Suppression of antiferromagnetic spin fluctuations in the collapsed phase of CaFe2As2

Inelastic neutron-scattering measurements of CaFe2As2 under applied hydrostatic pressure show that the antiferromagnetic spin fluctuations observed in the ambient pressure, paramagnetic, and tetragonal (T) phase are strongly suppressed, if not absent, in the collapsed tetragonal (cT) phase. These results are consistent with a quenched Fe moment in the cT phase and the strong decrease in resistivity observed upon crossing the boundary from the T to cT phases. The suppression or absence of static antiferromagnetic order and dynamic spin fluctuations in the nonsuperconducting cT phase supports the notion of a coupling between spin fluctuations and superconductivity in the iron arsenides.