  We use angle-resolved photoemission spectroscopy (ARPES) and density
functional theory (DFT) calculations to study the electronic structure of
CaFe$_2$As$_2$ in previously unexplored collapsed tetragonal (CT) phase. This
unusual phase of the iron arsenic high temperature superconductors was hard to
measure as it exists only under pressure. By inducing internal strain, via the
post growth, thermal treatment of the single crystals, we were able to
stabilize the CT phase at ambient-pressure. We find significant differences in
the Fermi surface topology and band dispersion data from the more common
orthorhombic-antiferromagnetic or tetragonal-paramagnetic phases, consistent
with electronic structure calculations. The top of the hole bands sinks below
the Fermi level, which destroys the nesting present in parent phases. The
absence of nesting in this phase along with apparent loss of Fe magnetic
moment, are now clearly experimentally correlated with the lack of
superconductivity in this phase.
