PT - JOURNAL ARTICLE AU - Yuan, Qian-Qian AU - Zhou, Liqin AU - Rao, Zhi-Cheng AU - Tian, Shangjie AU - Zhao, Wei-Min AU - Xue, Cheng-Long AU - Liu, Yixuan AU - Zhang, Tiantian AU - Tang, Cen-Yao AU - Shi, Zhi-Qiang AU - Jia, Zhen-Yu AU - Weng, Hongming AU - Ding, Hong AU - Sun, Yu-Jie AU - Lei, Hechang AU - Li, Shao-Chun TI - Quasiparticle interference evidence of the topological Fermi arc states in chiral fermionic semimetal CoSi AID - 10.1126/sciadv.aaw9485 DP - 2019 Dec 01 TA - Science Advances PG - eaaw9485 VI - 5 IP - 12 4099 - http://advances.sciencemag.org/content/5/12/eaaw9485.short 4100 - http://advances.sciencemag.org/content/5/12/eaaw9485.full SO - Sci Adv2019 Dec 01; 5 AB - Chiral fermions in solid state feature “Fermi arc” states, connecting the surface projections of the bulk chiral nodes. The surface Fermi arc is a signature of nontrivial bulk topology. Unconventional chiral fermions with an extensive Fermi arc traversing the whole Brillouin zone have been theoretically proposed in CoSi. Here, we use scanning tunneling microscopy/spectroscopy to investigate quasiparticle interference at various terminations of a CoSi single crystal. The observed surface states exhibit chiral fermion–originated characteristics. These reside on (001) and (011) but not (111) surfaces with p-rotation symmetry, spiral with energy, and disperse in a wide energy range from ~−200 to ~+400 mV. Owing to the high-energy and high-space resolution, a spin-orbit coupling–induced splitting of up to ~80 mV is identified. Our observations are corroborated by density functional theory and provide strong evidence that CoSi hosts the unconventional chiral fermions and the extensive Fermi arc states.