Generalized Langevin-Debye model of the field dependence of tilt, birefringence, and polarization current near the de Vries smectic-A∗ to smectic-C∗ liquid crystal phase transition

摘要

In chiral smectic-A (Sm-A) liquid crystals, an applied electric field induces a tilt of the optic axis from thenlayer normal. When these materials are of the de Vries type, the electroclinic tilt susceptibility is unusually large,nwith the field-induced director reorientation accompanied by a substantial increase in optical birefringence withnessentially no change in the smectic layer spacing. In order to account for the observed electro-optic behavior, wenassume that the molecular orientation distribution in the Sm-A has two degrees of freedom: azimuthal orientationnand tilt of the molecular long axis from the layer normal, with the tilt confined to a narrow range of angles.nWe present a generalized Langevin-Debye model of the response of this orientational distribution to appliednfield that gives a field-induced optic axis tilt, birefringence, and polarization dependence that agrees well withnexperimental measurements and reproduces the double-peaked polarization current response characteristic of anfirst-order Sm-A∗–Sm-C∗ transition. Additionally, we find that the measured field-induced polarization and thenLangevin-Debye model predictions can be quantitatively described as pre-transitional behavior near the tricriticalnpoint of a recently published generalized 3D XY model of interacting hard rods confined to reorient on a cone innthe presence of an applied field.