  Spin fluctuations are widely believed to play an important role in the
superconducting mechanisms of unconventional high-temperature superconductors.
Spin fluctuations have been observed in iron-based superconductors as well.
However, in some iron-based superconductors such as LaFePO$_{0.9}$, they have
not been observed by inelastic neutron scattering (INS). LaFePO$_{0.9}$ is an
iron-based superconductor with a low superconducting transition temperature
($T_{\rm c}$= 5 K), where line nodes are observed in the superconducting gap
function. The line-node symmetry typically originates from sign reversal of the
order parameter in spin-fluctuation-mediated superconductivity. This
contradiction has been a long-standing mystery of this superconductor. Herein,
spin fluctuations were found at high energies such as 30$-$50 meV with
comparable intensities to an optimally doped LaFeAs(O,F). Based on this
finding, the line-node symmetry can be explained naturally as
spin-fluctuation-mediated superconductivity.
