical topological analysis illustrate the physical mechanism underlying the formation of the topological nodal-line states and associated surface states for the first time, thus paving the way towards exploring the exotic properties of the topological nodal-line fermions in condensed matter systems.

Two-dimensional (2D) materials are not expected to be metals at low temperature due to electron localization [1]. Consistent with this, pioneering studies on thin films reported only superconducting and insulating ground states, with a direct transition between the two as a function of disorder or magnetic field [2][3][4][5][6]. However, more recent works have revealed the presence of an intermediate quantum metallic state occupying a substantial region of the phase diagram [7-10] whose nature is intensely debated [11][12][13][14][15][16][17]. Here, we observe such a state in the disorder-free limit of a crystalline 2D superconductor, produced by mechanical co-lamination of NbSe 2 in inert atmosphere. Under a small perpendicular magnetic field, we induce a transition from superconductor to the quantum metal. We find a unique power law scaling with field in this phase, which is consistent with the Bose metal model where metallic behavior arises from strong phase fluctuations caused by the magnetic field [11][12][13][14].Global superconductivity emerges in a sample when conduction electrons form Cooper pairs and condense into a macroscopic, phase-coherent quantum state. In two dimensions, the phase coherence can be disrupted even at zero temperature by increasing disorder, either by degrading crystal quality or applying magnetic fields to create vortices [2]. Granular or amorphous superconducting thin films, for which disorder levels can be controlled during growth, have thus provided an established platform for the study of quantum phase transitions in 2D superconductors. Within the conventional theoretical framework, increasing disorder or magnetic field perpendicular to a strongly disordered film at T = 0 induces a direct transition to an insulating state as the normal state sheet resistance approaches the pair quantum resistance h/(2e) 2 = 6.4 kΩ [2,4]. As film quality has improved over time, however, an intervening metallic phase with resistance much lower than the normal state resistance has been observed in several systems with generally less disorder [7][8][9][10]. Its origin is not well understood, and the various theoretical treatments can be generally divided between purely bosonic-based models, in which Cooper pairing persists in the metallic phase but phase coherence is lost [11][12][13][14], and models that also incorporate other fermionic degrees of freedom [15][16][17].Recently, mechanical exfoliation has emerged as a technique to produce ultra-clean, crystalline 2D materials, with graphene being a well-known example [18]. Like amorphous films, the thickness of these samples can be easily controlled down to the level of individual atomic layers. In contrast to amorphous films, a 2D superconductor exfoliated from a lay- Figure 1. Environmentally controlled device fabrication. a) Schematic of heterostructure assembly process. Boron nitride (BN)/graphite (G) on a polymer stamp (PDMS) is used to electrically contact and encapsulate NbSe2 in inert atmosphere. The heterostructure is lithographically patte...

Photoemission established tantalum phosphide as a Weyl semimetal, which hosts exotic Weyl fermion quasiparticles and Fermi arcs.

We report comprehensive studies of the single crystal growth and electrical transport properties for various samples of TaAs, the first experimentally confirmed inversion symmetry-breaking Weyl semimetal. The transport parameters for different samples are obtained through the fitting of the two band model and the analysis of Shubnikov de Haas oscillations. We find that the ratio factor of transport lifetime to quantum lifetime is intensively enhanced when the Fermi level approaches the Weyl node. This result is consistent with the side-jump interpretation derived from a chirality-protected shift in the scattering process for a Weyl semimetal.Comment: This is a modified version of arXiv:1502.0025

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We report electronic properties of superconductivity in single crystals of topological nodal-line semimetal PbTaSe2. Resistivity, magnetic susceptibility and specific heat measurements were performed on high-quality single crystals. We observed large, temperature-dependent anisotropy in its upper critical field (Hc2). The upper critical field measured for H ab plane shows a sudden upward feature instead of saturation at low temperatures. The specific heat measurements in magnetic fields reveal a full superconducting gap with no gapless nodes.