Credit: © 2010 NPG

Superconductivity in organic compounds is quite rare; important examples include charge-transfer salts and metal-doped all-carbon materials, such as graphite or fullerenes. Relatively high transition temperatures (Tc) have been reported for calcium-intercalated graphite (CaC6; Tc = 11.5 K) and RbCs2C60 (Tc = 33 K). It is notable that these examples all feature extensive π-electron networks.

Now, Yoshihiro Kubozono and colleagues from several institutions in Japan have discovered superconductivity in a simple hydrocarbon doped with an alkali metal. The hydrocarbon is picene, which is flat and resembles five benzene rings fused together. When crystals of picene were heated to 440 K with potassium metal in an evacuated glass tube, they changed from white to black and showed metallic properties. The researchers made a range of compositions from just one atom of potassium per picene molecule to over five. When cooled, the compositions with around three atoms of potassium showed superconducting behaviour, with Tc ranging from 6.5 to 18 K; although only accessible by liquid-helium cooling, this is still extremely high for organic superconductors.

The suddenness of the transition — the temperature range between the onset of the transition and its finish — for picenes (at around 0.5 K) is similar to that observed in CaC6, but much more rapid than for K3C60 (over 10 K) or the recently discovered pnictide superconductors. Kubonozo and colleagues have also investigated intercalating other alkali metals into picene, and although the sodium and rubidium materials did not show superconductivity, the caesium analogue did at 6.9 K.