Smectic dielectric properties

Dielectric properties of the smectic phase could be illustrated using the series of 4,4 -di-n-alkylazoxybenzene... 

Like the usual dielectrics, the ferroelectric smectic-C phase possesses contributions to its dielectric permittivity which are based on the deformation of molecular electron shells and the orientation of permanent molecular dipoles. The dielectric properties at low frequencies, however, are dominated by additional contributions, the Goldstone mode and the soft mode [43], which result from the presence of the spontaneous polarization P and the coupling between P and 9. 

For such materials, which may be smectic or nematic liquid crystals, the dielectric properties of the LC phase are anisotropic. Fora uniaxial LC phase, the dielectric tensor is diagonal such that... 

These changes reflect the molecular reorganization that takes place at transitions between different liquid crystal phases. The interpretation of the dielectric properties of smectic phases can be carried out using Eq. (34). Differences in orientational order in smectic phases are accounted for through the appropriate orientational order parameters given in Eq. (29), while other influences of the translational smectic order will affect the internal field factors and short range dipole-dipole interactions. For strongly... 

Most recent studies [28,29] of the dielectric properties of the SmC phase have focussed on the ferroelectric chiral smectic C phase, because of its importance in applications. The molecular interpretation of the principal permittivities is contained in Eqs. (27), with appropriate correlation fac-... 

The theory of the dielectric properties of chiral smectic liquid crystals is far from complete, particularly with respect to a molecular statistical approach. Simple Landau theory [31 ] gives expressions for the contributions of soft modes (jj g) and Goldstone modes (Xo) to the low frequency permittivity as ... 

In the standard description of the dielectric properties of the chiral tilted smectics worked out by Carlssonet al. [152], four independent modes are predicted. In the smectic C the collective excitations are the soft mode and the Goldstone mode. In the SmA phase the only collective relaxation is the soft mode. Two high frequency modes are connected to noncollective fluctuations of the polarization predicted by the theory. These two modes become a single noncollective mode in the smectic A phase. There is no consensus [153] as yet as to whether these polarization modes really exist. Investigations of the temperature dependence of the relaxation frequency for the rotation around the long axis show that it is a single Cole-Cole relaxation on both sides of the phase transition between smectic A and smectic C [154]. The distribution parameter a of the Cole-Cole function is temperature-dependent and increases linearly (a=a-pT+bj) with temperature. The proportionality constant uj increases abruptly at the smectic A to SmC transition. This fact points to the complexity of the relaxations in the smectic C phase. 

TABLE 1 Dielectric properties of typical liquid crystals. Cr crystal SmA, SmB etc. smectic subphases N nanatic I isotropic liquid,... 

This is a chiral smectic A with symmetry Dqo. Its properties are similar to those of the achiral SmA. However, close to the transition to the smectic C phase, the chiral smectic A phase shows interesting pretransitional phenomena in the dielectric and electrooptical effects (the so-caUed soft dielectric mode and electroclinic effect). They will be discussed in Chapter 13. 

Pozhidayev, E.P., Blinov, L.M., Beresnev, L.A., Belyayev, V.V. The dielectric anomaly near the transition from the smectic A to smectic C phase and visco-elastic properties of ferroelectric liquid crystals. Mol. Cryst. Liq. Cryst. 124, 359-377 (1985)... 

To some extent, development of smectic materials has been slow and usually come as an off shoot of work on nematics. For example, the binary mixture shown in (38) exhibits a room temperature smectic A, a short range nematic, and is of positive dielectric anisotropy.Such properties may be used in thermally addressed displays where a transition from a scattering to a clear state forms the optical effect. This transition may also be effected by an electric field. Obviously, more work on useful smectics is required although their inherent high viscosity is a problem. 

Let us keep to the simple case of the smectic A phase. The discrete symmetries which leave smectic A liquid crystals invariant are clearly the same as those for nematic phases. Optical, magnetic, dielectric and other properties will simply... 

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