Undulation instabilities in smecticCphases

Abstract

Microscopic studies on thermally induced undulation instabilities of the smectic $C$ phase of pentyloxybenzylideneheptylaniline ($\overline{5}\mathrm{BA}7$) and pentylphenyloctylthiobenzoate ($5\mathrm{PTB}\overline{8}$) are reported. The $A-C$ transitions of these compounds appear to be second order. The tilt angle in smectic $C$ is strongly temperature dependent. The biaxial order is small. Layer undulations can be produced with suitable samples by a stepwise reduction of the temperature. A few hundredths of a degree are sufficient. The preferred undulation mode has a wave vector parallel to the $C_2$ axis. A theoretical analysis and comparison with data from the smectic $A$ phase shows that the dilation modulus of smectic $C$ is about two orders of magnitude smaller than that of smectic $A$. The temperature dependence of the modulus near the transition to smectic $A$ is discussed theoretically on the basis of a Landau expansion.