Critical Fluctuations in Triglycene Sulfate

Critical-point polarization fluctuations in ferroelectric triglycene sulfate have been observed at the Curie temperature by measuring the random noise voltages appearing between electrodes applied to the crystal. These noise voltages are interpreted in terms of the Nyquist noise associated with the crystal conductance and therefore can be used to determine the true crystal impedance at and near the transition temperature where the crystal is highly nonlinear. The polarization noise density at the Curie point has a simple relaxation spectrum with a time constant of 5.3×${10}^{-3\mathrm{}}$ sec, which furthermore is relatively independent of temperature near the Curie temperature. A thermal Barkhausen noise observed in barium titanate as the crystal is passed slowly through the transition temperature appears to be associated with the inherent domain structure of the sample.