Rk. Subramanian et al., IMPURITY AGAINST NONLOCALITY IN THE INCOMMENSURATE PHASE OF RB2ZNCL4 - A CL-35 NQR STUDY, Journal of physics. Condensed matter, 6(44), 1994, pp. 9437-9449
The influence of impurities on the phason and amplitudon dynamics in t
he incommensurate phase of (Rb1-xCsx2ZnCl4 (for x = 0.005 and 0.01) is
investigated using Cl-35 nuclear quadrupole resonance frequency (nu)
and spin lattice relaxation time (T1) measurements as a function of te
mperature. While the paraelectric-to-incommensurate transition tempera
ture (T(I)) is suppressed by about 8 K, the lock-in transition tempera
ture (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 th
e manifestation of non-locality of the modulation wave with respect to
the region of influence of the nuclear spin probe. The phason and amp
litudon contributions to the spin lattice relaxation, T1phi and T1A re
spectively, are measured by selective excitation of the broadened NQR
spectra. While T1A is insensitive to impurities, T1phi is strongly dep
endent on temperature and impurity concentration. The temperature depe
ndence of T1phi is explained as being due to the underdamping of phono
n modes in the presence of impurities.