AMORPHOUS DIELECTRIC BEHAVIOR OF INCOMMENSURATE FERROELECTRIC (PB0.45SN0.55)(2)P2SE6 AT LOW-TEMPERATURES

The complex dielectric susceptibility, epsilon=epsilon'+i epsilon '', of the incommensurate ferroelectric (Pb0.45Sn0.55)(2)P2Se6 has been me asured at various frequencies (300 Hz-100 kHz) and excitation voltages (0.15-7.5 V/mm) in the temperature range from 10 to 500 mK. It is fou nd that epsilon' of the crystal studied, similarly to amorphous materi als, exhibits a minimum in the millikelvin temperature region. The rea l part of the dielectric constant at the high-temperature side of this minimum shows an appreciable frequency dependence, and the temperatur e where the minimum occurs, T-min, obeys a power-law dependence on the frequency. This amorphous dielectric behavior of an incommensurately modulated crystal can be described using the two-level system model. W e have also observed a strong dependence of epsilon' on the magnitude of the measuring electric field at the low-temperature side of the min imum. We suggest that this nonlinear dielectric behavior is associated with a variation of the height of the potential barriers in the two-l evel systems as a function of the electric-field strength.