International audienceThe intercalation ofEu underneathGr on Ir(111) is comprehensively investigated by microscopic, magnetic, and spectroscopic measurements, as well as by density functional theory. Depending on the coverage, the intercalated Eu atoms form either a (2x2) or a (√3x√3)R30° superstructure with respect to Gr. We investigate the mechanisms of Eu penetration through a nominally closed Gr sheet and measure the electronic structures and magnetic properties of the two intercalation systems. Their electronic structures are rather similar. Compared to Gr on Ir(111), the Gr bands in both systems are essentially rigidly shifted to larger binding energies resulting in n doping. The hybridization of the Ir surface state S1 with Gr states is lifted, and the moir´e super periodic potential is strongly reduced. In contrast, the magnetic behavior of the two intercalation systems differs substantially, as found by x-ray magnetic circular dichroism. The (2x2) Eu structure displays plain paramagnetic behavior, whereas for the (√3x√3)R30° structure the large zero-field susceptibility indicates ferromagnetic coupling, despite the absence of hysteresis at 10 K. For the latter structure, a considerable easy-plane magnetic anisotropy is observed and interpreted as shape anisotropy

We use scanning tunneling microscopy to visualize and thermal desorption spectroscopy to quantitatively measure that the binding of naphthalene molecules to graphene (Gr), a case of pure van der Waals (vdW) interaction, strengthens with n-and weakens with p-doping of Gr. Density functional theory calculations that include the vdW interaction in a seamless, ab initio way accurately reproduce the observed trend in binding energies. Based on a model calculation, we propose that the vdW interaction is modified by changing the spatial extent of Gr's π orbitals via doping.One of the key properties of graphene (Gr) is the wide-range tunability of its Fermi level and corresponding charge carrier concentration, either by a gate electrode [1], substitutional doping [2], adsorption [3, 4], or charge transfer from a supporting material or intercalation layer [5][6][7][8]. The tunability of the Fermi level through the otherwise rigid band structure results from the material being atomically thin and having a negligible density of states near the Dirac point.In recent years, interest has arisen in using this tunability to control adsorption: For the case of ionic adsorbates, Brar et al. [9] demonstrated a dependence of the ionization state of a Co adatom on the Gr Fermi level position, and Schumacher et al.[10] found a doping-dependent binding energy E b of ionic adsorbates to Gr, with a shift in E b on the order of the shift in the Fermi level induced by doping. For the case of radicals and based on ab initio calculations, Wehling et al.[11] predict dopingdependent adsorbate phase transitions for hydrogenated as well as fluorinated Gr, while Huang et al. [12] find a stronger binding of isolated H radicals for larger magnitudes of doping.For the case of van der Waals (vdW) interaction, the effect of the Gr doping level on the binding energy of adsorbates has not yet been explored. This is surprising, given that the adsorption of simple hydrocarbons to graphite or Gr has been used as a model system to study vdW interactions [13][14][15][16]. Here, we investigate this case with the help of epitaxial Gr on Ir(111), which can be doped from the backside by intercalation of highly electropositive (e. g. Cs, Eu) or -negative (e.g. O) elements into its interface with the substrate, while Gr's other side remains available for the adsorption experiment itself. This strategy not only enables us to achieve large Fermi level shifts on the order of ±1 eV, but also to visualize doping-induced binding energy differences by making using of intercalation patterns [10]. Naphthalene is chosen as a test molecule, since its binding to Gr is a pure vdW case studied previously, both experimentally [14] and theoretically [13]. Our experiments are complemented by density functional theory (DFT) calculations that include the vdW interaction in a seamless, ab initio way (for a recent review, see Ref.[17]).For this paradigmatic case we find in excellent agreement of experiment and theory an increase of the vdW binding energy when changing from p-to n-d...

In the standard model of charge density wave (CDW) transitions, the displacement along a single phonon mode lowers the total electronic energy by creating a gap at the Fermi level, making the CDW a metal–insulator transition. Here, using scanning tunneling microscopy and spectroscopy and ab initio calculations, we show that VS2 realizes a CDW which stands out of this standard model. There is a full CDW gap residing in the unoccupied states of monolayer VS2. At the Fermi level, the CDW induces a topological metal-metal (Lifshitz) transition. Non-linear coupling of transverse and longitudinal phonons is essential for the formation of the CDW and the full gap above the Fermi level. Additionally, x-ray magnetic circular dichroism reveals the absence of net magnetization in this phase, pointing to coexisting charge and spin density waves in the ground state.

We demonstrate a new synthesis route for the growth of organometallic sandwich molecular nanowires, taking the example of Eu-cyclooctatetraene (EuCot), a predicted ferromagnetic semiconductor. We employ simultaneous exposure of Cot molecules and Eu vapor in ultrahigh vacuum to an inert substrate, such as graphene. Using a Cot excess under temperature conditions of a finite residence time of the molecule, the reactand diffusion confined to two dimensions results in a clean product of ultralong wires. In situ scanning tunneling microscopy reveals not only their molecular structure but also a rich and intriguing growth morphology. The new on-surface synthesis permits experimental access to a largely unexplored class of one-dimensional organometallic systems with potential for exciting electronic and magnetic properties.

Using scanning tunneling microscopy, the oxygen adsorbate superstructures on bare Ir(111) are identified and compared to the ones formed by intercalation in between graphene and the Ir(111) substrate. For bare Ir(111) we observe O-(2 × 2) and O-(2 × 1) structures, thereby clarifying a persistent uncertainty about the existence of these structures and the role of defects for their stability. For the case of graphene-covered Ir(111), oxygen intercalation superstructures can be imaged through the graphene monolayer by choosing proper tunneling conditions. Depending on the pressure, temperature and duration of O2 exposure as well as on the graphene morphology, O-(2 × 2), O-(√3×√3)-R30°, O-(2 × 1) and O-(2√3 × 2√3)-R30° superstructures with respect to Ir(111) are observed under the graphene cover. Two of these structures, the O-(√3 × √3)-R30° and the (2√3 × 2√3)-R30° structure are only observed when the graphene layer is on top. Phase coexistence and formation conditions of the intercalation structures between graphene and Ir(111) are analyzed. The experimental results are compared to density functional theory calculations including dispersive forces. The existence of these phases under graphene and their absence on bare Ir(111) are discussed in terms of possible changes in the adsorbate-substrate interaction due to the presence of the graphene cover.