RAHNEJAT, KC; (2013) Scanning tunnelling microscopy studies of the graphitic superconductor CaC6. Doctoral thesis , UCL (University College London).

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Abstract

Graphitic systems have an electronic structure that can be readily manipulated through electrostatic or chemical doping, resulting in a rich variety of electronic ground states. One such class of materials are graphite intercalation compounds, a signi cant number of which exhibit superconductivity at low temperatures. The optimal superconductor in this class is CaC6, which superconducts at 11.5 K. This thesis presents the rst atomic-resolution surface studies of CaC6 revealing not only the surface structure but also, via detailed spectroscopic imaging and Fourier transform techniques, a stripe phase. This stripe phase corresponds to a charge density wave with a period three times that of the Ca superlattice. The work presented in this thesis is the rst such reporting and detailed characterization of a charge density wave in a graphitic material. Of further interest is the fact that the stripe phase is found to modulate the Ca lattice but no distortion of the graphene lattice is found indicating the graphene sheets may host the ideal, purely electronic, charge density wave. The implications for the understanding of superconductivity in CaC6, graphene and the apparent ubiquity of CDWs with superconductivity in anisotropic layered materials are discussed.

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