NON-LINEAR DIELECTRIC PROPERTIES OF KTa_(1-x)Nb_xO_3

摘要

We report measurements of the non-linear dielectric constant in the mixed crystal KTa_(1-x)Nb_xO_3 with x = 15.7% and 1.2%. For both concentrations the dielectric peak is observed to decrease with increasing field and to shift towards higher temperature. The dispersion ε' (ω), is also suppressed by the field. On the framework of Landau theory, we fit ε'(E) with an expansion in a power series of the applied dc field E. ε~((2)) is seen to exhibit two distinct temperature dependencies. At high temperatures, it scales as (T-T_c)~(-γ) where γ is found to be in fair agreement with Landau theory for low concentrations. At lower temperatures it then crosses over to a much weaker dependence. This crossover is interpreted as a transition from a weakly correlated (dynamic) regime to a strongly correlated (quasistatic) regime of polar clusters. The non-analyticity due to these polar clusters is discussed.