  Strain engineering provides the ability to control the ground states and
associated phase transition in the epitaxial films. However, the systematic
study of intrinsic characters and their strain dependency in transition-metal
nitrides remains challenging due to the difficulty in fabricating the
stoichiometric and high-quality films. Here we report the observation of
electronic state transition in highly crystalline antiferromagnetic CrN films
with strain and reduced dimensionality. Shrinking the film thickness to a
critical value of ~ 30 unit cells, a profound conductivity reduction
accompanied by unexpected volume expansion is observed in CrN films. The
electrical conductivity is observed surprisingly when the CrN layer as thin as
single unit cell thick, which is far below the critical thickness of most
metallic films. We found that the metallicity of an ultrathin CrN film recovers
from an insulating behavior upon the removal of as-grown strain by fabrication
of first-ever freestanding nitride films. Both first-principles calculations
and linear dichroism measurements reveal that the strain-mediated orbital
splitting effectively customizes the relatively small bandgap at the Fermi
level, leading to exotic phase transition in CrN. The ability to achieve highly
conductive nitride ultrathin films by harness strain-controlling over competing
phases can be used for utilizing their exceptional characteristics.
