Creep dynamics of structural defects in ferroelectric liquid crystals with chevron geometry

Abstract

Electric-field induced motions of zigzag walls forming defects in the chevron structure of surface stabilized ferroelectric liquid crystals are investigated. An explicit experimental evidence, including direct microscopic observation, is reported that the walls display viscous creep motions at small length scales, being dependent on the amplitude U and frequency f of an applied voltage. The relaxation-to-creep transition is analyzed using both the electro-optical response and dynamic hysteresis data recorded at different values of U and f. It is shown that, at any fixed temperature below the chevron-to-bookshelf transition point, there exists a critical line in the U-f plane, separating the relaxation and creep dynamic regions. In contrast to field-activated nonlocal excitations discovered in various disordered media, the creep motions of zigzag walls are found to occur not only at low or very low field frequencies.

Comparison of dielectric and optical responses of chevron ferroelectric liquid crystals

“…6 is a critical line separating regions of different dynamic behaviours of defects. The region on the left side of the critical line refers to localized motions of molecules constituting defects, while the region on the right side of this line corresponds to creep and sliding motions of zig-zag walls [12,16]. It is remarkable that both the creep and sliding wall motions do not affect almost all the optical response at large voltage amplitudes (see Fig.…”

Perturbation theory for ac-driven interfaces in random media

“…In recent years, also the problem of ac-driven elastic interfaces in disordered systems has gained experimental interest [5][6][7]. In experiments, considerable attention is devoted to the behaviour of the ac-susceptibility of ferroelectric thin films or ultrathin ferromagnetic multilayers, which is believed to be related to the motion of domain walls in these systems.…”

Strongly nonlinear dynamics of ferroelectric liquid crystals

“…In particular, the dependence of the diagrams on U 0 is not monotonous for a given (small) frequency. This can be considered as a simple consequence of the fact that relatively large contributions to the low-frequency part of the nonlinear response of SSFLCs not only come from collective reorientations of molecules but also from other dynamic processes, such as ionic currents and movements of walls of zig-zag defects in the regular chevron structure of SSFLCs [27][28][29]. As well known, the chevron structure spontaneously forms in SSFLCs, when their thickness is not very small.…”

“…The effect of straightening of chevron smectic layers is especially distinct in the case of strong applied fields (constant or alternating) and then the smectic layers can be treated as being upright. Thus, the linear size of such uprighted smectic layers in the direction perpendicular to cell plates is equal to d. Spatial fluctuations of molecular orientations can be investigated separately within each of the smectic layers by determining the space dependence of the azimuthal angle φ between vectors of the local polarization and the external electric field [27][28][29]. Accordingly, the fluctuations of φ can be investigated only along one direction, say the x direction, perpendicular to plates confining the sample.…”

“…Generally, these coefficients do not have all identical functional form. Initial values of a i (in the absence of the external electric field) are determined by appropriate boundary conditions, involving surface anchoring interactions [27][28][29][30][31]. After the electric field is switched on, the coefficients a i change according to electric field oscillations.…”