Spin excitations in high-Tc superconductors : discussion of neutron and NMR experiments

The spin excitation spectrum as observed in neutron diffraction and NMR experiments in YBa2Cu3O6+x is theoretically examined on the basis of a bidimensionnal extended t-t'-J model in which next-nearest neighbour hopping term t' is considered in order to fit the shape of the Fermi surface observed in angle-resolved photoemission experiments. We study more specifically the superconducting phase in which d-wave pairing leads to the prediction of a resonance in S(Qo,ω) surimposed to a gap-structure in agreement with the neutron results. We also extend our calculations to the study of the RVB (resonant-valence-bond) singlet state and show that the latter is a good candidate to describe the normal phase above Tc of heavily-doped systems. (orig.)