NUCLEATION OF FOCAL CONIC DOMAINS IN SMECTIC-A LIQUID-CRYSTALS

The first-order structural transitions caused by an external magnetic field or by a surface anchoring are considered for a smectic A liquid crystal in the restricted geometry of a flat cell. The transition occu rs by nucleation of focal conic domains. The free energy of the system is calculated as a function of the order parameter rho = domain radiu s/cell thickness for finite value of the splay (K) and a saddle-splay (($) over bar K) elastic constants and the anchoring coefficient. For small rho the behavior of the system is defined by the balance of the stabilising elastic term and destabilising field (or anchoring) term. Homogeneous nucleation from ideal uniform state is hindered by high en ergetic barrier unless ($) over bar K is positive and comparable (or l arger) than K. A few possible scenarios of heterogeneous nucleation ha ve been considered, among them the nucleation at local layer undulatio ns and the nucleation at field-induced dislocations. The most effectiv e and general scenario is the nucleation at distortions (dislocations) caused by bulk or surface irregularities. The expansion of the domain (rho >> 1) is governed by the balance between the field and surface a nchoring and does not depend directly on the elastic constants. The sa turation field that provides the domain expansion can be smaller than the threshold field for other known mechanisms of the SmA instabilitie s.