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Publication about this research: S.-Y. Xu et al., “Discovery of a Weyl Weyl Discovered After 85 Years semimetal and topological Fermi arcs,” Science 349, 6248 An international team led (2015); and S.-M. Huang et al., “A by Princeton University scien- Weyl Fermion semimetal with tists has discovered an elusive surface Fermi arcs theoretically massless particle first theo- predicted in the transition metal rized 85 years ago: the Weyl monopnictide TaAs class,” Nat. fermion. It was detected as Commun. 6, 7373 (2015). an emergent in synthetic crystals of the Research conducted by: S.-Y. Xu, I. semimetal, tantalum arsenide Belopolski, N. Alidoust, G. Bian, H. (TaAs). Using angle-resolved Zheng, D.S. Sanchez, P.P. Shibayev, photoemission spectroscopy and M.Z. Hasan (Princeton Univ.); (ARPES), the researchers studied M. Neupane (Princeton Univ. and the surface and bulk band Los Alamos National Laboratory); structure of TaAs. The results C. Zhang and Z. Yuan, (Peking Univ.); exhibit the features—cones, R. Sankar and F. Chou (National nodes, and arcs—that establish Taiwan Univ.); G. Chang, C.-C. Lee, the presence of Weyl fermions. S.-M. Huang, and H. Lin (National These exotic particles, if applied Left: Crystal structure of TaAs, shown as stacked Ta and As Univ. of Singapore); J. Ma (Oak Ridge to next-generation electronics, layers. The lattice of TaAs does not exhibit space inversion National Laboratory); B.K. Wang could allow for the nearly free . Right: Photo of TaAs single crystals. (National Univ. of Singapore and and efficient flow of electricity, Northeastern Univ.); A. Bansil and thus greater power, in (Northeastern Univ.); and S. Jia devices such as computers. cist in 1929 as generates heat in normal elec- (Peking Univ. and Collaborative More generally, the work paves an alternative to Einstein’s tronic materials. Innovation Center of Quantum the way for the realization of theory of relativity. Although According to established Matter, Beijing). many fascinating topological that application never panned theory, a Weyl semimetal state quantum phenomena. out, the characteristics of his could only arise in a crystal Research funding: Gordon and Weyl fermions were proposed theoretical particle intrigued where either time-reversal sym- Betty Moore Foundation; Singapore by the mathematician and physi- physicists for nearly a century. metry or inversion symmetry is National Research Foundation; Unlike electrons, Weyl fermions broken. Initially, the team National Basic Research Program are massless and possess a high researched hundreds of crystal of China; Taiwan Ministry of Science degree of mobility. They also structures and simulated dozens and Technology; and the U.S. possess a form of in of likely structures. They pub- Department of Energy (DOE), Office which their spin is either in lished an earlier paper theorizing of Basic Energy Sciences (BES). the same direction as motion that Weyl fermions could exist Operation of the ALS is supported (right-handed) or in the opposite in a class of materials composed by DOE BES. direction (left-handed). Their of a transition metal (e.g. Ta or basic nature means that Nb) and a Group 15 element Weyl fermions move very (e.g. As or P). They synthesized 12.0.1) as well as other synchro- quickly on the surface of the asymmetrical TaAs crystals tron light sources (Stanford crystal with no backscattering, and brought them to the ALS Synchrotron Radiation Light- which hinders efficiency and (Beamlines 4.0.3, 10.0.1, and source, Diamond Light Source, Materials By Design

Stone, bronze, iron, silicon: in ages past and present, materials have had the power to shape civilization. Now, some believe we are on the verge of a golden age of materials science, in which new materials are rationally designed rather than discovered by accident or through painstaking trial and error. Converging advances in supercomputing, nanofabrication, and materials characterization (at facilities Princeton team members (from left to right): Ilya Belopolski, such as the ALS) are enabling new materials specifically Daniel Sanchez, Guang Bian, M. Zahid Hasan, and Hao Zheng. designed for artificial photosynthesis, for example, or for (Photo by Danielle Alio, Office of Communications.) processing more information in less space at lower power.

In this work, Xu et al. sifted through hundreds of materials point, it is a Weyl semimetal, and surface–bulk correspondence) of with the desired structural characteristics, identified the the point is called a Weyl node. the Weyl semimetal state in TaAs. ones likely to be semimetals or narrow-bandgap semicon- Superimposing the SX-ARPES Although practical technolo- ductors, and performed systematic calculations of their band bulk data containing the Weyl gies are still a long way off, structures. “The nature of this research and how it emerged nodes onto the VUV-ARPES publications about Weyl is really different and more exciting than most of other work data containing the surface Fermi fermions have increased rapidly we have done before,” Xu said. “Usually, theorists tell us arcs to scale, the researchers noted since the discovery was that some compound might show some new or interesting that all the arc terminations and reported, signifying the opening properties, then we as experimentalists grow that sample projected Weyl nodes match of an exciting new frontier in and perform experiments to test the prediction. In this case, within the experimental resolu- condensed matter physics. “The we came up with the theoretical prediction ourselves and tion. Because the SX-ARPES bulk physics of the Weyl fermion are then performed the experiments. This makes the final suc- data and the VUV-ARPES surface so strange,” said M. Zahid cess even more exciting and satisfying than before.” data are completely independent Hasan, the Princeton physics measurements obtained at two professor who led the research different beamlines, the fact team, “there could be many and the Swiss Light Source) to a semimetal (insulators and that they match well provides things that arise from this perform ARPES experiments. semiconductors have bandgaps). another piece of evidence of particle that we’re just not Using a combination of When the tips meet at a single the topological nature (the capable of imagining now.” vacuum ultraviolet (VUV) and soft x-ray (SX) ARPES, the researchers systematically studied the surface and bulk electronic structure of TaAs. The surface-sensitive VUV-ARPES measurements demonstrated the existence of Fermi-arc surface states, consistent with earlier band calculations. The SX-ARPES measurements, which are reasonably bulk sensitive, revealed 3D (bulk) Weyl cones with linear dispersions, similar (a) ARPES Fermi-surface map showing horseshoe-shaped Fermi arcs. (b) ARPES bulk dispersion to the 2D (surface) Dirac cones map showing two linearly dispersive Weyl cones. Plus and minus signs denote right- and left- associated with topological handed chirality. (c) Correspondence between Fermi-arc endpoints and Weyl fermion nodes insulators and . When bolsters the case for Weyl fermions. (d) Illustration of the simplest Weyl semimetal state with two the tips of the cones overlap Weyl nodes with opposite (±1) chiral charges in the bulk. A corresponding surface Fermi arc is slightly, there is no bandgap, shown above. (Images and data by Su-Yang Xu and M. Zahid Hasan.) and the material is considered

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