Superconductivity and electron-phonon interactions in graphite intercalation compounds

• Main Author: Dean, M. P. M.
• Published: University of Cambridge 2010
• Subjects: 541.37
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598476

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 <i>T_c </i>from &lt; 80 mK to 11.5 K. It was possible to correlate the increase in <i>T_c</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 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. <i>T_c </i>initially increases consistent with the idea that moving the graphene layers closer increases <i>T_c</i>, however, at higher pressures <i>T_c</i> decreases disappearing at 7 GPa. These effects are discussed in light of a possible valence transition in YbC₆.