Graphite and intercalated compound superconductors: Atomic and electronic structures

Graphite is a carbon allotrope with a unique anisotropy. The in-plane bonds of carbon have a strong covalent bonding characteristic, while out-of-plane bonding of carbon is due to the weak van der Waals interaction. Graphite can be easily exfoliated, i.e., a sheet of graphite called graphene can be easily separated. By intercalating metallic atoms, the property of graphite can be drastically altered. For example, CaC₆ is known as a graphite intercalation compound (GIC) superconductor having transition temperature of 11.5 K at ambient pressure. In the first half of this chapter, the brief history of GIC discovery as a superconducting material is outlined, and then the current understanding of GIC superconducting mechanism is described. In the second half of this chapter, the experimental verification of the atomic and electronic structures of the pristine graphite and GIC using photoelectron spectroscopy and diffraction is introduced. Finally, this chapter is summarized and expanded into future prospects.