SPIN EXCITATIONS IN HIGH-TC SUPERCONDUCTORS - DISCUSSION OF NEUTRON AND NMR EXPERIMENTS

The spin excitation spectrum as observed in neutron diffraction and NM R experiments in YBa2Cu3O6+x is theoretically examined on the basis of a bidimensionnal extended t-t'-J model in which next-nearest neighbou r hopping term t' is considered in order to fit the shape of the Fermi surface observed in angle-resolved photoemission experiments. We stud y more specifically the superconducting phase in which d-wave pairing leads to the prediction of a resonance in S(Q(o), omega) surimposed to a gap-structure in agreement with the neutron results. We also extend our calculations to the sudy of the RVB (resonant-valence-bond) singl et state and show that the latter is a good candidate to describe the normal phase above T(c) of heavily-doped systems.