  Nonequilibrium phase transition plays a pivotal role in a broad physical
context from condensed matter to cosmology. Tracking the formation of
non-equilibrium phases in condensed matter is challenging and requires a
resolution of the long-range cooperativity on ultra-short timescales. Here, we
study the spontaneous symmetry breaking transformation of a charge-density wave
system from a stripe phase into a checkerboard state. Such a state is
thermodynamically forbidden, but is introduced through a system quench using
ultrashort, intense laser pulses. The dynamics is mediated by the soft modes
that unfold spontaneously and order the field on a timescale ~1 ps. Using the
coherent electron diffraction with ~100 fs resolution, we capture the entire
course and demonstrate nonergodic behavior proximal to symmetry breaking that
is crucial for stabilizing the hidden states. Remarkably, the thermalization
due to carriers cooling arrests the remnants of the transient orders into the
topological defects in the eventual state with distinct new properties that
last for more than 1 ns. The fundamental dynamics observed here opens an
intriguing perspective of controlling phase transitions in quantum materials
far from equilibrium.

Mesoscale and Nanoscale Physics