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 grapheneThe authors acknowledge funding from the Spanish Ministry of Science and Innovation MCIN/AEI/10.13039/297 501100011033 though grants # PID2020-115171GB-I00, PID2020-114880GB-I00, PID2019-107338RB-C61 and the “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M, CEX2020-001038-M), the Comunidad de Madrid NMAT2D-CM program under grant S2018/NMT-4511, the Comunidad de Madrid, the Spanish State and the European Union by the Recovery, Transformation and Resilience Plan “Materiales Disruptivos Bidimensionales (2D)” (MAD2D-CM)-UAM3 and the European Union through the Next Generation EU funds and the Horizon 2020 FET-Open project SPRING (No. 863098). J. C. C. thanks the German Science Foundation DFG and SFB 1432 for sponsoring his stay at the University of Konstanz as a Mercator Fello

Van Hove singularities in doped twisted graphene bilayers studied by scanning tunneling spectroscopy

International audienceThe effect of electron doping on the van Hove singularities (vHs) which develop in twisted graphene bilayers (tBLs) is studied for a broad range of rotation angles θ(1.5°<θ<15°) by means of scanning tunneling microscopy and spectroscopy. Bilayer and trilayer graphene islands were grown on the 6H-SiC(000-1) (3×3) surface, which results in tBLs doped in the 10 E12 cm−2 range by charge transfer from the substrate. For large angles, doping manifests in a strong asymmetry of the positions of the upper (in empty states) and lower (in occupied states) vHs with respect to the Fermi level. The splitting of these vHs energies is found essentially independent of doping for the whole range of θ values, but the center of theses vHs shifts towards negative energies with increasing electron doping. Consequently, the upper vHs crosses the Fermi level for smaller angles (around 3°). The analysis of the data performed using tight-binding calculations and simple electrostatic considerations shows that the interlayer bias remains small (<100mV) for the doping level resulting from the interfacial charge transfer (≃5×10E12 cm−2)

Friedel oscillations in graphene-based systems probed by Scanning Tunneling Microscopy

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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.This work was supported by AEI and FEDER under projects MAT2016-80907-P and MAT2016-77852-C2-2-R (AEI/FEDER, UE), by the Fundación Ramón Areces, and by the Comunidad de Madrid NMAT2D-CM program under grant S2018/NMT-4511. J.F.R. acknowledges financial support European Regional Development Fund Project No. NORTE-01-50145- FEDER-000019, and the UTAPEXPL/NTec/0046/2017 projects, as well as Generalitat Valenciana funding Prometeo2017/139 and MINECO Spain (Grant No. MAT2016-78625-C2). J.L.L is grateful for financial support from the Academy of Finland Projects Nos. 331342 and 336243

Shaping graphene superconductivity with nanometer precision

International audienceGraphene holds great potential for superconductivity due to its pure two-dimensional nature, the ability to tune its carrier density through electrostatic gating, and its unique, relativistic-like electronic properties. At present, we are still far from controlling and understanding graphene superconductivity, mainly because the selective introduction of superconducting properties to graphene is experimentally very challenging. Here, we have developed a method 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. We first grow Pb nano-islands 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, we perform a series of experiments, 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

Structural paradox in submonolayer chlorine coverage on Au(111)

Équipe 102 : Surfaces et SpectroscopiesInternational audienceIn this work, we present a combined low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) study of chlorine adsorption on Au(111) at low coverages. Our STM study of Cl/Au(111) system has shown that at submonolayer coverages (theta < 0.1 ML) chlorine atoms form chainlike structures with abnormally short distances of 3.8 angstrom between them. Our DFT calculations have shown that chlorine atoms can interact with each other through distortion of the substrate and this indirect elastic interaction is strong enough to affect their arrangement in the chainlike structures

Self-Organization of Gold Chloride Molecules on Au(111) Surface

Adsorption of molecular chlorine on Au(111) has been studied with a low-temperature (5 K) scanning tunneling microscope in combination with density functional theory calculations. The formation of AuCl₂ quasi-molecules was detected at chlorine coverage exceeding 0.33 ML. The self-organization of the AuCl₂ species into the ordered “honeycomb” structure was clearly demonstrated for coverages close to saturation

Correction to Weakly Trapped, Charged, and Free Excitons in Single-Layer MoS 2 in the Presence of Defects, Strain, and Charged Impurities

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