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Achievement 1

Ripples and charge puddles in screened graphene

            Although renown for its extraordinary properties, graphene exhibits spatial doping disorder, the so-called electron-hole puddles. These local charge inhomogeneities are directly linked to properties appearing at the Dirac point. They also limit how close this point can actually be approached in a macroscopic sample. On a dielectric substrate, several scenarii, such as charged impurities trapped below the graphene as well as the local graphene curvature, have been pointed out to be at the origin of these puddles.

            In a scanning tunneling microscopy and spectroscopy (STM/STS) experiment, a metallic tip is approached to a few Angstroms to a surface. The tunnel current flowing contains information both about the topography and the local density of states. We have performed STM/STS on graphene grown by chemical vapor deposition on Ir(111). In this system, graphene can - under some circumstances - be very weakly coupled to its metallic substrate. We observe the existence of both ripples (topographic disorder) and charge puddles (doping disorder) here and show that they are correlated: the hills are more p-doped than the valleys (Figure 1). These observations are well explained by a model combining electrostatic charge transfer and a very small wave function overlap between graphene and its substrate.

            We further map the interferences of electrons scattered by the doping disorder. This allows us to reconstruct the dispersion relation E(k) of the graphene. We not only observe E(k) to be linear - a proof that the "Dirac physics" are preserved - but also a renormalization of the Fermi velocity to lower values. This later finding is a strong indication that electron-electron interactions in the graphene are modified by the presence of the nearby metallic substrate.  This system shows that the exciting electronic properties of graphene are well preserved but under strong electrostatic influence of the substrate.

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Figure 1 : Ripples (topography, vertical axis) and puddles (doping level, color code) in graphene weakly bound to its Ir(111) substrate.

Référence:

"Disorder and screening in decoupled graphene on a metallic substrate", S. Martin, B. Sacépé, A. Kimouche, J. Coraux, F. Fuchs, B. Grévin, H. Courtois, and C.B. Winkelmann, arxiv:1304:1183, Phys. Rev. B Rapid comm. 91, 041406 (2015).

 

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