  The collective dynamics of topological structures have been of great interest
from both fundamental and applied perspectives. For example, the studies of
dynamical properties of magnetic vortices and skyrmions not only deepened the
understanding of many-body physics but also led to potential applications in
data processing and storage. Topological structures constructed from electrical
polarization rather than spin have recently been realized in ferroelectric
superlattices, promising for ultrafast electric-field control of topological
orders. However, little is known about the dynamics of such complex extended
nanostructures which in turn underlies their functionalities. Using
terahertz-field excitation and femtosecond x-ray diffraction measurements, we
observe ultrafast collective polarization dynamics that are unique to polar
vortices, with orders of magnitude higher frequencies and smaller lateral size
than those of experimentally realized magnetic vortices. A previously unseen
soft mode, hereafter referred to as a vortexon, emerges as transient arrays of
nanoscale circular patterns of atomic displacements, which reverse their
vorticity on picosecond time scales. Its frequency is significantly reduced at
a critical strain, indicating a condensation of structural dynamics.
First-principles-based atomistic calculations and phase-field modeling reveal
the microscopic atomic arrangements and frequencies of the vortex modes. The
discovery of subterahertz collective dynamics in polar vortices opens up
opportunities for applications of electric-field driven data processing in
topological structures with ultrahigh speed and density.
