Suppression of antiferromagnetic spin fluctuations in the collapsed phase of CaFe 2 As 2

Inelastic neutron-scattering measurements of ${\text{CaFe}}_{2}{\text{As}}_{2}$ 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.