Superconductivity in aromatic hydrocarbons

'Aromatic hydrocarbon' implies an organic molecule that satisfies the (4n + 2) π-electron rule and consists of benzene rings. Doping solid aromatic hydrocarbons with metals provides the superconductivity. The first discovery of such superconductivity was made for K-doped picene (K_xpicene, five benzene rings). Its superconducting transition temperatures (T_c's) were 7 and 18 K. Recently, we found a new superconducting K_xpicene phase with a T_c as high as 14 K, so we now know that K_xpicene possesses multiple superconducting phases. Besides K_xpicene, we discovered new superconductors such as Rb_xpicene and Ca_xpicene. A most serious problem is that the shielding fraction is ≤15% for K_xpicene and Rb_xpicene, and it is often ∼1% for other superconductors. Such low shielding fractions have made it difficult to determine the crystal structures of superconducting phases. Nevertheless, many research groups have expended a great deal of effort to make high quality hydrocarbon superconductors in the five years since the discovery of hydrocarbon superconductivity. At the present stage, superconductivity is observed in certain metal-doped aromatic hydrocarbons (picene, phenanthrene and dibenzopentacene), but the shielding fraction remains stubbornly low. The highest priority research area is to prepare aromatic superconductors with a high superconducting volume-fraction. Despite these difficulties, aromatic superconductivity is still a core research target and presents interesting and potentially breakthrough challenges, such as the positive pressure dependence of T_c that is clearly observed in some phases of aromatic hydrocarbon superconductors, suggesting behavior not explained by the standard BCS picture of superconductivity. In this article, we describe the present status of this research field, and discuss its future prospects.