Organic Superconductor - Doped Buckminster Fullerenes

Doped Buckminster Fullerenes

Superconducting fullerenes (based on the Buckminster fullerene C₆₀) are fairly different from other organic superconductors. The building molecules are no longer manipulated hydrocarbons but pure carbon molecules. In addition these molecules are no longer flat but bulky which gives rise to a three dimensional, isotropic superconductor. The pure C₆₀ grows in a fcc-lattice and is an insulator. By placing alkali atoms in the interstitials the crystal becomes metallic and eventually superconducting at low temperatures.

Unfortunately C₆₀ crystals are not stable at ambient atmosphere. They are grown and investigated in closed capsules, limiting the measurement techniques possible. The highest transition temperature measured so far was T_C = 33 K for Cs₂RbC₆₀.The highest measured transition temperature of an organic superconductor was found in 1995 in Cs₃C₆₀ pressurized with 15 kbar to be T_C = 40 K. Under pressure this compound shows a unique behaviour. Usually the highest T_C is achieved with the lowest pressure necessary to drive the transition. Further increase of the pressure reduces the transition temperature usually. Different in Cs₃C₆₀: Superconductivity sets in at very low pressures of several 100 bar and the transition temperature keeps increasing with increasing pressure. This indicates a completely different mechanism then just broadening of the bandwidth.