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Title: Superconductivity and electron-phonon interactions in graphite intercalation compounds
Author: Dean, M. P. M.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2010
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Systematic Raman scattering experiments were performed to compare the phonon properties in series of GICs BaC6, SrC6, YbC6 and CaC6. The key difference induced by changing the ions in this order is a reduction in the separation of the graphene layers, which is concurrent with an increase in the superconducting transition temperature Tc from < 80 mK to 11.5 K. It was possible to correlate the increase in Tc with a softening of an out-of-plane carbon related phonon, which was explained in terms of increased charge in the carbon-related electronic band. This provides evidence that the carbon-related phonons and electronic-bands are crucial to the superconductivity in these compounds. An in-plane carbon phonon was also measured, which was shown not to follow the Born-Oppenheimer approximation. Recent theoretical attempts to explain these effects cannot fully account for the observed electron-phonon scattering rate. Neutron scattering was also used to measure the high energy carbon-related phonons in CaC6. Due to the highly textured nature of the samples, special analytical techniques were developed to allow for the comparison between experiment and density functional theory (DFT). Overall, a good level of agreement between experiment and theory is found, which is significant in light of several other measurements of phonon related properties of CaC6, which disagree with the theoretical predictions. YbC6 was studied as a function of pressure to investigate the changes induced by reducing the layer separation. Tc initially increases consistent with the idea that moving the graphene layers closer increases Tc, however, at higher pressures Tc decreases disappearing at 7 GPa. These effects are discussed in light of a possible valence transition in YbC6.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available