Ju n 20 04 Direct relation between the low-energy spin excitations and superconductivity of overdoped highT c superconductors

The dynamic spin susceptibility, χ′′(ω), has been measured over the energy range of 2 ≤ ω ≤ 10 meV for overdoped La2−xSrxCuO4. Incommensurate (IC) spin excitations are observed at 8 K for all superconducting samples for 0.25 ≤ x ≤ 0.28 with χ′′ peaking at ∼ 6 meV. The IC peaks at 6 meV become smaller in intensity with increasing x and, finally, become unobservable for a sample with x = 0.30 which has no bulk superconductivity. The maximum χ′′ decreases linearly with Tc(onset) in the overdoped region, implying a direct cooperative relation between the spin fluctuations and the superconductivity.