Observation of unconventional chiral fermions with long Fermi arcs in CoSi

Zhicheng Rao, Hang Li, Tiantian Zhang, Shangjie Tian, Chenghe Li, Binbin Fu, Cenyao Tang, Le Wang, Zhilin Li, Wenhui Fan, Jiajun Li, Yaobo Huang, Zhehong Liu, Youwen Long, Chen Fang, Hongming Weng, Youguo Shi, Hechang Lei (), Yujie Sun (), Tian Qian () and Hong Ding
Additional contact information
Zhicheng Rao: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Hang Li: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Tiantian Zhang: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Shangjie Tian: Renmin University of China
Chenghe Li: Renmin University of China
Binbin Fu: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Cenyao Tang: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Le Wang: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Zhilin Li: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Wenhui Fan: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Jiajun Li: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Yaobo Huang: Chinese Academy of Sciences
Zhehong Liu: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Youwen Long: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Chen Fang: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Hongming Weng: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Youguo Shi: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Hechang Lei: Renmin University of China
Yujie Sun: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Tian Qian: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences
Hong Ding: Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences

Nature, 2019, vol. 567, issue 7749, 496-499

Abstract: 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 fermions1–4, theorists have proposed a number of unconventional (that is, beyond the standard model) chiral fermions in condensed matter systems5–8, 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.

Date: 2019
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DOI: 10.1038/s41586-019-1031-8

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