Critical Slowing Down of Orientational Fluctuations in a Plastic Crystal

For the first time a critical slowing down of orientational fluctuations on approaching the order-disorder transition of a plastic crystal has been detected. The observed inverse relaxation rates ${\ensuremath{\Gamma}}^{\ensuremath{-}1}$ in solid ${\mathrm{CD}}_{4}$ range from ${10}^{\ensuremath{-}11}$ sec (11\ifmmode^\circ\else\textdegree\fi{} above ${T}_{0}=27$ K) to 2.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}10}$ sec (close to ${T}_{0}$). The temperature dependence of $\ensuremath{\Gamma}$ can be fitted by a power law with an exponent $\ensuremath{\rho}=1.13\ifmmode\pm\else\textpm\fi{}0.13$. For the $q$ dependence of $\ensuremath{\Gamma}$ an exponent $z=1.90\ifmmode\pm\else\textpm\fi{}0.14$ is found close to ${T}_{0}$.