Soliton dynamics in a type-II incommensurate-commensurate system: Betaine calcium chloride dihydrate (BCCD)

The results of dielectric experiments on pure and Br-doped betaine calcium chloride dihydrate (BCCD) at variable temperatures T, hydrostatic pressures p, and quasistatic electric fields E are presented, which show the occurrence of field-induced commensurate (c) phases proving the existence of a square-wave modulated polarization at low T. With increasing T the shape of the modulation wave turns rather smoothly into an almost harmonic wave in the incommensurate (ic) region. Near the transition temperature c(delta=1/4)-->ic at T-1/4 indications for the onset of a gliding polarization wave (phason) in the ic phase, but also in the narrow c phases above T-1/4 with an almost harmonic modulation were detected in BCCD. This range extends up to T-i.