Systematic Raman scattering experiments were performed to compare the phonon properties in series of GICs BaC<sub>6</sub>, SrC<sub>6</sub>, YbC<sub>6</sub> and CaC<sub>6</sub>. 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 <i>T<sub>c</sub> </i>from < 80 mK to 11.5 K. It was possible to correlate the increase in <i>T<sub>c </sub></i>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 CaC<sub>6</sub>. 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 CaC<sub>6</sub>, which disagree with the theoretical predictions. YbC<sub>6</sub> was studied as a function of pressure to investigate the changes induced by reducing the layer separation. <i>T<sub>c</sub> </i>initially increases consistent with the idea that moving the graphene layers closer increases <i>T<sub>c</sub></i>, however, at higher pressures <i>T<sub>c</sub></i> decreases disappearing at 7 GPa. These effects are discussed in light of a possible valence transition in YbC<sub>6</sub>.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:598476 |
| Date | January 2010 |
| Creators | Dean, M. P. M. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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