Topological nodal line semimetals, a novel quantum state of materials, possess topologically nontrivial valence and conduction bands that touch at a line near the Fermi level. The exotic band structure can lead to various novel properties, such as long-range Coulomb interaction and flat Landau levels. Recently, topological nodal lines have been observed in several bulk materials, such as PtSn4, ZrSiS, TlTaSe2 and PbTaSe2. However, in two-dimensional materials, experimental research on nodal line fermions is still lacking. Here, we report the discovery of two-dimensional Dirac nodal line fermions in monolayer Cu2Si based on combined theoretical calculations and angle-resolved photoemission spectroscopy measurements. The Dirac nodal lines in Cu2Si form two concentric loops centred around the Γ point and are protected by mirror reflection symmetry. Our results establish Cu2Si as a platform to study the novel physical properties in two-dimensional Dirac materials and provide opportunities to realize high-speed low-dissipation devices.

The physical property investigation (like transport measurements) and ultimate application of the topological insulators usually involve surfaces that are exposed to ambient environment (1 atm and room temperature). One critical issue is how the topological surface state will behave under such ambient conditions. We report high resolution angle-resolved photoemission measurements to directly probe the surface state of the prototypical topological insulators, Bi 2 Se 3 and Bi 2 Te 3 , upon exposing to various environments. We find that the topological order is robust even when the surface is exposed to air at room temperature. However, the surface state is strongly modified after such an exposure. Particularly, we have observed the formation of two-dimensional quantum well states near the exposed surface of the topological insulators. These findings provide key information in understanding the surface properties of the topological insulators under ambient environment and in engineering the topological surface state for applications.T he topological insulators represent a novel state of matter where the bulk is insulating but the surface is metallic, which is expected to be robust due to topological protection (1-5). The topological surface state exhibits unique electronic structure and spin texture that provide a venue not only to explore novel quantum phenomena in fundamental physics (6-10) but also to show potential applications in spintronics and quantum computing (2,5,11). The angle-resolved photoemission spectroscopy (ARPES) is a powerful experimental tool to directly identify and characterize topological insulators (12). A number of three-dimensional topological insulators have been theoretically predicted and experimentally identified by ARPES (13-21); some of their peculiar properties have been revealed by scanning tunneling microscopy (STM) (22-26). The application of the topological surface states depends on the surface engineering that can be manipulated by incorporation of nonmagnetic (27-31) or magnetic (27, 28, 31-33) impurities or gas adsorptions (27,(33)(34)(35). While the ARPES and STM measurements usually involve the fresh surface obtained by cleaving samples in situ under ultrahigh vacuum, for the transport and optical techniques, which are widely used to investigate the intrinsic quantum behaviors of the topological surface state (36-40), and particularly the ultimate applications of the topological insulators, the surface is usually exposed to ambient conditions (1 atm air and room temperature) or some gas protection environment. It is therefore crucial to investigate whether the topological order can survive under the ambient conditions and, furthermore, whether and how the surface state may be modified after such exposures. Results and DiscussionWe start by first looking at the electronic structure of the prototypical topological insulators Bi 2 ðSe;TeÞ 3 under ultrahigh vacuum. The Fermi surface and the band structure of the Bi 2 ðSe 3−x Te x Þ topological insulators depend sensitively on ...