  The origin of unconventional superconductivity, including high-temperature
and heavy-fermion superconductivity, is still a matter of controversy. Spin
excitations instead of phonons are thought to be responsible for the formation
of Cooper pairs. Using inelastic neutron scattering, we present the first
in-depth study of the magnetic excitation spectrum in momentum and energy space
in the superconducting and the normal states of CeCu2Si2. A clear spin
excitation gap is observed in the superconducting state. We determine a
lowering of the magnetic exchange energy in the superconducting state, in an
amount considerably larger than the superconducting condensation energy. Our
findings identify the antiferromagnetic excitations as the major driving force
for superconducting pairing in this prototypical heavy-fermion compound located
near an antiferromagnetic quantum critical point.
