Observation of Kekulé vortices around hydrogen adatoms in graphene.

Fractional charges are one of the wonders of the fractional quantum Hall effect. Such objects are also anticipated in two-dimensional hexagonal lattices under time reversal symmetry-emerging as bound states of a rotating bond texture called a Kekulé vortex. However, the physical mechanisms inducing such topological defects remain elusive, preventing experimental realization. Here, we report the observation of Kekulé vortices in the local density of states of graphene under time reversal symmetry. The vortices result from intervalley scattering on chemisorbed hydrogen adatoms. We uncover that their 2π winding is reminiscent of the Berry phase π of the massless Dirac electrons. We can also induce a Kekulé pattern without vortices by creating point scatterers such as divacancies, which break different point symmetries. Our local-probe study thus confirms point defects as versatile building blocks for Kekulé engineering of graphene's electronic structure.

Shaping Graphene Superconductivity with Nanometer Precision.

Graphene holds great potential for superconductivity due to its pure 2D nature, the ability to tune its carrier density through electrostatic gating, and its unique, relativistic-like electronic properties. At present, still far from controlling and understanding graphene superconductivity, mainly because the selective introduction of superconducting properties to graphene is experimentally very challenging. Here, a method is developed that enables shaping at will graphene superconductivity through a precise control of graphene-superconductor junctions. The method combines the proximity effect with scanning tunnelling microscope (STM) manipulation capabilities. Pb nano-islands are first grown that locally induce superconductivity in graphene. Using a STM, Pb nano-islands can be selectively displaced, over different types of graphene surfaces, with nanometre scale precision, in any direction, over distances of hundreds of nanometres. This opens an exciting playground where a large number of predefined graphene-superconductor hybrid structures can be investigated with atomic scale precision. To illustrate the potential, a series of experiments are performed, rationalized by the quasi-classical theory of superconductivity, going from the fundamental understanding of superconductor-graphene-superconductor heterostructures to the construction of superconductor nanocorrals, further used as "portable" experimental probes of local magnetic moments in graphene.

Observation of Yu-Shiba-Rusinov States in Superconducting Graphene.

When magnetic atoms are inserted inside a superconductor, the superconducting order is locally depleted as a result of the antagonistic nature of magnetism and superconductivity. Thereby, distinctive spectral features, known as Yu-Shiba-Rusinov states, appear inside the superconducting gap. The search for Yu-Shiba-Rusinov states in different materials is intense, as they can be used as building blocks to promote Majorana modes suitable for topological quantum computing. Here, the first observation of Yu-Shiba-Rusinov states in graphene, a non-superconducting 2D material, and without the participation of magnetic atoms, is reported. Superconductivity in graphene is induced by proximity effect brought by adsorbing nanometer-scale superconducting Pb islands. Using scanning tunneling microscopy and spectroscopy the superconducting proximity gap is measured in graphene, and Yu-Shiba-Rusinov states are visualized in graphene grain boundaries. The results reveal the very special nature of those Yu-Shiba-Rusinov states, which extends more than 20 nm away from the grain boundaries. These observations provide the long-sought experimental confirmation that graphene grain boundaries host local magnetic moments and constitute the first observation of Yu-Shiba-Rusinov states in a chemically pure system.

Quantum Confinement of Dirac Quasiparticles in Graphene Patterned with Sub-Nanometer Precision.

Quantum confinement of graphene Dirac-like electrons in artificially crafted nanometer structures is a long sought goal that would provide a strategy to selectively tune the electronic properties of graphene, including bandgap opening or quantization of energy levels. However, creating confining structures with nanometer precision in shape, size, and location remains an experimental challenge, both for top-down and bottom-up approaches. Moreover, Klein tunneling, offering an escape route to graphene electrons, limits the efficiency of electrostatic confinement. Here, a scanning tunneling microscope (STM) is used to create graphene nanopatterns, with sub-nanometer precision, by the collective manipulation of a large number of H atoms. Individual graphene nanostructures are built at selected locations, with predetermined orientations and shapes, and with dimensions going all the way from 2 nm up to 1 µm. The method permits the patterns to be erased and rebuilt at will, and it can be implemented on different graphene substrates. STM experiments demonstrate that such graphene nanostructures confine very efficiently graphene Dirac quasiparticles, both in 0D and 1D structures. In graphene quantum dots, perfectly defined energy bandgaps up to 0.8 eV are found that scale as the inverse of the dot's linear dimension, as expected for massless Dirac fermions.

Selective Hydrogen Adsorption in Graphene Rotated Bilayers.

The absorption energy of atomic hydrogen at rotated graphene bilayers is studied using ab initio methods based on the density functional theory including van der Waals interactions. We find that, due to the surface corrugation induced by the underneath rotated layer and the perturbation of the electronic density of states near the Fermi energy, the atoms with an almost AA stacking are the preferential ones for hydrogen chemisorption. The adsorption energy difference between different atoms can be as large as 80 meV. In addition, we find that, due to the logarithmic van Hove singularities in the electronic density of states at energies close to the Dirac point, the adsorption energy of either electron or hole doped samples is substantially increased. We also find that the adsorption energy increases with the decrease of the rotated angle between the layers. Finally, the large zero point energy of the C-H bond (∼0.3 eV) suggests adsorption and desorption of atomic hydrogen and deuterium should behave differently.

Graphene Tunable Transparency to Tunneling Electrons: A Direct Tool To Measure the Local Coupling.

The local interaction between graphene and a host substrate strongly determines the actual properties of the graphene layer. Here we show that scanning tunneling microscopy (STM) can selectively help to visualize either the graphene layer or the substrate underneath, or even both at the same time, providing a comprehensive picture of this coupling with atomic precision and high energy resolution. We demonstrate this for graphene on Cu(111). Our spectroscopic data show that, in the vicinity of the Fermi level, graphene π bands are well preserved presenting a small n-doping induced by Cu(111) surface state electrons. Such results are corroborated by Angle-Resolved Photoemission Spectra (ARPES) and Density Functional Theory with van der Waals (DFT + vdW) calculations. Graphene tunable transparency also allows the investigation of the interaction between the substrate and foreign species (such as atomic H or C vacancies) on the graphene layer. Our calculations explain graphene tunable transparency in terms of the rather different decay lengths of the graphene Dirac π states and the metal surface state, suggesting that it should apply to a good number of graphene/substrate systems.

Atomic-scale control of graphene magnetism by using hydrogen atoms.

Isolated hydrogen atoms absorbed on graphene are predicted to induce magnetic moments. Here we demonstrate that the adsorption of a single hydrogen atom on graphene induces a magnetic moment characterized by a ~20-millielectron volt spin-split state at the Fermi energy. Our scanning tunneling microscopy (STM) experiments, complemented by first-principles calculations, show that such a spin-polarized state is essentially localized on the carbon sublattice opposite to the one where the hydrogen atom is chemisorbed. This atomically modulated spin texture, which extends several nanometers away from the hydrogen atom, drives the direct coupling between the magnetic moments at unusually long distances. By using the STM tip to manipulate hydrogen atoms with atomic precision, it is possible to tailor the magnetism of selected graphene regions.

Ethylene irradiation: a new route to grow graphene on low reactivity metals.

A novel technique for growing graphene on relatively inert metals, consisting in the thermal decomposition of low energy ethylene ions irradiated on hot metal surfaces in ultrahigh vacuum, is reported. By this route, we have grown graphene monolayers on Cu(111) and, for the first time, on Au(111) surfaces. For both noble metal substrates, but particularly for Au(111), our scanning tunneling microscopy and spectroscopy measurements provide sound evidence of a very weak graphene-metal interaction.

Observation of shell effects in superconducting nanoparticles of Sn.

In a zero-dimensional superconductor, quantum size effects (QSE) not only set the limit to superconductivity, but are also at the heart of new phenomena such as shell effects, which have been predicted to result in large enhancements of the superconducting energy gap. Here, we experimentally demonstrate these QSE through measurements on single, isolated Pb and Sn nanoparticles. In both systems superconductivity is ultimately quenched at sizes governed by the dominance of the quantum fluctuations of the order parameter. However, before the destruction of superconductivity, in Sn nanoparticles we observe giant oscillations in the superconducting energy gap with particle size leading to enhancements as large as 60%. These oscillations are the first experimental proof of coherent shell effects in nanoscale superconductors. Contrarily, we observe no such oscillations in the gap for Pb nanoparticles, which is ascribed to the suppression of shell effects for shorter coherence lengths. Our study paves the way to exploit QSE in boosting superconductivity in low-dimensional systems.

Tip and surface determination from experiments and simulations of scanning tunneling microscopy and spectroscopy.

We present a very efficient and accurate method to simulate scanning tunneling microscopy images and spectra from first-principles density functional calculations. The wave functions of the tip and sample are calculated separately on the same footing and propagated far from the surface using the vacuum Green function. This allows us to express the Bardeen matrix elements in terms of convolutions and to obtain the tunneling current at all tip positions and bias voltages in a single calculation. The efficiency of the method opens the door to real time determination of both tip and surface composition and structure, by comparing experiments to simulated images for a variety of precomputed tips. Comparison with the experimental topography and spectra of the Si111-(7 x 7) surface shows a much better agreement with Si than with W tips, implying that the metallic tip is terminated by silicon.