PULSED PHOTOCONDUCTIVITY OF UNDOPED SOLID C-60 WITH AN ANOMALY AT PHASE-TRANSITION

We have studied pulsed photoconductivity Q(T, lambda) of undoped solid C60 at T=4.2-300 K and lambda=355-780 nm. An observation of the Dember effect at lambda=355 nm indicates that dominant photocarriers are electrons. At 250 K, we observed a significant anomaly in Q(T, lambda) at the structural phase transition (from sc to fcc) due to the orientational order-disorder transformation. This suggests that the mechanism of photoconductivity sensitively reflects the change of electronic structure due to rotational motion of C60 molecules. We have also recognized the critical energy gap at 2.0 eV in pulsed photoconductivity of undoped C60 via the temperature dependence of Q(T, lambda) above 250 K excited at various photon energies.