In 1929, H. Weyl proposed that the massless solution of the Dirac equation represents a pair of a new type of particles, the so-called Weyl fermions 1 . However, their existence in particle physics remains elusive after more than eight decades. Recently, significant advances in both topological insulators and topological semimetals have provided an alternative way to realize Weyl fermions in condensed matter, as an emergent phenomenon: when two non-degenerate bands in the three-dimensional momentum space cross in the vicinity of the Fermi energy (called Weyl nodes), the low-energy excitations behave exactly as Weyl fermions. Here we report the direct observation in TaAs of the long-sought-after Weyl nodes by performing bulk-sensitive soft X-ray angle-resolved photoemission spectroscopy measurements. The projected locations at the nodes on the (001) surface match well to the Fermi arcs, providing undisputable experimental evidence for the existence of Weyl fermionic quasiparticles in TaAs.The massless Dirac equation in the three-dimensional (3D) momentum space can be regarded as the overlap of two Weyl fermions with opposite chirality 1,2 . The Dirac fermionic quasiparticle is stable under the protection of some crystal symmetry in topological Dirac semimetals such as Na 3 Bi (ref. 3) and Cd 3 As 2 (ref. 4). However, a separated single Weyl node is much more robust and requires no protection of crystal symmetry. An isolated Weyl node is a sink or source of gauge field of Berry curvature, like a monopole in momentum space, and the chirality corresponds to its topological charge [5][6][7] . Weyl nodes appear in pairs of opposite chirality in a real material due to the 'No-go theorem' 8,9 . To obtain isolated Weyl nodes, the spin degeneracy of the electronic bands has to be removed by breaking either inversion symmetry or time-reversal symmetry. Although non-degenerate band crossing is not rare, finding a material with only Weyl nodes near the Fermi energy (E F ) is a big challenge. Recently, the noncentrosymmetric and non-magnetic transition-metal monoarsenide TaAs has been predicted to be a Weyl semimetal (WSM), and twelve pairs of Weyl nodes are expected in its 3D Brillouin zone (BZ; refs 10,11). Compared with other proposals 6,7,12-22 for realizing a Weyl state, the TaAs family features easy sample fabrication, a non-magnetic state and no fine-tuning of the electronic states is necessary, making experimental studies of Weyl semimetals possible. Many exotic properties induced by the Weyl nodes have been predicted and observed recently, such as surface states with Fermi arcs 23,24 and a negative magneto-resistivity 25,26 due to the chiral anomaly 27-29 . However, crucial evidence for Weyl nodes in the bulk states has not been observed. In this paper, by using soft X-ray angle-resolved photoemission spectroscopy (ARPES), which is sensitive to the bulk states, we report the first experimental observation of Weyl nodes in TaAs.TaAs crystallizes in a body-centred-tetragonal structure with the nonsymmorphic space group...

A Weyl semimetal possesses spin-polarized band-crossings, called Weyl nodes, connected by topological surface arcs. The low-energy excitations near the crossing points behave the same as massless Weyl fermions, leading to exotic properties like chiral anomaly. To have the transport properties dominated by Weyl fermions, Weyl nodes need to locate nearly at the chemical potential and enclosed by pairs of individual Fermi surfaces with non-zero Fermi Chern numbers. Combining angle-resolved photoemission spectroscopy and first-principles calculation, here we show that TaP is a Weyl semimetal with only a single type of Weyl fermions, topologically distinguished from TaAs where two types of Weyl fermions contribute to the low-energy physical properties. The simple Weyl fermions in TaP are not only of fundamental interests but also of great potential for future applications. Fermi arcs on the Ta-terminated surface are observed, which appear in a different pattern from that on the As-termination in TaAs and NbAs.

Topological Kondo insulators have been proposed as a new class of topological insulators in which non-trivial surface states reside in the bulk Kondo band gap at low temperature due to strong spin-orbit coupling. In contrast to other three-dimensional topological insulators, a topological Kondo insulator is truly bulk insulating. Furthermore, strong electron correlations are present in the system, which may interact with the novel topological phase. By applying spin-and angle-resolved photoemission spectroscopy, here we show that the surface states of SmB 6 are spin polarized. The spin is locked to the crystal momentum, fulfilling time reversal and crystal symmetries. Our results provide strong evidence that SmB 6 can host topological surface states in a bulk insulating gap stemming from the Kondo effect, which can serve as an ideal platform for investigating of the interplay between novel topological quantum states with emergent effects and competing orders induced by strongly correlated electrons.

Recent theoretical calculations and experimental results suggest that the strongly correlated material SmB6 may be a realization of a topological Kondo insulator. We have performed an angleresolved photoemission spectroscopy study on SmB6 in order to elucidate elements of the electronic structure relevant to the possible occurrence of a topological Kondo insulator state. The obtained electronic structure in the whole three-dimensional momentum space reveals one electron-like 5d bulk band centred at the X point of the bulk Brillouin zone that is hybridized with strongly correlated f electrons, as well as the opening of a Kondo bandgap (∆B ∼ 20 meV) at low temperature. In addition, we observe electron-like bands forming three Fermi surfaces at the centerΓ point and boundaryX point of the surface Brillouin zone. These bands are not expected from calculations of the bulk electronic structure, and their observed dispersion characteristics are consistent with surface states. Our results suggest that the unusual low-temperature transport behavior of SmB6 is likely to be related to the pronounced surface states sitting inside the band hybridisation gap and/or the presence of a topological Kondo insulating state. A three-dimensional (3D) topological insulator (TI) is an unusual topological quantum state associated with unique metallic surface states that appear within the bulk bandgap [1,2]. Owing to the peculiar spin texture protected by time-reversal symmetry, the Dirac fermions in TIs are forbidden from scattering due to nonmagnetic impurities and disorder [3,4]. Hence they carry dissipationless spin current [5], making it possible to explore fundamental physics, spintronics, and quantum computing [1,2]. However, even after extensive materials synthesis efforts [6][7][8][9][10], impurities in the bulk of these materials make them metallic, prompting us to search for new types of TIs with truly insulating bulks.The 3D Kondo insulator SmB 6 may open a new route to realizing topological surface states. SmB 6 is a typical heavy fermion material with strong electron correlation. Localized f electrons hybridize with conduction electrons, leading to a narrow bandgap on the order of 10 meV opening at low temperatures, with the chemical potential lying in the gap [11][12][13][14]. Due to the opening of the bandgap, the conductivity changes from metallic to insulating behavior with decreasing temperature. It saturates to a constant value below about 1 K, which is thought to be caused by in-gap states [15]. Theoretical studies have proposed that SmB 6 may host threedimensional topological insulating phases [16,17]. Recently, transport experiments employing a novel geometry [18] showed convincing evidence of a distinct surface contribution to the conductivity that is unmixed with the bulk contribution, suggesting SmB 6 is an ideal topological insulator with a perfectly insulating bulk. Pointcontact spectroscopy revealed that the low-temperature Kondo insulating state harbors conduction states on the surface, in support of predict...