  Nanoscale ferroelectric topologies such as vortices, anti-vortices, bubble
patterns etc. are stabilized in thin films by a delicate balance of both
mechanical and electrical boundary conditions. A systematic understanding of
the phase stability of bubble domains, particularly when the above factors act
simultaneously, remains elusive. Here we present first-principle-based
simulations in combination with scanning probe microscopy of ultrathin
epitaxial (001) PbZr0.4Ti0.6O3 heterostructures to address this gap. The
simulations predict that as-grown labyrinthine domains will transform to
bubbles under combinations of reduced film thickness, increased mechanical
pressure and/or improved electrical screening. These topological transitions
are explained by a common fundamental mechanism. Namely, we argue that,
independently of the nature of the driving force, the evolution of the domain
morphology allows the system to conserve its original residual depolarization
field. Thereby, the latter remains pinned to a value determined by an external
or built-in electric bias. To verify our predictions, we then exploit
tomographic atomic force microscopy to achieve the concurrent effect of
reducing film thickness and increased mechanical stimulus. The results provide
a systematic understanding of phase stability and demonstrate controlled
manipulation of nanoscale ferroelectric bubble domains.
