  A stochastical description is applied in order to understand how
ferroelectric structures can be formed. The predictions are compared with
experimental data of the so-called electrical fixing: Domains are patterned in
photorefractive lithium niobate crystals by the combination of light-induced
space-charge fields with externally applied electrical fields. In terms of our
stochastical model the probability for domain nucleation is modulated according
to the sum of external and internal fields. The model describes the shape of
the domain pattern as well as the effective degree of modulation.
