Collective and non-collective excitations in antiferroelectric and ferrielectric liquid crystals studied by dielectric relaxation spectroscopy and electro-optic measurements

Abstract

Collective and non-collective excitations in antiferroelectric and ferrielectric liquid crystals studied by dielectric relaxation spectroscopy and electro-optic measurements, Liquid Crystals, 23:5, 723-739,The dynamics of diVerent molecular modes in four antiferroelectric liquid crystal substances have been studied by a combination of spectroscopic methods.The fastest motion is the reorientation around the molecular long axis, here found in the low GHz range by time domain spectroscopy. The reorientation around the short axis has a characteristic frequency of about 10 kHz and is detected by frequency domain spectroscopy in the homeotropic con® guration. As for the collective excitations, the Goldstone and soft modes, characteristic of the ferroelectric phase, have counterparts in the antiferroelectric phase which appear very diVerent. There are two characteristic peaks in the spectrum, one at high frequency, about 100 kHz, the other at low frequency, about 10 kHz. The latter has often been mistaken for short axis reorientation and both have been attributed to soft modes. By combining diVerent experimental techniques and diVerent geometries it can be shown that neither is a soft mode, but both are collective modes of diVerent character: the high frequency mode corresponds tō uctuations where molecules in neighbouring layers are moving in opposite phase, the low frequency mode to phase¯uctuations in the helicoidal superstructure. In materials exhibiting a C* phase in addition to the C * a or C * c phases, an additional strong peak appears in at least one lower-lying phase adjacent to the C* phase. We show that this peak, which we call a hereditary peak, has nothing to do with the antiferroelectric or ferrielectric order, but is

Dielectric spectroscopy of liquid crystals. Electrodes resistivity and connecting wires inductance influence on dielectric measurements

“…This model gives us good results for high fre− quency modes detected in ITO cells [11,12]. Additionally, previously used the special shape function Af n [7] for esti− mation of high frequency losses can be derived using this model [13]. In ITO cells, the main problem is the high elec− trodes resistivity R. When ITO in measuring cell is replaced with gold, the next problem related to the wires inductance L appears.…”

“…In Ref. 7, the proposed way to improve it is the addition of special (phe− nomenological) part in imaginary part of dielectric spec− trum. In Ref.…”

“…In many papers, difficulties with high frequency measure− ments are indicated which are related to finite conductivity of ITO electrodes in measuring cells [1][2][3][4][5][6][7][8][9]. In Ref.…”

Electrooptic and dielectric properties of new antiferroelectric liquid crystal mixtures

“…The second curve is the amplitude of the low frequency response in the antiferroelectric phase. The mechanism behind this response is still controversial but can be in- terpreted as a modulation of the effective birefringence of the liquid crystal, since it is most clearly seen in samples with random planar alignment, where the linear response is suppressed by spatial averaging [8].…”

Dielectric spectroscopy of liquid crystals. Theoretical model of ITO electrodes influence on dielectric measurements

“…In many papers difficulties with high frequency measure− ments are indicated which are related to finite resistivity of ITO electrodes in measuring cells [1][2][3][4]. In Ref.…”