  There are increasing evidences that ferroelectric states at the nanoscale can
exhibit fascinating topological structures including polar vortices and
skyrmions,akin to those observed in the ferromagnetic systems. Here we report
the discovery of a new type of polar topological structure,ordered array of
nanoscale spirals in the tricolor ferroelectric superlattice system via phase
field simulations. This polar spiral structure is composed of fine ordered
semivortex arrays with vortex cores forming a wavy distribution. It is
demonstrated that this tricolor system has an ultrahigh Curie temperature of
1000 K and a temperature of 650 K for the phase transformation from spiral
structure to inplane orthorhombic domain structure, showing a greatly enhanced
thermal stability than the recently discovered polar vortex lattices in the
superlattice system. Moreover, the spiral structure has a net inplane
polarization that could be switched by an experimentally feasible irrotational
inplane field. The switching process involves a metastable vortex state, and is
fully reversible.
