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Register a new userTunable Kondo Effect in Graphene with Defects
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Graphene is a model system for the study of electrons confined to a strictly two-dimensional layer1 and a large number of electronic phenomena have been demonstrated in graphene, from the fractional2, 3 quantum Hall effect to superconductivity4. However, the coupling of conduction electrons to local magnetic moments5, 6, a central problem of condensed matter physics, has not been realized in graphene, and, given carbons lack of d or f electrons, magnetism in graphene would seem unlikely. Nonetheless, magnetism in graphitic carbon in the absence of transition-metal elements has been reported7-10, with explanations ranging from lattice defects11 to edge structures12, 13 to negative curvature regions of the graphene sheet14. Recent experiments suggest that correlated defects in highly-ordered pyrolytic graphite (HOPG) induced by proton irradiation9 or native to grain boundaries7, can give rise to ferromagnetism. Here we show that point defects (vacancies) in graphene15 are local moments which interact strongly with the conduction electrons through the Kondo effect6, 16-18 providing strong evidence that defects in graphene are indeed magnetic. The Kondo temperature TK is tunable with carrier density from 30-90 K; the high TK is a direct consequence of strong coupling of defects to conduction electrons in a Dirac material18. The results indicate that defect engineering in graphene could be used to generate and control carrier-mediated magnetism, and realize all-carbon spintronic devices. Furthermore, graphene should be an ideal system in which to probe Kondo physics in a widely tunable electron system.
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Tailoring electron transfer dynamics across solid-liquid interfaces is fundamental to the interconversion of electrical and chemical energy. Stacking atomically thin layers with a very small azimuthal misorientation to produce moire superlattices enables the controlled engineering of electronic band structures and the formation of extremely flat electronic bands. Here, we report a strong twist angle dependence of heterogeneous charge transfer kinetics at twisted bilayer graphene electrodes with the greatest enhancement observed near the magic angle (~1.1 degrees). This effect is driven by the angle-dependent tuning of moire-derived flat bands that modulate electron transfer processes with the solution-phase redox couple. Combined experimental and computational analysis reveals that the variation in electrochemical activity with moire angle is controlled by atomic reconstruction of the moire superlattice at twist angles <2 degrees, and topological defect AA stacking regions produce a large anomalous local electrochemical enhancement that cannot be accounted for by the elevated local density of states alone. Our results introduce moire flat band materials as a distinctively tunable paradigm for mediating electrochemical transformations.
We study the magnetic properties in the vicinity of a single carbon defect in a monolayer of graphene. We include the unbound $sigma$ orbital and the vacancy induced bound $pi$ state in an effective two-orbital single impurity model. The local magnetic moments are stabilized by the Coulomb interaction as well as a significant ferromagnetic Hunds rule coupling between the orbitals predicted by a density functional theory calculation. A hybridization between the orbitals and the Dirac fermions is generated by the curvature of the graphene sheet in the vicinity of the vacancy. We present results for the local spectral function calculated using Wilsons numerical renormalization group approach for a realistic graphene band structure and find three different regimes depending on the filling, the controlling chemical potential, and the hybridization strength. These different regions are characterized by different magnetic properties. The calculated spectral functions qualitatively agree with recent scanning tunneling spectra on graphene vacancies.
Understanding the coupling of graphene with its local environment is critical to be able to integrate it in tomorrows electronic devices. Here we show how the presence of a metallic substrate affects the properties of an atomically tailored graphene layer. We have deliberately introduced single carbon vacancies on a graphene monolayer grown on a Pt(111) surface and investigated its impact in the electronic, structural and magnetic properties of the graphene layer. Our low temperature scanning tunneling microscopy studies, complemented by density functional theory, show the existence of a broad electronic resonance above the Fermi energy associated with the vacancies. Vacancy sites become reactive leading to an increase of the coupling between the graphene layer and the metal substrate at these points; this gives rise to a rapid decay of the localized state and the quenching of the magnetic moment associated with carbon vacancies in free-standing graphene layers.
The Kondo effect has been observed in a single gate-tunable atom. The measurement device consists of a single As dopant incorporated in a Silicon nanostructure. The atomic orbitals of the dopant are tunable by the gate electric field. When they are tuned such that the ground state of the atomic system becomes a (nearly) degenerate superposition of two of the Silicon valleys, an exotic and hitherto unobserved valley Kondo effect appears. Together with the regular spin Kondo, the tunable valley Kondo effect allows for reversible electrical control over the symmetry of the Kondo ground state from an SU(2)- to an SU(4) -configuration.
We address local inelastic scattering from vibrational impurity adsorbed onto graphene and the evolution of the local density of electron states near the impurity from weak to strong coupling regime. For weak coupling the local electronic structure is distorted by inelastic scattering developing peaks/dips and steps. These features should be detectable in the inelastic electron tunneling spectroscopy, $d^2I/dV^2$, using local probing techniques. Inelastic Friedel oscillations distort the spectral density at energies close to the inelastic mode. In the strong coupling limit, a local negative $U$-center forms in the atoms surrounding the impurity site. For those atoms, the Dirac cone structure is fully destroyed, that is, the linear energy dispersion as well as the V-shaped local density of electron states is completely destroyed. We further consider the effects of the negative $U$ formation and its evolution from weak to strong coupling. The negative $U$-site effectively acts as local impurity such that sharp resonances appear in the local electronic structure. The main resonances are caused by elastic scattering off the impurity site, and the features are dressed by the presence of vibrationally activated side resonances. Going from weak to strong coupling, changes the local electronic structure from being Dirac cone like including midgap states, to a fully destroyed Dirac cone with only the impurity resonances remaining.
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