Weyl semimetals are a class of materials that can be regarded as three-dimensional analogs of graphene upon breaking time-reversal or inversion symmetry. Electrons in a Weyl semimetal behave as Weyl fermions, which have many exotic properties, such as chiral anomaly and magnetic monopoles in the crystal momentum space. The surface state of a Weyl semimetal displays pairs of entangled Fermi arcs at two opposite surfaces. However, the existence of Weyl semimetals has not yet been proved experimentally. Here, we report the experimental realization of a Weyl semimetal in TaAs by observing Fermi arcs formed by its surface states using angle-resolved photoemission spectroscopy. Our first-principles calculations, which match remarkably well with the experimental results, further confirm that TaAs is a Weyl semimetal.

We report the temperature evolution of the detailed electronic band structure in FeSe singlecrystals measured by angle-resolved photoemission spectroscopy (ARPES), including the degeneracy removal of the dxz and dyz orbitals at the Γ/Z and M points, and the orbital-selective hybridization between the dxy and d xz/yz orbitals. The temperature dependences of the splittings at the Γ/Z and M points are different, indicating that they are controlled by different order parameters. The splitting at the M point is closely related to the structural transition and is attributed to orbital ordering defined on Fe-Fe bonds with a d-wave form in the reciprocal space that breaks the rotational symmetry. In contrast, the band splitting at the Γ/Z points remains at temperature far above the structural transition. Although the origin of this latter splitting remains unclear, our experimental results exclude the previously proposed ferro-orbital ordering scenario.

We demonstrate, using first-principles calculations, that the electronic structure of FeSe1−xTex (x=0.5) is topologically non-trivial, characterized by an odd Z2 invariant and Dirac cone type surface states, in sharp contrast to the end member FeSe (x=0). This topological state is induced by the enhanced three-dimensionality and spin-orbit coupling due to Te substitution (compared to FeSe), characterized by a band inversion at the Z point of the Brillouin zone, which is confirmed by our ARPES measurements. The results suggest that the surface of FeSe0.5Te0.5 may support a non-trivial superconducting channel in proximity to the bulk.PACS numbers: 74.70.Xa, Among the Fe-based superconductors, the FeSe 1−x Te x family of compounds [1-4] is of particular interest. First, it has the simplest PbO structure (space group P 4/nmm) with Se (or Te) atoms forming distorted tetrahedra around Fe (see Fig. 1(a)) similar to the structure of FeAs planes in the families of FeAs-based high T c superconductors [5]. Second, the internal parameters can be systematically tuned by the substitution of Se by Te [6][7][8], which provides us a platform for in-depth study of possible superconducting mechanisms and topological characters. Thirdly, superconductivity has been observed for a wide range of composition x [2-4], and the transition temperature T c can be further enhanced by pressure [9][10][11]. More recently, superconductivity with T c higher than 77 K was suggested for single unit cell FeSe films [12] epitaxially grown on SrTiO 3 substrates.Despite these interesting properties though, the particularities of the system have still not been fully explored. Earlier studies, both theoretical and experimental, suggest the similarity of the electronic structures of the Fe chalcogenides (FeSe, FeTe) [13][14][15] and the FeAsbased [16][17][18] superconductors. Indeed, the low-energy physics around the Fermi level is dominated by the Fe-3d states, and the morphology of the Fermi surfaces is similar. On the other hand, a surprisingly stable (no splitting under external magnetic field) zero-energy bound state (ZBS) at randomly distributed interstitial excess Fe sites was observed in very recent scanning tunneling microscopy (STM) measurements on the surface of superconducting Fe(Te,Se) [19], suggesting possible topological feature of its electronic structure. Obviously, the * * these authors contributed equally to this work. 5p orbitals of Te are more extended and have stronger spin-orbit coupling (SOC) than the 4p orbitals of Se. The consequences of Te substitution, particularly for the bulk topological character of FeSe 1−x Te x , have been largely ignored in the literature and will be the main purpose of the present paper. Based on first-principles calculations combined with angle resolved photoemission spectroscopy (ARPES) measurements, here we report that the electronic structure of FeSe 0.5 Te 0.5 is topologically non-trivial, in sharp contrast to its end member FeSe. The topological properties of FeSe 0.5 Te 0.5 can be characterized ...