Rotational modes, ferroelectrics

It has to be noted that Kremer et al conducted their studies on purposely unoriented samples. They did so because the presence of the optically active end groups in the side mesogen chains usually leads to chirality of the mesophase. As a result, polymers 31, 37, 39, 41 and 43 exhibit the cholesteric (chiral nematic) phase, and polymer 40, 41 and 43 the chiral smectic C phase.Since the smectic C shows ferroelectric-ity, in order to separate the molecular rotational modes from the ferroelectric Goldstone and soft modes (cf. Section 4.3), samples should be unoriented. On the other hand, measurements performed on the oriented sample of the chiral C phase of polymer 43 led Vallerien et al to the observation of ferroelectric modes. 

One can also wonder, which is the original cause of the ferroelectric-para-electric phase transitions in KDP-type crystals the H-hopping or the vibrations (rotations) of the H-bonded molecules. It has been shown that the angular displacements and the H-sites are coupled, thus the vibrations of molecules destabilize and facilitate the H-hopping. This coupling is essential for understanding the interactions between the lattice-mode vibrations in crystals, and the transformations in hydrogen bonds. Also other features of the KDP crystals, like the existence of soft modes, can be explained in this way. 

We can answer the last question if consider a constraction of the so-called surface stabilised ferroelectric liquid crystal cell or simply SSFLC ceU [9]. Such SSFLC cell is only few micrometers thin and, due to anchoring of the director at the surfaces, the intrinsic helical stmcture of the SmC is unwound by boundaries but a high value of the spontaneous polarisation is conserved. The cell is con-stracted in a way to realise two stable states of the smectic C liquid crystal using its interaction with the surfaces of electrodes, see Fig. 13.6a. First of all, in the SSFLC cell, the so-called bookshelf geometry is assumed the smectic layers are vertical (like books) with their normal h parallel the z-axis. Then the director is free to rotate along the conical surface about the h axis as shown in Fig. 13.6b (Goldstone mode). It is important that, to have a bistability, the director should be properly... 

Gouda, F., Skarp, K., Andersson, G., Lagerwall, S. T., Stebler. B., and Helgee, B., Soft mode response and rotational viscosity of a ferroelectric liquid crystal polymer, Jpn. J. Appl. Phys., 34, 5653-5657 (1995). 

In the case of a helical pitch shorter than the wavelength of visible light, colouration due to selective reflection from the helical structure disappears and the rotation of the optical axis by deformation of the helical structure can be used for optical switching. This mode is called the Deformed-Helix Ferroelectric (DHF) liquid crystal mode. The DHF mode can realize stable continuous grey scale. Moreover, the viewing angle dependence of the contrast is small even in... 

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