Electrical transport measurements on tdae-c60 single crystals

DC and AC electrical conductivity measurements on TDAE-C60 single crystals along different crystallographic axes are presented. The conductivity shows activated behavior between room temperature and 100 K with no reentrant conductivity down to 4 K, indicating an insulating electronic ground state. The activation energy changes from 0.3 eV above 150 K to 0.1 eV below this temperature. This change is accompanied by a change in the AC response. We attribute it to the freezing of the rotational degrees of freedom and consequently reduced randomness in low-temperature phase. AC conductivity may be written in two-component form σDC (T)+σAC (ω) where frequency-dependent part has exponential dependence σAC (ω)=Bωs typical for disordered systems.