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.

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Photoemission, from core levels and valence band, and low-energy electron diffraction (LEED) have been employed to investigate the electronic and structural properties of novel graphene-ferromagnetic (G-FM) systems,\ud obtained by intercalation of one mono-layer (1ML) and several layers (4ML) of Co on G grown on Ir(111).\ud Upon intercalation of 1ML of Co, the Co lattice is resized to match the Ir-Ir lattice parameter, resulting in a\ud mismatched G/Co/Ir(111) system. The intercalation of further Co layers leads to a relaxation of the Co lattice\ud and a progressive formation of a commensurate G layer lying on top. We show the C 1s line shape and the band\ud structure of G in the two artificial phases, mismatched and commensurate G/Co, through a comparison with the\ud electronic structure of G grown directly on a Co thick film. Our results show that while the G valence band\ud mainly reflects the hybridization with the d states of Co, regardless of the structural phase, the C 1s line shape\ud is very sensitive to the rumpling of the G layer and the coordination of carbon atoms with the underlying Co.\ud Even in the commensurate (1x1) G/Co phase, where graphene is in register with the Co film, from the angular\ud dependence of the C 1s core level we infer the presence of a double component, due to in-equivalent adsorption\ud sites of carbon sub-lattices

Angle-resolved photoemission spectroscopy (ARPES) and ab initio band structure calculations have been used to study the detailed valence band structure of molybdenite, MoS(2) and MoSe(2). The experimental band structure obtained from ARPES has been found to be in good agreement with the theoretical calculations performed using the linear augmented plane wave (LAPW) method. In going from MoS(2) to MoSe(2), the dispersion of the valence bands decreases along both k(parallel) and k(perpendicular), revealing the increased two-dimensional character which is attributed to the increasing interlayer distance or c/a ratio in these compounds. The width of the valence band and the band gap are also found to decrease, whereas the valence band maxima shift towards the higher binding energy from MoS(2) to MoSe(2).

VS 2 is a challenging material to prepare stoichiometrically in the bulk, and the single layer has not been successfully isolated before now. Here we report the first realization of single-layer VS 2 , which we have prepared epitaxially with high quality on Au(111) in the octahedral (1T) structure. We find that we can deplete the VS 2 lattice of S by annealing in vacuum so as to create an entirely new twodimensional compound that has no bulk analogue. The transition is reversible upon annealing in an H 2 S gas atmosphere. We report the structural properties of both the stoichiometric and S-depleted compounds on the basis of low-energy electron diffraction, X-ray photoelectron spectroscopy and diffraction, and scanning tunneling microscopy experiments.