M. Papagno

Near-edge x-ray absorption fine-structure investigation of graphene

We report the near-edge x-ray absorption fine-structure (NEXAFS) spectrum of a single layer of graphite (graphene) obtained by micromechanical cleavage of highly ordered pyrolytic graphite on a SiO2 substrate. We utilized a photoemission electron microscope to separately study single-, double-, and few-layers graphene samples. In single-layer graphene we observe a splitting of the pi resonance and a clear signature of the predicted interlayer state. The NEXAFS data illustrate the rapid evolution of the electronic structure with the increased number of layers.

Large band gap opening between graphene Dirac cones induced by Na adsorption onto an Ir superlattice.

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 ~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 band gap of Δ(Na/Ir/G) = 740 meV, comparable to the one of Ge and with preserved high group velocity of the charge carriers.

Artificially lattice-mismatched graphene/metal interface: Graphene/Ni/Ir(111)

We report the structural and electronic properties of an artificial graphene/Ni(111) system obtained by the intercalation of a monoatomic layer of Ni in graphene/Ir(111). Upon intercalation, Ni grows epitaxially on Ir(111), resulting in a lattice mismatched graphene/Ni system. By performing Scanning Tunneling Microscopy (STM) measurements and Density Functional Theory (DFT) calculations, we show that the intercalated Ni layer leads to a pronounced buckling of the graphene film. At the same time an enhanced interaction is measured by Angle-Resolved Photo-Emission Spectroscopy (ARPES), showing a clear transition from a nearly-undisturbed to a strongly-hybridized graphene -band. A comparison of the intercalation-like graphene system with flat graphene on bulk Ni(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.

Hybridization of graphene and a Ag monolayer supported on Re(0001)

We have investigated the electronic structure of graphene supported on Re(0001) before and after the intercalation of one monolayer of Ag by means of angle-resolved photoemission spectroscopy measurements and density functional theory calculations. The intercalation of Ag reduces the graphene-Re interaction and modifies the electronic band structure of graphene. Although the linear dispersion of the {\pi} state of graphene in proximity of the Fermi level highlights a rather weak graphene-noble metal layer interaction, we still observe a significant hybridization between the Ag bands and the {\pi} state in lower energy regions. These results demonstrate that covering a surface with a noble metal layer does decouple the electronic states, but still leads to a noticeable change in the electronic structure of graphene.

Complex Magnetic Exchange Coupling between Co Nanostructures and Ni(111) across Epitaxial Graphene.

We report on the magnetic coupling between isolated Co atoms as well as small Co islands and Ni(111) mediated by an epitaxial graphene layer. X-ray magnetic circular dichroism and scanning tunneling microscopy combined with density functional theory calculations reveal that Co atoms occupy two distinct adsorption sites, with different magnetic coupling to the underlying Ni(111) surface. We further report a transition from an antiferromagnetic to a ferromagnetic coupling with increasing Co cluster size. Our results highlight the extreme sensitivity of the exchange interaction mediated by graphene to the adsorption site and to the in-plane coordination of the magnetic atoms.

Two Distinct Phases of Bilayer Graphene Films on Ru(0001)

By combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy we reveal the structural and electronic properties of multilayer graphene on Ru(0001). We prove that large ethylene exposure allows the synthesis of two distinct phases of bilayer graphene with different properties. The first phase has Bernal AB stacking with respect to the first graphene layer and displays weak vertical interaction and electron doping. The long-range ordered moiré pattern modulates the crystal potential and induces replicas of the Dirac cone and minigaps. The second phase has an AA stacking sequence with respect to the first layer and displays weak structural and electronic modulation and p-doping. The linearly dispersing Dirac state reveals the nearly freestanding character of this novel second-layer phase.

Multiple coexisting Dirac surface states in three-dimensional topological insulator PbBi6Te10

By means of angle-resolved photoemission spectroscopy (ARPES) measurements, we unveil the electronic band structure of three-dimensional PbBi6Te10 topological insulator. ARPES investigations evidence multiple coexisting Dirac surface states at the zone-center of the reciprocal space, displaying distinct electronic band dispersion, different constant energy contours, and Dirac point energies. We also provide evidence of Rashba-like split states close to the Fermi level, and deeper M- and V-shaped bands coexisting with the topological surface states. The experimental findings are in agreement with scanning tunneling microscopy measurements revealing different surface terminations according to the crystal structure of PbBi6Te10. Our experimental results are supported by density functional theory calculations predicting multiple topological surface states according to different surface cleavage planes.

Optimizing long-range order, band gap, and group velocities for graphene on close-packed metal surfaces

We compare different growth methods with the aim of optimizing the long-range order of a graphene layer grown on Ru(0001). Combining chemical vapor deposition with carbon loading and segregation of the surface layer leads to autocorrelation lengths of 240 Å. We present several routes to band gap and charge carrier mobility engineering for the example of graphene on Ir(111). Ir cluster superlattices self-assembled onto the graphene moiré pattern produce a strong renormalization of the electron group velocity close to the Dirac point, leading to highly anisotropic Dirac cones and the enlargement of the gap from 140 to 340 meV. This gap can further be enhanced to 740 meV by Na co-adsorption onto the Ir cluster superlattice at room temperature. This value is close to that of Ge, and the high group velocity of the charge carriers is fully preserved. We also present data for Na adsorbed without the Ir clusters. In both cases we find that the Na is on top of the graphene layer.

Photoemission as a probe of coexisting and conflicting periodicities in low-dimensional solids

When two different periodic potentials are present at the same time in a solid, the electron wavefunctions must conform to the resulting overall periodicity. It is the case of the broken-symmetry phases which are often observed in low-dimensional systems. The rearrangement of the electronic states has some interesting and perhaps unexpected consequences on the momentum distribution of the spectral weight, which can be measured in an ARPES experiment.

Massless Dirac cones in graphene: Experiments and theory

The opening of a gap in single-layer graphene is often ascribed to the breaking of the equivalence between the two carbon sublattices. It is shown by angle-resolved photoemission spectroscopy that Ir- and Na-modified graphene grown on the Ir(111) surface presents a very large unconventional gap that can be described in terms of a phenomenological massless Dirac model. The consequences and differences of this model are discussed in comparison of the standard massive gap model, and the conditions under which such anomalous gap can arise from a spontaneous symmetry breaking are investigated.