ABSTRACT
An effective search would require a comprehensive description of the normal and superconducting properties of the compounds hitherto reported. In fact, within the frame of the above BCS theory, Tc changes involve a delicate interplay of a number of electronic and transport properties, notably the frequency of the phonons relevant
to the pairing, the electron-phonon matrix elements, and the density of states at the Fermi level. As a matter of fact, even qualitative predictions are known to be extremely difficult for real systems due to the low-energy scale involved and the radical simplifications introduced by the BCS theory and its generalizations, such as the Eliashberg theory. However, the reliability of theoretical predictions have been recently improved thanks to the development of powerful ab initio computational methods like the superconducting density functional theory [13,14]. Nowadays, these methods enable the reliable ab initio computation of all BCS parameters in relatively simple compounds, such as elemental metals [14,15]. Thanks to these theoretical developments and to a wide range of advanced experimental investigations, in this chapter, we show that a comprehensive description of the superconducting properties of CaC6 is achieved within the frame of the BCS theory based on the conventional electron-phonon pairing mechanism.In this chapter, we shall focus on the following three salient aspects of superconductivity in CaC6: (i) the normal state transport properties enabling to single-out the electron-phonon modes responsible for the electron-phonon coupling, (ii) the large pressure-induced Tc enhancement up to 15.1 K, the highest Tc value hitherto reported for GICs, and (iii) the evidence of a latent lattice instability of the Ca sublattice which limits Tc in the high pressure regime. We believe that the scenario proposed below may give hints as to the possibility of further enhancing Tc in GICs. 6.1.1 Crystal and Electronic Structures
of Graphite-Intercalated CompoundsGICs [6] are interesting because intercalation is found to radically alter the electronic properties of pristine graphite. Depending on the position of the intercalant atom within the van der Waals gap between adjacent graphene layers and on the intercalant concentration, various polytypes are formed [6]. The three different positions of the intercalant are labeled as αβγ (see Fig. 6.1). In the following, we shall restrict our considerations to the AC6 family (A = alkaline earth, such as Ca, Sr, Ba, or rare earth, such as Yb). For this family, two structures have been reported [16], as shown in Fig. 6.1. The first structure with hexagonal P63/mmc symmetry is found in BaC6,
SrC6, and YbC6 and corresponds to the αβ sequence, whilst the second one with rhombohedral R-3m symmetry is found in CaC6and corresponds to the αβγ stacking sequence. As expected from general considerations of solid state chemistry, ab initio calculations of the electronic structures [17-20] confirm that the above different structures have in fact comparable free energies differences and lead to similar electronic structures.
