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.

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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.