Superconductivity in single crystals of the fullerene C-70 (Retracted article. See vol 422 pg 92 2003)

The observation of superconductivity in doped C-60 has attracted much attention(1-3), as these materials represent an entirely new class of superconductors. A maximum transition temperature (Tc) of 40 K has been reported(4) for electron-doped C-60 crystals, while a Tc of 52 K has been seen 5 in hole-doped crystals; only the copper oxide superconductors have higher transition temperatures. The results for C-60 raise the intriguing questions of whether conventional electron-phonon coupling alone(1) can produce such high transition temperatures, and whether even higher transition temperatures might be observed in other fullerenes(6-8). There have, however, been no conrrmed reports of superconductivity in other fullerenes, though it has recently been observed in carbon nanotubes(9). Here we report the observation of superconductivity in single crystals of electric-field-doped C-70. The maximum transition temperature of about 7 K is achieved when the sample is doped to approximately four electrons per C-70 molecule, which corresponds to a half-filled conduction band. We anticipate superconductivity in smaller fullerenes at temperatures even higher than in C-60 if the right charge density can be induced.