Superconductivity and Electron-Phonon Coupling in Graphite Intercalation Compunds

Abstract

In graphite intercalation compounds (GIC), the intercalation of various atomic or molecular species in between graphene layers in graphite leads to novel properties and a very rich physics, including superconductivity (Dresselhaus & Dresselhaus, 2002). In graphite intercalated with alkaline metals, superconductivity has been known for decades (Hannay et al., 1965), but after recent discovery of relatively high Tc superconductivity in CaC6 (Tc = 11.5 K)(Emery et al., 2005; Weller et al., 2005) research in this field has been intensified. In conventional metals, the electron-phonon coupling has long been known to be the pairing interaction responsible for the superconductivity. The strength of this interaction essentially determines the superconducting transition temperature Tc. Even though the electron-phonon coupling is most likely responsible for pairing in GICs (Hinks et al., 2007; Kim et al., 2006; Lamura et al., 2006), it is still not clear what electronic states, intercalantor graphenederived ones, and what phonons are responsible for pairing (Boeri et al., 2007; Calandra M Mazin et al., 2007; Mazin, 2005). Due to differences in structure and composition, no clear trends have been identified that could unambiguously resolve these issues. For example, KC8 is a superconductor and LiC6 is not. Further, in GICs intercalated with alkaline earths, Tc ranges from zero to 11.5K, even though they share the same chemical formula MC6, where M is an alkaline earth atom. This obviously represents a serious problem to the proposal that superconductivity originates from graphene sheets and that the only role of intercalants is to provide the charge to the graphene bands. Further, band structure calculations show that in graphite and GICs, an interlayer state exists above π∗ band (Holzwarth et al., 1984; Posternak et al., 1983), prompting some researchers to propose that its partial filling and coupling to soft intercalant phonons induces superconductivity in GICs (Csanyi et al., 2005; Mazin, 2005). The experimental situation is still inconclusive, with strong advocates for intercalant (Hinks et al., 2007) and graphene dominated superconductivity (Dean et al., 2010; Gruneis et al., 2009; Kim et al., 2006; Pan et al., 2010; Valla et al., 2009). Recent angle resolved photoemission spectroscopy study on CaC6 (Valla et al., 2009) reported that the electron-phonon coupling on graphene-derived Fermi surface to graphene phonons is strong enough to explain a Tc in the range of tens of Kelvin, indicating that graphene sheets provide crucial ingredients for superconductivity in GICs. However, to test this idea, it would be important to extend similar studies to GICs with different Tc. One manifestation of electron-phonon coupling is a renormalization of the electronic dispersion or a kink at the energy scale associated with the phonons. This renormalization Superconductivity and Electron-Phonon Coupling in Graphite Intercalation Compunds 19