  The relationship between antiferromagnetic spin fluctuations and
superconductivity has become a central topic of research in studies of
superconductivity in the iron pnictides. We present unambiguous evidence of the
absence of magnetic fluctuations in the non-superconducting collapsed
tetragonal phase of CaFe2As2 via inelastic neutron scattering time-of-flight
data, which is consistent with the view that spin fluctuations are a necessary
ingredient for unconventional superconductivity in the iron pnictides. We
demonstrate that the collapsed tetragonal phase of CaFe2As2 is non-magnetic,
and discuss this result in light of recent reports of high-temperature
superconductivity in the collapsed tetragonal phase of closely related
compounds.
