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.

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The complex electronic properties of ZrTe5 have recently stimulated in-depth investigations that assigned this material to either a topological insulator or a 3D Dirac semimetal phase. Here we report a comprehensive experimental and theoretical study of both electronic and structural properties of ZrTe5, revealing that the bulk material is a strong topological insulator (STI). By means of angle-resolved photoelectron spectroscopy, we identify at the top of the valence band both a surface and a bulk state. The dispersion of these bands is well captured by ab initio calculations for the STI case, for the specific interlayer distance measured in our x-ray diffraction study. Furthermore, these findings are supported by scanning tunneling spectroscopy revealing the metallic character of the sample surface, thus confirming the strong topological nature of ZrTe5.The discovery of topological insulators (TIs), characterized by metallic spin-polarized surface states connecting the bulk valence and conduction bands [1], has stimulated the search for novel topological phases of matter [2][3][4][5][6]. ZrTe 5 has recently emerged as a challenging system with unique, albeit poorly understood, electronic properties [7][8][9][10][11][12][13][14][15][16][17]. Magneto-transport [11], magneto-infrared [13] and optical spectroscopy [14] studies describe ZrTe 5 in terms of a 3D Dirac semimetal. Theoretical calculations have predicted its bulk electronic properties to lie in proximity of a topological phase transition between a strong and a weak TI (STI and WTI, respectively), where only the former displays topologically protected surface states at the experimentally accessible (010) surface [10]. The monolayer is also computed to be a 2D TI [10] and scanning tunneling microscopy/spectroscopy (STM/STS) experiments suggest the existence of topologically protected states at step edges [18,19]. However, the unambiguous identification of the topological phase of ZrTe 5 is still lacking.In this Letter we report on the STI character of the bulk ZrTe 5 by combining ab initio calculations and multiple experimental techniques, at temperature both above and below the one of the resistivity peak, T * ∼ 160 K [7][8][9]15]. Angleresolved photoelectron spectroscopy (ARPES) experiments in the ultraviolet (UV) and soft x-ray (SX) energy ranges reveal the presence of two distinct states at the top of the valence band (VB). On the basis of photon energy dependent studies, we ascribe the origin of these two states to the bulk and crystal surface, respectively. We have performed ab initio calculations of the topological phase diagram of ZrTe 5 , as a function of the interlayer distance b/2. Our measured band dispersion is in agreement with the calculations and it is consistent with the STI case for b/2 = 7.23 ± 0.02Å. This value has been confirmed for our specimen by x-ray diffraction (XRD) measurements. Furthermore, the 3D Dirac semimetal phase is not protected by crystalline symmetries, and it manifests only for the specific b/2 = 7.35Å at the boundar...

Macroscopic ferroelectric order in α-GeTe with its noncentrosymmetric lattice structure leads to a giant Rashba spin splitting in the bulk bands due to strong spin-orbit interaction. Direct measurements of the bulk band structure using soft x-ray angle-resolved photoemission (ARPES) reveals the three-dimensional electronic structure with spindle torus shape. By combining high-resolution and spin-resolved ARPES as well as photoemission calculations, the bulk electronic structure is disentangled from the two-dimensional surface electronic structure by means of surface capping, which quenches the complex surface electronic structure. This unravels the bulk Rashba-split states in the ferroelectric Rashba α-GeTe(111) semiconductor exhibiting a giant spin splitting with Rashba parameter α R around 4.2 eVÅ, the highest of so-far known materials. DOI: 10.1103/PhysRevB.94.205111 In spintronics an important goal is to be able to control the spin of the electron in solids without applying magnetic fields [1,2]. The most promising mechanism is based on the Rashba effect [3] and the subsequent spin precession induced in such systems [4]. While most research has previously focused on 2D electron systems [5,6], recently a threedimensional (3D) form of such Rashba effect was found in a series of bismuth tellurohalides BiTeX (X = I, Br, or Cl) [7][8][9][10][11][12]. Although these materials exhibit a very large spin splitting, they lack an important property concerning functionalization, namely, the possibility to switch or tune the spin texture. This limitation can be overcome in a new class of functional materials displaying Rashba splitting coupled to ferroelectricity, the so-called ferroelectric Rashba semiconductors (FERS) [13,14].Recent photoemission experiments on α-GeTe-the stable rhombohedral room temperature configuration of the GeTe phase change material [15]-indicate that this system is a hallmark candidate for entanglement of the ferroelectric and spin-orbit order [16,17]. Due to the giant Rashba splitting spin injection from magnetic systems into GeTe appears viable in order to achieve spin-to-charge conversion [18]. Therefore, ferroelectric [13] or multiferroic [19] Rashba semiconductors bring new multifunctional assets for spintronic devices. A crucial issue for the understanding of FERS is to disentangle the Rashba effect in the bulk caused by the bulk ferroelectric lattice distortion and surface effects arising from particular surface terminations and/or possible band bendings. This represents a major challenge for surface sensitive techniques such as angle-resolved photoemission (ARPES). For this reason up to now ARPES measurements on α-GeTe surfaces performed in the surface-sensitive UV regime have been dominated by surface effects [16,17] and clear information of the three-dimensional bulk electronic structure and its spin texture has not been obtained.