Electrical conductivity in dynamically orientationally disordered systems: ac and dc measurements in ferromagnetic single crystals of TDAE-C-60

In spite of a metallic C-60 molecular ground state, electrical conductivity measurements performed on single crystals of ferromagnetic TDAE-C-60 fail to show evidence of metallic behavior. Instead, the material appears to be an insulter exhibiting conductivity of the form sigma(omega,T) = sigma(dc)(T) + sigma(ac)(omega). Phonon-assisted activated polaronic hopping and interbuckyball tunneling, respectively, are proposed as the mechanisms for the two processes. The transition from a dynamically disordered state (rotating buckyballs) to a quasiordered state (frozen buckyballs) is clearly visible as a change in activation energy in the conductivity at T-0 = 150 K.