Dielectric study of the modulated smectic C*‐uniform smectic C transition in a magnetic field

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Abstract
The in‐plane component of the dielectric constant of chiral smectic p‐decyloxybenzilidene‐p'‐amino‐2‐methylbutyl cinnamate is measured as a function of temperature and magnetic field applied parallel to the smectic layers. The transition from the modulated smectic C* to the uniform smectic C phase is accompanied by a drop in the dielectric constant in analogy to the incommensurate‐commensurate transition in ferroelectrics. The C*‐C transition line exhibits a large hysteresis and the critical field for the unwinding of the helix increases with decreasing temperature except close to the λ‐line. The obtained results are in qualitative disagreement with the predictions of the Landau‐de Gennes model and seem to suggest a different mechanism for the unwinding of the smectic C* helix.