We report on the electronic properties of straight, 1.6 nm wide, silicene nanoribbons on Ag(110), arranged in a one-dimensional grating with a pitch of 2 nm, whose high-resolution scanning tunneling microscopy images reveal a honeycomb geometry. Angle-resolved photoemission shows quantum confined electronic states of one-dimensional character. The silicon band dispersion along the direction of the nanoribbons suggests a behavior analogous to the Dirac cones of graphene on different substrates.

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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.

Silicene, the silicon equivalent of graphene, is attracting increasing scientific and technological attention in view of the exploitation of its exotic electronic properties. This novel material has been theoretically predicted to exist as a free-standing layer in a low-buckled, stable form, and can be synthesized by the deposition of Si on appropriate crystalline substrates. By employing low-energy electron diffraction and microscopy, we have studied the growth of Si on Ag(1 1 1) and observed a rich variety of rotationally non-equivalent silicene structures. Our results highlight a very complex formation diagram, reflecting the coexistence of different and nearly degenerate silicene phases, whose relative abundance can be controlled by varying the Si coverage and growth temperature. At variance with other studies, we find that the formation of single-phase silicene monolayers cannot be achieved on Ag(1 1 1).

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We present a new method to engineer the charge carrier mobility and its directional asymmetry in epitaxial graphene by using metal cluster superlattices self-assembled onto the moiré pattern formed by graphene on Ir(111). Angle-resolved photoemission spectroscopy reveals threefold symmetry in the band structure associated with strong renormalization of the electron group velocity close to the Dirac point giving rise to highly anisotropic Dirac cones. We further find that the cluster superlattice also affects the spectral-weight distribution of the carbon bands as well as the electronic gaps between graphene states. DOI: 10.1103/PhysRevLett.105.246803 PACS numbers: 73.20.Àr, 73.21.Cd, 73.22.Pr, 79.60.Ài Graphitic materials have attracted strong scientific interest because they exhibit exotic phenomena such as superconductivity or the anomalous quantum Hall effect [1]. Graphene (gr) is the building block of these materials; it is wrapped up into fullerenes, rolled up into carbon nanotubes, or stacked into 3D graphite. It presents a model system to investigate the influence of many-body interactions on the electron dynamics in these materials. In addition, the exceptional electronic mobility makes graphene a candidate material for next generation electronic devices [2]. Freestanding graphene is a zero-gap semiconductor. Because most electronic applications require a gap between valence and conduction bands, considerable effort has been spent to induce and control the opening of such a band gap [3][4][5][6].A related, and for applications equally relevant, issue is the ability to tailor the band dispersion. The speed with which information can be transmitted along a graphene layer depends on the charge carrier group velocity. Close to the K points, the bands of freestanding graphene have a linear dispersion well described by the relativistic Dirac equation for massless neutrinos. The resulting Dirac cones are trigonally warped due to the chiral nature of graphene charge carriers in the equivalent A and B carbon sublattices [7]. The ability to increase and tailor this anisotropy would open a manifold of new applications. Several theoretical studies suggest that this goal can be reached by applying an external periodic potential with nanometer period giving rise to highly anisotropic Dirac cones [8][9][10][11].Epitaxial graphene layers grown on lattice mismatched close-packed metal substrates, such as Pt (111) Here we demonstrate with angle-resolved photoemission spectroscopy (ARPES) that an Ir cluster superlattice grown on the gr=Irð111Þ-(9:32 AE 0:15 Â 9:32 AE 0:15) moiré structure [13,15] gives rise to significant group velocity and Dirac cone asymmetries. We attribute this to a much stronger periodic potential caused by the cluster superlattice than by the moiré itself. H decorated gr=Irð111Þ has been reported to give rise to band gap opening but not to the Dirac cone asymmetries reported here [6]. We thereby confirm theoretical predictions and present a new way to tailor the directionality of carrier mobilit...

We investigate the effects of Na adsorption on the electronic structure of bare and Ir cluster superlattice covered epitaxial graphene on Ir(111) using angle-resolved photoemission spectroscopy and scanning tunneling microscopy. At Na saturation coverage a massive charge migration from sodium atoms to graphene raises the graphene Fermi level by about 1.4 eV relative to its neutrality point. We find that Na is adsorbed on top of the graphene layer and when coadsorbed onto an Ir cluster superlattice it results in the opening of a large bandgap of Δ Na/Ir/G = 740 meV comparable to the one of Ge and with preserved high group velocity of the charge carriers.

We report the structural and electronic properties of an artificial graphene/Ni (111) (111), and mildly corrugated graphene on Ir(111), allows to disentangle the two key properties which lead to the observed increased interaction, namely lattice matching and electronic interaction. Although the latter determines the strength of the hybridization, we find an important influence of the local carbon configuration resulting from the lattice mismatch.