Three types of fermions play a fundamental role in our understanding of nature: Dirac, Majorana and Weyl. Whereas Dirac fermions have been known for decades, the latter two have not been observed as any fundamental particle in high-energy physics, and have emerged as a much-sought-out treasure in condensed matter physics. A Weyl semimetal is a novel crystal whose low-energy electronic excitations behave as Weyl fermions. It has received worldwide interest and is believed to open the next era of condensed matter physics after graphene and three-dimensional topological insulators. However, experimental research has been held back because Weyl semimetals are extremely rare in nature. Here, we present the experimental discovery of the Weyl semimetal state in an inversion-symmetry-breaking single-crystalline solid, niobium arsenide (NbAs). Utilizing the combination of soft X-ray and ultraviolet photoemission spectroscopy, we systematically study both the surface and bulk electronic structure of NbAs. We experimentally observe both the Weyl cones in the bulk and the Fermi arcs on the surface of this system. Our ARPES data, in agreement with our theoretical band structure calculations, identify the Weyl semimetal state in NbAs, which provides a real platform to test the potential of Weyltronics.
Work at Princeton University and Princeton-led synchrotron-based ARPES measurements were supported by the Gordon and Betty Moore Foundations EPiQS Initiative through Grant GBMF4547 (M.Z.H.). First-principles band structure calculations at National University of Singapore were supported by the National Research Foundation, Prime Minister’s Office, Singapore under its NRF fellowship (NRF Award No. NRF-NRFF2013-03). Single-crystal growth was supported by National Basic Research Program of China (Grant Nos. 2013CB921901 and 2014CB239302) and by DE-FG-02-05ER46200. T.-R.C. and H.-T.J. were supported by the National Science Council, Taiwan. H.-T.J. also thanks National Center for High-Performance Computing (NCHC), Computer and Information Network Center National Taiwan University (CINC-NTU), and National Center for Theoretical Sciences (NCTS), Taiwan, for technical support. L.H. is supported by CEM, an NSF MRSEC, under grant DMR-1420451. Experiments at the Ames Laboratory in the Iowa State University were supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Contract No. DE-AC02-07CH11358. The work at Northeastern University was supported by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences grant number DE-FG02-07ER46352, and benefited from Northeastern University’s Advanced Scientific Computation Center (ASCC) and the NERSC Supercomputing Center through DOE grant number DE-AC02-05CH11231. We gratefully thank S.-k. Mo, J. Denlinger, A. V. Fedorov, M. Hashimoto, M. Hoesch and T. Kim for their beamline assistance at the Advanced Light Source, the Stanford Synchrotron Radiation Lightsource and the Diamond Light Source. We thank D. Huse, I. Klebanov, A. Polyakov, P. Steinhardt, H. Verlinde and A. Vishwanath for discussions. T.-R.C. and H.L. acknowledge visiting scientist support from Princeton University. We also thank C.-H. Hsu for technical assistance in the theoretical calculations.