Polarity sensitive electric responses in a twisted smectic-C liquid crystal

摘要

The observation of two polarity-sensitive electrical responses found in the low-frequency (<1 Hz) regimenof a square wave field is reported for an achiral rodlike smectic-C liquid crystal with negative dielectric andnconductivity anisotropies and in the 90◦-twisted configuration. The first involves a transient director modulationnappearing at each polarity reversal and vanishing under steady field conditions. The instability is polaritynsensitive, with the maximum distortion localized near the negative electrode instead of the sample midplane.nThis is inferred from the wave-vector orientation alternating in the two halves of the driving cycle between thenalignment directions at the two substrates. Various electro-optic characteristics of this temporal phenomenon arenalso described. Following a similar observation in nematic liquid crystals, we associate the transient periodic ordernwith the Carr-Helfrich mechanism assisted by quadrupolar flexoelectric polarization obtaining under electric fieldngradients. The second polarity-sensitive effect manifests in the relative shift of the periodic Fr´eedericksz patternnupon field reversal. The shift, which is linear in field for lowfields, tends to saturate for large fields. It is interpretednas due to flexoelectric polarization associated primarily with the c director twist about the layer normal. A modelninvolving a periodic wedgelike band, which has the twist localized within it and is flanked by two uniformly andntransversely aligned regions, accounts for the flexoelectric shift of the optical pattern.