Superconductivity in graphite intercalation compounds

Superconductivity in graphite intercalation compounds (GIC) has been studied for more than 40 years and it is still not fully understood, despite the recent progress and the discovery of relatively high Tc superconductivity in CaC6 and YbC6. Recent studies now suggest that the electron-phonon coupling (EPC) is most likely responsible for pairing. However, it is still not clear whether the graphene-derived electronic states and vibrations, or the intercalant-derived ones play a more important role. Most of the theoretical studies suggest that the intercalantderived Fermi surface (FS) and intercalant-derived soft vibrations are the key for superconductivity, while some suggest that the intercalant states couple also to the out-of-plane graphene vibrations. However, all of them agree that the graphene electrons are only very weakly coupled to in-plane graphene vibrations, rendering this interaction virtually irrelevant for superconductivity.

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Figure: (a) 1–9: photoemission spectra from CaC6 along several momentum lines near the K point as indicated in (b). (b) Photoemission intensity from CaC6 from a narrow energy interval around the Fermi level (! ¼10 meV) near the K point of the BZ. Thin bars indicate momentum lines probed in (a). (c) Fermi surface of CaC6 obtained by sixfold symmetrization of intensity shown in (b). White lines correspond to the doping of 1=2 electrons per C atom, at which the van Hove singularity in the band sits at the Fermi level. (d) ARPES spectrum from a thin pristine graphite flake from the momentum line going through the K point orthogonal to the K-M line. (e) photoemission intensity from the pristine graphite flake around K point at ! ¼ 0 and (f ) at ! ¼1 eV. All the spectra in (a), (b), (d), (e), and (f ) are shown on the same momentum and energy scales and all were taken at T ¼ 14 K, in the normal state of CaC6