The origin of unconventional superconductivity, including high-temperatureand heavy-fermion superconductivity, is still a matter of controversy. Spinexcitations instead of phonons are thought to be responsible for the formationof Cooper pairs. Using inelastic neutron scattering, we present the firstin-depth study of the magnetic excitation spectrum in momentum and energy spacein the superconducting and the normal states of CeCu2Si2. A clear spinexcitation gap is observed in the superconducting state. We determine alowering of the magnetic exchange energy in the superconducting state, in anamount considerably larger than the superconducting condensation energy. Ourfindings identify the antiferromagnetic excitations as the major driving forcefor superconducting pairing in this prototypical heavy-fermion compound locatednear an antiferromagnetic quantum critical point.
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