Discovery of a Weyl Semimetal in non-Centrosymmetric Compound TaAs

Institut fu¨r Anorganische Chemie und Analytische Chemie,Johannes Gutenberg-Universtit¨at, 55099 Mainz, Germany(Dated: July 3, 2015)Three-dimensional (3D) topological Weyl semimetals (TWSs) represent a novel state of quantummatter with unusual electronic structures that resemble both a 3D graphene and a topologicalinsulator by possessing pairs of Weyl points (through which the electronic bands disperse linearlyalong all three momentum directions) connected by topological surface states, forming the uniqueFermi-arc type Fermi-surface (FS). Each Weyl point is chiral and contains half of the degrees offreedom of a Dirac point, and can be viewed as a magnetic monopole in the momentum space.Here, by performing angle-resolved photoemission spectroscopy on non-centrosymmetric compoundTaAs, we observed its complete band structures including the unique Fermi-arc FS and linear bulkband dispersion across the Weyl points, in excellent agreement with the theoretical calculations[1, 2]. This discovery not only confirms TaAs as the first 3D TWS, but also provides an idealplatform for realizing exotic physical phenomena (e.g. negative magnetoresistance, chiral magneticeffects and quantum anomalous Hall effect) which may also lead to novel future applications.INTRODUCTION