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In magnetic topological insulators (TIs), the interplay between magnetic order and nontrivial topology can induce fascinating topological quantum phenomena, such as the quantum anomalous Hall effect, chiral Majorana fermions and axion electrodynamics. Recently, a great deal of attention has been focused on the intrinsic magnetic TIs, where disorder effects can be eliminated to a large extent, which is expected to facilitate the emergence of topological quantum phenomena. In despite of intensive efforts, the experimental evidence of topological surface states (SSs) remains elusive. Here, by combining first-principles calculations and angle-resolved photoemission spectroscopy (ARPES) experiments, we have revealed that EnSn2As2 is an antiferromagnetic TI with observation of Dirac SSs consistent with our prediction.We also observed gapless Dirac SSs in another antiferromagnetic TI MnBi2Te4, which were missed in previous ARPES study. These results provide clear evidence for nontrivial topology of the two intrinsic magnetic TIs. Moreover, the topological SSs show no observable changes across the magnetic transitions within the experimental resolution, indicating that the magnetic order has limited effect on the topological SSs, which can be attributed to weak hybridization between the magnetic states and the topological electronic states. This provides insights for further studies that the correlations between magnetism and topological states need to be strengthened to induce larger gaps of the topological SSs, which will facilitate the realization of topological quantum phenomena at higher temperatures.

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 ...