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 interfacial charge transfer from the substrate may influence the electronic structure of the epitaxial van der Waals (vdW) monolayers and thus their further technological applications. For instance, the freestanding Sb monolayer in puckered honeycomb phase (α-antimonene), the structural analog of black phosphorene, was predicted to be a semiconductor, but the epitaxial one behaves as a gapless semimetal when grown on the T d -WTe 2 substrate. Here, we demonstrate that interface engineering can be applied to tune the interfacial charge transfer and thus the electron band of epitaxial monolayer. As a result, the nearly freestanding (semiconducting) α-antimonene monolayer with a band gap of ~170 meV was successfully obtained on the SnSe substrate. Furthermore, a semiconductor-semimetal crossover is observed in the bilayer α-antimonene. This study paves the way towards modifying the electron structure in twodimensional vdW materials through interface engineering.

Puckered honeycomb Sb monolayer, the structural analog of black phosphorene, has been recently successfully grown by means of molecular beam epitaxy. However, little is known to date about the growth mechanism for such puckered honeycomb monolayer. In this study, by using scanning tunneling microscopy in combination with firstprinciples density functional theory calculations, we unveil that the puckered honeycomb Sb monolayer takes a kinetics-limited two-step growth mode. As the coverage of Sb increases, the Sb atoms firstly form the distorted hexagonal lattice as the half layer, and then the distorted hexagonal half-layer transforms into the puckered honeycomb lattice as the full layer. These results provide the atomic-scale insight in understanding the growth mechanism of puckered honeycomb monolayer, and can be instructive to the direct growth of other monolayers with the same structure.