Critical proton and deuteron spin-lattice relaxation at the phase transition inp-terphenyl

The spin-lattice relaxation around the order-disorder transition in p-terphenyl has been studied at different frequencies. The data are analyzed in terms of the critical slowing down for the librational fluctuations of the phenyl rings, providing interesting insights on the temperature behavior of the characteristic frequency width ${\Gamma }_c$ of the central peak in the dynamical structure factor S(q→,ω). This peak has been recently observed in the deuterated crystal through high-resolution neutron backscattering techniques. Values of ${\Gamma }_c$ are obtained from proton $T_1$ without adjustable parameters over a wide temperature range. These data are in agreement with the previous estimates from neutron scattering in the deuterated crystal and demonstrate the occurrence of critical slowing down. A critical exponent n=γ-ν(d-z)≃1 is derived for T→$T_c^{+}$, indicating the failure of any mean-field description and suggesting a two-dimensional correlation in the fluctuations. Below $T_c$ the order parameter, as approximately derived from the $T_1$ data, exhibits a very rapid rise indicative of a small value of the critical exponent β, as also observed by superlattice neutron diffraction and as expected for low-dimensional systems. The data for the deuteron spin-lattice relaxation confirm the conclusion from proton $T_1$ and support the hypothesis of a negligible contribution of quantum tunneling to the critical dynamics driving the phase transition.