Condensed-Matter Physics & Materials Science Seminar

The discovery of superconductivity in CaC6 and YbC6 at 11.5 K and 6.5 K respectively has reinvigorated the interest in superconductivity in graphite intercalation compounds. Several different techniques are brought together to study the phonons, the electron-phonon interaction and the superconducting properties of these compounds. Raman scattering was used to perform a comparative study of BaC6, SrC6, YbC6 and CaC6. We show that the superconducting transition temperature increases in compounds with more electrons in the carbon-related electronic states, which implies that the carbon-related electronic states play a crucial role in the superconductivity. It is also demonstrated that the high-energy carbon phonons in these compounds are subject to non-adiabatic effects, which cause a large shift in the phonon energy and an increase in the phonon line width. To further investigate these effects, inelastic x-ray and neutron scattering is employed to measure the phonon dispersions in CaC6. The results of which are compared to theoretical predictions. High-pressure studies are also underway to examine the interplay between superconductivity and the Yb ion valence transition in YbC6.