Impurity against non-locality in the incommensurate phase of Rb2ZnCl4: a 35Cl NQR study

Abstract

The influence of impurities on the phason and amplitudon dynamics in the incommensurate phase of (Rb1-xCsx)2ZnCl4 (for x=0.005 and 0.01) is investigated using 35Cl nuclear quadrupole resonance frequency ( nu ) and spin lattice relaxation time (T1) measurements as a function of temperature. While the paraelectric-to-incommensurate transition temperature (TI) is suppressed by about 8 K, the lock-in transition temperature (observed at 192 K in the pure compound) is not observed down to 77 K for x=0.01. The observation of an additional singularity in the NQR spectrum of the x=0.01 compound is explained as being due to the manifestation of non-locality of the modulation wave with respect to the region of influence of the nuclear spin probe. The phason and amplitudon contributions to the spin lattice relaxation, T1 phi and T1A respectively, are measured by selective excitation of the broadened NQR spectra. While T1A is insensitive to impurities, T1 phi is strongly dependent on temperature and impurity concentration. The temperature dependence of T1 phi is explained as being due to the underdamping of phonon modes in the presence of impurities.