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.
At a glance
- Dirac, Majorana and Weyl fermions. Am. J. Phys. 79, 485–498 (2011).
- Elektron und gravitation. I. Z. Phys. 56, 330–352 (1929).
- Weyl electrons kiss. Physics 4, 36 (2011).
- Why are there analogies between condensed matter and particle theory? Phys. Today 51, 11–13 (1998).
- 2013). & in Topological Insulators (eds Franzand, M. & Molenkamp, L.) (Elsevier,
- ” to the bulk Fermi surface: “Fermi-level plumbing” in topological metals. Preprint at http://arXiv.org/abs/1401.0529 (2014). Attachment of surface “Fermi arcs
- 55–100 (John Wiley, 2015). , & in Topological Insulators: Fundamentals and Perspectives (eds Ortmann, F., Roche, S. & Valenzuela, S. O.)
- Topological semimetal and Fermi-arc surface states in the electronic structure of pyrochlore iridates. Phys. Rev. B 83, 205101 (2011). , , &
- Phase transition between the quantum spin Hall and insulator phases in 3D: Emergence of a topological gapless phase. New J. Phys. 9, 356 (2007).
- Weyl semimetal in a topological insulator multilayer. Phys. Rev. Lett. 107, 127205 (2011). &
- Chern semi-metal and quantized anomalous Hall effect in HgCr2Se4. Phys. Rev. Lett. 107, 186806 (2011). et al.
- Topological electronic structure and Weyl semimetal in the TlBiSe2 class of semiconductors. Phys. Rev. B 86, 115208 (2012). et al.
- Prediction of a Weyl semimetal in Hg1−x−yCdxMnyTe. Phys. Rev. B 89, 081106(R) (2014). , &
- Weyl semimetals from noncentrosymmetric topological insulators. Phys. Rev. B 90, 155316 (2014). &
- Topological phase transition and texture inversion in a tunable topological insulator. Science 332, 560–564 (2011). et al.
- The Adler–Bell–Jackiw anomaly and Weyl fermions in a crystal. Phys. Lett. B 130, 389–396 (1983). &
- Topological response in Weyl semimetals and the chiral anomaly. Phys. Rev. B 86, 115133 (2012). &
- Excitonic phases from Weyl semimetals. Phys. Rev. Lett. 109, 196403 (2012). , &
- Probing the chiral anomaly with nonlocal transport in three-dimensional topological semimetals. Phys. Rev. X 4, 031035 (2014). , , , &
- Magneto-optical conductivity of Weyl semimetals. Phys. Rev. B 87, 245131 (2013). &
- Electromagnetic response of Weyl semimetals. Phys. Rev. Lett. 111, 027201 (2013). &
- Helical Fermi arcs and surface states in time-reversal invariant Weyl semimetals. Phys. Rev. B 87, 245112 (2013).
- Friedel oscillations due to Fermi arcs in Weyl semimetals. Phys. Rev. B 86, 195102 (2012).
- Quantum oscillations from surface Fermi-arcs in Weyl and Dirac semi-metals. Nature Commun. 5, 5161 (2014). , &
- An inversion breaking Weyl semimetal state in the TaAs material class. Nature Commun. 6, 7373 (2015). et al.
- Weyl semimetal phase in non-centrosymmetric transition metal monophosphides. Phys. Rev. X 5, 011029 (2015). , , , &
- Discovery of a Weyl Fermion semimetal and topological Fermi arcs. Science doi:10.1126/science.aaa9297 (2015) et al.
- http://arXiv.org/abs/1502.04684 (2015). et al. Discovery of Weyl semimetal TaAs. Preprint at
- The crystal structure of NbAs. Acta Crystallogr. 17, 1077–1078 (1964). &
- The transposition structure of NbAs and of similar monophosphides and arsenides of niobium and tantalum. Acta Crystallogr. 16, 1095–1101 (1963). &
- Soft-X-ray ARPES facility at the ADRESS beamline of the SLS: Concepts, technical realisation and scientific applications. J. Synchrotron Radiat. 21, 32–44 (2014). et al.
- Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865–3868 (1996). , &
- Revisiting magnetic coupling in transition-metal-benzene complexes with maximally localized Wannier functions. Phys. Rev. B 79, 235118 (2009). , &