Discommensurations and pinning of modulation waves in the incommensurate phase of thiourea

Abstract

Dielectric measurements have been made in the incommensurate phase of SC(ND₂)₂ and S_1–x O _x C(ND₂)₂. At relatively fast scanning rates of temperature, new features of memory effects are observed in the mixed crystal. Very slow measurements (0.5 or 0.25 mK/min) have revealed discontinuous step-wise behaviour in ϵ′ and ϵ″ in a quasi-periodic way (0.6₂ K for the pure crystal and 0.7₀ K for the mixed one). The jumps, δϵ′/ϵ′, are roughly ten times larger for the mixed crystal. The series of anomalies are due to discommensurations pinned by impurities. The dependence of dielectric constants on scanning rates shows the very slow relaxation of modulation wave to equilibrium in the incommensurate phase, particularly for the pure crystal. The interesting new features are considered to be a delicate balance of pinning effects and slow relaxational process.