If non-chiral smectic C host phases are doped with chiral guest molecules, helically structured Sc* phases are formed by symmetry breaking. If the chiral dopants bear transverse dipoles a spontaneous polarization, Ps, will occur in these so-called induced Sc* phases. In 'classical' dopants the chiral centre(s) as well as the transverse dipole(s) are situated in alkyl side groups (type I dopants). The tilt-angle, reduced polarization, P,=P,/si n 0 @=tilt angle), in these mixtures are independent of the host with a linear dependence on the guest mole fraction, xG. As a material parameter the polarization power, 6, = (dP0/axG),,, has been introduced and is discussed in terms of the molecular structure of the dopant. Deviations of PO(xG) from linearity can be considered by a local field correction. Novel chiral dopants bearing the chiral centre@) and the transverse dipole(s) in a rigid core (type II dopants) exhibit a significant dependence of the magnitude, and sometimes even of the sign, of Po on the nature of the host. The different behaviours of the two dopant types are discussed by means of a microscopic model and can be understood in terms of a hard core guest-host interaction which influences the rotational distribution of the dopant molecules with respect to their long molecular axes.In 1980 we showed that ferroelectricity can be observed not only in neat smectic C* phases (like DOBAMBC) but also in achiral smectic C host phases which are doped with chiral guest molecules bearing sufficiently large transverse dipoles.' We call these mixed systems 'induced smectic C* phases'. The guest molecules need not necessarily form mesophases by themselves. However, owing to the pronounced sensitivity of the Sc phase stability towards impurities, in the case of nonmesogenic chiral dopants the range of induced S,* phases is often restricted to mole fractions, xG, of the guest molecules up to 0.2 at most (for example, see the phase diagram shown in Fig. 12). Therefore, the most desirable dopants are those which form cholesteric or even Sc* phases by themselves. In the latter case, mixed systems of guest and host molecules over the whole range 0 < xG < 1 could be obtained (Fig. 2).It will be of great interest to determine whether the spontaneous polarization, P,, of induced Sc* phases will be determined only by the transverse dipoles of the chiral guest molecules, or whether the dipolar properties of the host molecules also contribute to the value of P,.3 In this paper 100 I 1 I , , , I , , , 1 90 80 t ./ I WCh 9 70 i2 60 50 1 1 " " " ' 1 1 1 1 0 0.05 0.1 0.15 XG Fig. 1 Phase diagram of a mixed system of the glucopyranoside S 262 as a non-mesogenic chiral dopant in the achiral smectic C host phase 8007298 A A 0 0.2 0.4 0.6 0.8 1 XG 20 Fig. 2 Phase diagram of a mesogenic oxirane W 46 as chiral dopant in the achiral host phase 800721we will show that the answer to this question depends significantly on the general molecular structure of the guest molecules. We will further investigate how the polarization depends on the mole ...

Piezoelectric thin film AlN has great potential for on-chip devices such as thin-film resonator (TFR)-based bandpass filters. The AlN electromechanical coupling constant, K(2), is an important material parameter that determines the maximum possible bandwidth for bandpass filters. Using a previously published extraction technique, the bulk c-axis electromechanical coupling constant was measured as a function of the AlN X-ray diffraction rocking curve [full width at half maximum (FWHM)]. For FWHM values of less than approximately 4 degrees , K (2) saturates at approximately 6.5%, equivalent to the value for epitaxial AlN. For FWHM values >4 degrees , K(2) gradually decreases to approximately 2.5% at a FWHM of 7.5 degrees . These results indicate that the maximum possible bandwidth for TFR-based bandpass filters using polycrystalline AlN is approximately 80 MHz and that, for 60-MHz bandwidth PCS applications, an AlN film quality of >5.5 degrees FWHM is required.

Electrohydrodynamic convection due to the Carr-Helfrich (CH) effect has been extensively investigated in nematics. It has been predicted also for freely suspended smectic C and C* films. On the other hand, in smectic A films another type of electroconvection (vortex flow) has been described (Morris S. W., De Bruyn J. R. and May A., Phys. Rev. Lett. 65 (1990) 2378) but is not sufficiently understood by theory so far. We report experiments on freestanding smectic C films in lateral AC electric fields. A convective instability bifurcating continuously from the homogeneous ground state is found. Evidently the mechanism of the instability is of a similar type as in SA. The director field in the convection vortices is directly visualized in the optical texture.

The Goldstone mode has been measured in a homeotropically oriented ferroelectric liquid crystal in SSFLC cells and in free standing films by means of optical reflectivity. In cells the Goldstone mode was found only in the smectic C* phase as expected. In the free standing films, however, the Goldstone mode was still detectable several degrees above the bulk phase transition temperature to the smectic A phase. This effect is due to a molecular tilt in the boundary layers of the smectic A film caused by surface ordering.

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