  Nucleation processes of mixed-phase states are an intrinsic characteristic of
first-order phase transitions, typically related to local symmetry breaking.
Direct observation of emerging mixed-phase regions in materials showing a
first-order metal-insulator transition (MIT) offers unique opportunities to
uncover their driving mechanism. Using photoemission electron microscopy, we
image the nanoscale formation and growth of insulating domains across the
temperature-driven MIT in NdNiO3 epitaxial thin films. Heteroepitaxy is found
to strongly determine the nanoscale nature of the phase transition, inducing
preferential formation of striped domains along the terraces of atomically flat
stepped surfaces. We show that the distribution of transition temperatures is
an intrinsic local property, set by surface morphology and stable across
multiple temperature cycles. Our data provides new insights into the MIT of
heteroepitaxial nickelates and points to a rich, nanoscale phenomenology in
this strongly correlated material.
