Shuttleworth, NE; (2015) Gap Phenomena in Graphite and its MC6 Intercalation Compounds. Doctoral thesis , UCL (University College London).

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Abstract

The physical and electronic structure of graphite lend it a huge amount of functional flexibility. Through charge doping of its layers by intercalation or dimensional constriction diverse electronic phenomena can be observed. This thesis constitutes the study of two aspects of the excitations of graphite in two extremes: electron-phonon coupling in the strongly doped bulk and electronic transport in the semi-metallic sub-micron regime. Measurements of the heat capacity of the graphite intercalation compounds CaC6, SrC6 and BaC6 have been performed between 300K and 390mK. The onset anomoly of a charge density wave state recently detected in CaC6 at 78K has not been observed signifying a transition temperature above 300K. The superconducting phase of CaC6 has been characterised as BCS-like with intermediate coupling and mild gap anisotropy, while the inaccuracy of predictions of the superconducting heat capacity anomoly has been suggested as due to Fermi surface competition with the charge density wave state. The electron-phonon coupling strength has been found to be in agreement with predictions for CaC6 and SrC6, while for BaC6 it is measured as half the predicited value, explaining the failure of previous experiments to observe superconductivity at the predicted TC 230mK. Micron-sized graphitic devices created by focused ion beam micro-machining have been found to exhibit ballistic transport behaviour up to 250K. Below 40K a Coulomb-like pseudogap has been observed in agreement with previous devices, suggested as due to electron-electron interactions within the device. Below 10K further complex conductance features have been observed with unknown cause.

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