Critical slowing down of low-frequency dielectric relaxation in incommensurate

Abstract

The complex dielectric constant of tetramethylammonium tetrachlorozincate (TMA-${\mathrm{ZnCl}}_{4}$) in the incommensurate phase has been measured in the frequency range from 5 kHz to 13 MHz. Two relaxation processes were observed, one fast and one slow, which is common in ${\mathit{A}}_{2}$${\mathit{BX}}_{4}$ crystals. In contrast to earlier investigations of TMA-${\mathrm{ZnCl}}_{4}$, however, we find that both the slow relaxation process, which dominates \ensuremath{\epsilon}(0) near the lock-in transition, and the fast process exhibit critical slowing down. This may indicate the coexistence of incommensurate and commensurate domains, or be related to a recent suggestion of Holakovsky, Dvorak, Fousek, and Kroupa concerning the pinning of solitons.