  In conventional Bardeen-Cooper-Schrieffer (BCS) superconductors,
superconductivity occurs when electrons form coherent Cooper pairs below the
superconducting transition temperature Tc. Although the kinetic energy of
paired electrons increases in the superconducting state relative to the normal
state, the reduction in the ion lattice energy is sufficient to give the
superconducting condensation energy. For iron pnictide superconductors derived
from electron or hole doping of their antiferromagnetic (AF) parent compounds,
the microscopic origin for supercnductivity is unclear. Here we use neutron
scattering to show that high-Tc superconductivity only occurs for iron
pnictides with low-energy itinerant electron-spin excitation coupling and high
energy spin excitations. Since our absolute spin susceptibility measurements
for optimally hole-doped iron pnictide reveal that the change in magnetic
exchange energy below and above Tc can account for the superconducting
condensation energy, we conclude that the presence of both high-energy spin
excitations giving rise to a large magnetic exchange coupling J and low-energy
spin excitations coupled to the itinerant electrons is essential for high-Tc
superconductivity in iron pnictides.
