Rb87nuclear magnetic resonance evidence for solitons and phasons inRb2ZnBr4

Abstract

The temperature and angular dependences of the $^{87}\mathrm{Rb}$ \textonehalf{}\ensuremath{\rightarrow}-\textonehalf{} NMR spectra and spin-lattice relaxation times have been studied in the paraelectric ($P$), the incommensurate ($I$), and the ferroelectric commensurate ($C$) phases of ${\mathrm{Rb}}_{2}$Zn${\mathrm{Br}}_{4}$, and the distributions of the electric-field-gradient (EFG) tensors at the Rb sites have been determined in the $P$ and $I$ phases. The NMR line shapes can be described by "broad solitons" over most of the $I$ phase whereas the soliton width becomes small as compared to the intersoliton distance close to the transition into the $C$ phase. The temperature dependence of the soliton density has been determined. The spin-lattice relaxation rate in the $I$ phase is dominated by phasons if the nuclear displacement is small, whereas it is dominated by amplitudons if the nuclear displacement from the paraelectric site is large. Soliton pinning by defects has been confirmed.