  The glassy dynamics of plastic-crystalline cyclo-octanol and ortho-carborane,
where only the molecular reorientational degrees of freedom freeze without
long-range order, is investigated by nonlinear dielectric spectroscopy. Marked
differences to canonical glass formers show up: While molecular cooperativity
governs the glassy freezing, it leads to a much weaker slowing down of
molecular dynamics than in supercooled liquids. Moreover, the observed
nonlinear effects cannot be explained with the same heterogeneity scenario
recently applied to canonical glass formers. This supports ideas that molecular
relaxation in plastic crystals may be intrinsically non-exponential. Finally,
no nonlinear effects were detected for the secondary processes in
cyclo-octanol.
