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2 Chiral Fermions existed as quasiparticles in solid state feature the surface "Fermi arc" states, which connect the surface projections of the bulk chiral nodes with opposite Chern numbers. The surface Fermi arc is experimentally accessible as one of the most significant signature to manifest the nontrivial bulk topology. Aside from the Weyl nodes as firstly uncovered with Chern number C = ±1, chiral fermions carrying larger Chern number in CoSi family candidates have been theoretically proposed. Distinctly, the bulk chiral nodes in CoSi are enforced at high symmetric momenta in Brillouin zone by nonsymmorphic crystalline symmetry, and thus an extensive Fermi arc traversing the whole Brillouin zone is expected. Herein, we use scanning tunneling microscopy / spectroscopy (STM / STS) to investigate the quasiparticle interference (QPI) at various terminations of CoSi single crystal. The observed surface states exhibit the chiral fermion-originated characteristics. For instance, they are found to reside on (001) and (011) but not (111) surfaces with π-rotation symmetry, to spiral with energy, and to disperse in a wide energy range from ~ -200 mV to ~ +400 mV. Owing to the high energy and space resolution, a spin-orbit coupling induced splitting of up to ~ 80 mV is identified for the first time. Our experimental observations are corroborated by density functional theory (DFT) simulation, and thus provide a strong evidence that CoSi hosts the unconventional chiral fermions and extensive surface Fermi arc states. 3 INTRODUCTIONRecently, great progress has been achieved in condensed matter physics in search of the analog of the elementary particles as described in high-energy physics. The three types of fundamental fermions-Dirac, Weyl and Majorana-have been discovered in solids, in the form of low-energy fermionic excitations near the topologically or symmetrically protected band crossing (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)). Because these Fermionic excitations are constrained by the crystalline symmetry much lower than the Poincare symmetry in high-energy physics, new types of Fermions that have no high-energy counterparts have also been proposed and found in condensed matter materials(18-30), including spin-3/2 Rarita-Schwinger Weyl (RSW) excitations(26, 27), three-fold nexus fermions(22, 24), spin-1 Weyl fermions(28), double Weyl fermions(29) and double Dirac fermions(30) etc. These unconventional chiral fermions may exhibit fantastic physical properties, such as the helical surface states(31, 32), unusual magnetotransport(33-35), and the circular photogalvanic effect(36, 37), etc. The chiral crystalline family of transition metal silicides, including CoSi, RhSi, RhGe, and CoGe, has been recently proposed as ideal candidates to host unconventional chiral Fermion quasiparticles through ab-initio calculations(38-40).They are expected to have numbers of advantages against the previously explored Weyl semimetals. For example, multiple types of topological chiral nodes coexist and locate close to ...

The cuprate superconductors distinguish themselves from the conventional superconductors in that a small variation in the carrier doping can significantly change the superconducting transition temperature ( ), giving rise to a superconducting dome where a pseudogap 1,2 emerges in the underdoped region and a Fermi liquid appears in the overdoped region. Thus a systematic study of the properties over a wide doping range is critical for understanding the superconducting mechanism. Here, we report a new technique to continuously dope the surface of Bi 2 Sr 2 CaCu 2 O 8+x through an ozone/vacuum annealing method. Using in-situ ARPES, we obtain precise quantities of energy gaps and the coherent spectral weight over a wide range of doping. We discover that the d-wave component of the quasiparticle gap is linearly proportional to the Nernst temperature that is the onset of superconducting vortices 3 , strongly suggesting that the emergence of superconducting pairing is concomitant with the onset of free vortices, with direct implications for the onset of superconducting phase coherence at and the nature of the pseudogap phenomena.Bi 2 Sr 2 CaCu 2 O 8+x (Bi2212) single crystals have been extensively studied by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling spectroscopy (STS) 4,5 , two of the major experimental techniques for probing the cuprates. However, high-quality Bi2212 crystals can be only obtained within a narrow doping range. Moreover, surface cleaving, necessary for surface techniques such as ARPES and STS, posses a serious problem for quantitative comparisons from sample to sample due to variation of surface conditions. Realizing that the doping level in this material is solely controlled by the excess oxygen concentration, we use ozone/vacuum annealing to continuously change the doping level of the surface layers, which are subsequently measured by in-situ ARPES (Figs. 1a-c).