Field Dependence of Helicoidal Pitch and Its Phenomenological Theory

The field dependence of the helicoidal pitch in the ferroelectric liquid crystal DOBAMBC are studied. Divergence of the pitch is observed at a certain critical field, and two critical fields exist when the field is varied up and down. The dependence of both critical fields on temperature is also obtained experimentally, but the ratio of the critical fields, 3/2, is almost independent of temperature. The Landau expansion of the free energy involving a helicoidal structure is used to interpret the experimental results.