Superconductivity in charge transfer complexes of C60

Doping by alkali metals into single crystals of neutral charge transfer complexes of C60 has been attempted under mild conditions to ionize C60 in a packing manner different from that in the alkali-doped C60. By potassium or rubidium doping, the OMTTF·C60·benzene complex showed bulk superconductivity with Tc at 17–18.8 K for potassium doping, 23–26 K for rubidium doping, respectively, where OMTTF is octamethylene-tetrathiafulvalene (~10% volume fraction in the rubidium doping). Although an expansion of the unit cell volume has been observed in the case of the potassium doping, such an increase of the cell volume on rubidium doping was very small. After single crystal x-ray experiments on the rubidium-doped crystals, magnetization measurements showed superconductivity in one crystal (3–7% volume fraction) out of the three examined, suggesting that the doping was not uniform