Abstract

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.

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Acknowledgements

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.

Author information

Author notes

    • Su-Yang Xu
    • , Nasser Alidoust
    •  & Ilya Belopolski

    These authors contributed equally to this work.

Affiliations

  1. Laboratory for Topological Quantum Matter and Spectroscopy (B7), Department of Physics, Princeton University, Princeton, New Jersey 08544, USA

    • Su-Yang Xu
    • , Nasser Alidoust
    • , Ilya Belopolski
    • , Guang Bian
    • , Tay-Rong Chang
    • , Hao Zheng
    • , Daniel S. Sanchez
    •  & M. Zahid Hasan
  2. Princeton Center for Complex Materials, Princeton Institute for Science and Technology of Materials, Princeton University, Princeton, New Jersey 08544, USA

    • Su-Yang Xu
    • , Nasser Alidoust
    • , Ilya Belopolski
    •  & M. Zahid Hasan
  3. International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China

    • Zhujun Yuan
    • , Chenglong Zhang
    •  & Shuang Jia
  4. Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan

    • Tay-Rong Chang
    •  & Horng-Tay Jeng
  5. Paul Scherrer Institute, Swiss Light Source, CH-5232 Villigen PSI, Switzerland

    • Vladimir N. Strocov
  6. Centre for Advanced 2D Materials and Graphene Research Centre National University of Singapore, 6 Science Drive 2 Singapore 117546, Singapore

    • Guoqing Chang
    • , Chi-Cheng Lee
    • , Shin-Ming Huang
    • , BaoKai Wang
    •  & Hsin Lin
  7. Department of Physics, National University of Singapore, 2 Science Drive 3 Singapore 117542, Singapore

    • Guoqing Chang
    • , Chi-Cheng Lee
    • , Shin-Ming Huang
    • , BaoKai Wang
    •  & Hsin Lin
  8. Division of Materials Science and Engineering, Ames Laboratory, US DOE Ames, Iowa 50011, USA

    • Daixiang Mou
    • , Yun Wu
    • , Lunan Huang
    •  & Adam Kaminski
  9. Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA

    • Daixiang Mou
    • , Yun Wu
    • , Lunan Huang
    •  & Adam Kaminski
  10. Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA

    • BaoKai Wang
    •  & Arun Bansil
  11. Institute of Physics, Academia Sinica, Taipei 11529, Taiwan

    • Horng-Tay Jeng
  12. Princeton Center for Theoretical Science, Princeton University, Princeton, New Jersey 08544, USA

    • Titus Neupert
  13. Collaborative Innovation Center of Quantum Matter, Beijing 100871, China

    • Shuang Jia

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Contributions

S.-Y.X., N.A., I.B., G.B. and D.S.S. conducted the ARPES experiments with assistance from H.Z., V.N.S., D.M., Y.W., L.H., A.K. and M.Z.H.; Z.Y., C.Z. and S.J. grew the single-crystal samples; H.Z. conducted the STM measurements with assistance from G.B., S.-Y.X. and D.S.S.; T.-R.C., G.C., C.-C.L., S.-M.H., B.W., A.B., H.-T.J. and H.L. performed first-principles band structure calculations; T.N. did theoretical analyses; M.Z.H. was responsible for the overall direction, planning and integration among different research units.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to M. Zahid Hasan.

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https://doi.org/10.1038/nphys3437

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