Critical Region in Ferroelectric Triglycine Sulfate

Abstract

Careful measurements have been made for triglycine sulfate (TGS), in the vicinity of the critical point ${T}_{c}$, on the temperature dependence of the spontaneous polarization and the field dependence of the polarization at ${T}_{c}$. It is found that deviations from the mean-field theory occur only within ${T}_{c}\ensuremath{-}T<\ensuremath{\Delta}{T}_{c}\ensuremath{\approx}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}\ifmmode^\circ\else\textdegree\fi{}$C and $E<{E}_{c}\ensuremath{\approx}0.5$ V/cm. The coherence length ${\ensuremath{\xi}}_{0}$ at 0\ifmmode^\circ\else\textdegree\fi{}K is calculated on the basis of Menyh\'ard's theory, which gives a condition for the breakdown of the mean-field theory. The minimum values of ${\ensuremath{\xi}}_{0}$ obtained by $\ensuremath{\Delta}{T}_{c}$ and ${E}_{c}$ are 7 and 6 \AA{}, respectively, which are very close to the average distance, 6.8 \AA{}, between statistical units of TGS.