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# Three-component fermions with surface Fermi arcs in tungsten carbide

## Abstract

Topological Dirac and Weyl semimetals not only host quasiparticles analogous to the elementary fermionic particles in high-energy physics, but also have a non-trivial band topology manifested by gapless surface states, which induce exotic surface Fermi arcs1,2. Recent advances suggest new types of topological semimetal, in which spatial symmetries protect gapless electronic excitations without high-energy analogues3,4,5,6,7,8,9,10,11. Here, using angle-resolved photoemission spectroscopy, we observe triply degenerate nodal points near the Fermi level of tungsten carbide with space group $$P\bar{6}m2$$ (no. 187), in which the low-energy quasiparticles are described as three-component fermions distinct from Dirac and Weyl fermions. We further observe topological surface states, whose constant-energy contours constitute pairs of ‘Fermi arcs’ connecting to the surface projections of the triply degenerate nodal points, proving the non-trivial topology of the newly identified semimetal state.

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## Acknowl1edgements

We acknowledge G. Li for valuable discussion. This work was supported by the Ministry of Science and Technology of China (2016YFA0300600, 2016YFA0401000, 2016YFA0302400, 2015CB921300, 2013CB921700, 2016YFA0202301 and 2016YFA0300300), the National Natural Science Foundation of China (11622435, 11474340, 11422428, 11674369, 11234014, 11404175, 61725107 and 11674371) and the Chinese Academy of Sciences (XDB07000000 and XDB06). Y.-B.H. acknowledges funding from the CAS Pioneer Hundred Talents Program (type C). A portion of this work performed at the National High Magnetic Field Laboratory, Tallahassee, USA, is supported by the National Science Foundation Cooperative Agreement DMR-1157490 and the State of Florida.

## Author information

Authors

### Contributions

H.D. and T.Q. conceived the ARPES experiments; J.-Z.M. and T.Q. performed ARPES measurements with the assistance of B.-Q.L., L.-Y.K., X.G., L.-Y.R. and Y.-B.H.; Y.-F.X. and H.-M.W. performed ab initio calculations; J.-B.H., D.C., W.-L.Z. and G.-F.C. synthesized the single crystals; S.Z., D.C., C.-Y.X., E.S.C. and G.-F.C. performed quantum oscillation measurements; J.-Z.M., T.Q. and H.D. analysed the experimental data; J.-Z.M., Y.-F.X., T.Q. and J.-B.H. plotted the figures; X.D. discussed the experimental and calculated data; Y.S., J.-Z.M., Y.-L.W. and H.-J.G. performed STM experiments. T.Q., C.F., H.-M.W., H.D., J.-Z.M. and P.R. wrote the manuscript.

### Corresponding authors

Correspondence to G.-F. Chen, T. Qian or H. Ding.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

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## Supplementary information

### Supplementary Information

Supplementary Information, Supplementary Figures 1–3, Supplementary Table, References

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Reprints and Permissions

Ma, JZ., He, JB., Xu, YF. et al. Three-component fermions with surface Fermi arcs in tungsten carbide. Nature Phys 14, 349–354 (2018). https://doi.org/10.1038/s41567-017-0021-8

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