A new phenomenon—ferroelectric domain breakdown, or the for-mation of string-like domains—is observed in various bulk ferroelec-tric crystals under a high inhomogeneous electric field generated at
the tip of a high-voltage atomic force microscope. The domains have
a high aspect ratio when their length exceeds their lateral size by two
or three orders of magnitude. This chapter reviews some of the re-cent advances in understanding the nature of this phenomenon. The
main driving force for ferroelectric domain breakdown is not the tip’s
electric field, which is negligibly weak near the apex of the long do-main, but rather internal forces generated owing to the minimization
of depolarization field energy when the domain elongates. Domain
breakdown is considered an extreme manifestation of Coulomb in-stability, which is caused by repulsion of bound charges near the
inverted domain apex. This instability leads to strong pulling of the
domain apex into a region where the tip-induced electric field is
negligibly small.

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