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# Observation of unconventional chiral fermions with long Fermi arcs in CoSi

## Abstract

Chirality—the geometric property of objects that do not coincide with their mirror image—is found in nature, for example, in molecules, crystals, galaxies and life forms. In quantum field theory, the chirality of a massless particle is defined by whether the directions of its spin and motion are parallel or antiparallel. Although massless chiral fermions—Weyl fermions—were predicted 90 years ago, their existence as fundamental particles has not been experimentally confirmed. However, their analogues have been observed as quasiparticles in condensed matter systems. In addition to Weyl fermions1,2,3,4, theorists have proposed a number of unconventional (that is, beyond the standard model) chiral fermions in condensed matter systems5,6,7,8, but direct experimental evidence of their existence is still lacking. Here, by using angle-resolved photoemission spectroscopy, we reveal two types of unconventional chiral fermion—spin-1 and charge-2 fermions—at the band-crossing points near the Fermi level in CoSi. The projections of these chiral fermions on the (001) surface are connected by giant Fermi arcs traversing the entire surface Brillouin zone. These chiral fermions are enforced at the centre or corner of the bulk Brillouin zone by the crystal symmetries, making CoSi a system with only one pair of chiral nodes with large separation in momentum space and extremely long surface Fermi arcs, in sharp contrast to Weyl semimetals, which have multiple pairs of Weyl nodes with small separation. Our results confirm the existence of unconventional chiral fermions and provide a platform for exploring the physical properties associated with chiral fermions.

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## Acknowledgements

We thank Y. Zhong and J. Guan for assistance with the RHEED measurements. We thank H. Yao, H. Lu and Z. Wang for discussions. We thank N. B. M. Schröter, A. Chikina and V. N. Strocov for assistance with the ARPES measurements at the SLS. This work was supported by the Ministry of Science and Technology of China (2016YFA0401000, 2015CB921000, 2016YFA0300600, 2016YFA0300504, 2016YFA0302400, 2018YFA0305700 and 2017YFA0302901), the National Natural Science Foundation of China (11622435, U1832202, 11474340, 11822412, 11574371, 11674369, 11574394, 11774423 and 11774399), the Chinese Academy of Sciences (QYZDB-SSW-SLH043, XDB07000000 and XDB28000000), the Science Challenge Project (TZ2016004), the K. C. Wong Education Foundation (GJTD-2018-01), the Beijing Natural Science Foundation (Z180008) and the Beijing Municipal Science and Technology Commission (Z171100002017018, Z181100004218005 and Z181100004218001). Y.H. acknowledges support by the CAS Pioneer Hundred Talents Program (type C). Z. Li acknowledges support by the National Postdoctoral Program for Innovative Talents (BX20170012).

## Author information

T.Q. and Y. Sun supervised the project. Z.R., H. Li and T.Q. performed the ARPES measurements with the assistance of B.F., W.F., J.L. and Y.H.; Z.R., C.T. and Y. Sun processed the sample surfaces with the assistance of Z. Liu and Y.L.; T.Z. and H.W. performed ab initio calculations; S.T., C.L., H. Lei, L.W., Y. Shi and Z. Li synthesized the single crystals; Z.R, H. Li, T.Q. and Y. Sun analysed the experimental data; Z.R., T.Z., T.Q. and Y. Sun plotted the figures; T.Q., C.F. and H.D. wrote the manuscript.

### Competing interests

The authors declare no competing interests.

Correspondence to Hechang Lei or Yujie Sun or Tian Qian.

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