INELASTIC-NEUTRON-SCATTERING STUDY OF ANTIFERROMAGNETIC FLUCTUATIONS IN YBA2CU3O6.97

Inelastic-neutron-scattering experiments have been performed to study the spin-excitation spectrum in the overdoped regime of the high-T-c s uperconducting system YBa2Cu3O6+x, x=0.97. All the observed magnetic f luctuations are peaked at the antiferromagnetic wave vector. In the su perconducting state, the magnetic response is restricted over a limite d energy range ((h) over bar omega=33-46 meV). The imaginary part of t he magnetic susceptibility is characterized by two contributions defin ed by different q widths and different temperature dependences. A reso nant contribution, which displays a narrower q width, appears to be co nnected with superconductivity. The two contributions can be accounted for in the framework of the t-t'-J model in the presence of supercond uctivity. The main new feature in the overdoped regime is the partial disappearance of magnetic correlations for temperatures above the supe rconducting temperature.