An Abnormal Temperature Dependence of Conductivity in Fullerene Solids

An abnormal temperature dependence of conductivity has been observed in some fullerene solids, C 60 , C 70 , and C 60 H 36 within the temperature range 400--500 K. The temperature dependent conductivity measurements of these fullerene solids with various molecular diameters and moments of inertia indicate that the abnormal conductivity involves a molecular rotation effect in the fullerene solids. Conductivity measurements of the C 60 solid samples prepared using various pressures indicate that the abnormal conductivity is also related to a variation of the energy band structure. We are proposing a model to explain this abnormal conductivity observed as that, a reversible annealing effect results in the releasing or trapping of holes in the deep energy levels induced by lattice defects. The contribution of these holes leads to the abnormal variation in the conductivity with temperature.