35Cl NQR and 2H NMR analysis of the critical dynamics close to the displacive normal-incommensurate phase transition in an organic crystal

Nuclear magnetic resonance MMR) experiments, Cl-35 nuclear quadrupole resonance (NQR) and H-2 NMR, have been performed close to the displacive normal-incommensurate phase transition in the organic crystal of bis-(4-chlorophenyl)-sulfone. Calculations using coherent neutron scattering results show that the soft-mode contribution cannot explain the rapid increase of the spin-lattice relaxation rates close to the transition temperature. Calculations of the spectral densities taking into account the existence of a central-peak phenomenon describe both Cl-35 NQR and H-2 NMR spin-lattice relaxation rates on approaching the phase transition. In this way, the width of the central peak can be estimated to be in the range of several gigahertz.