SPIN-GAP AND MAGNETIC EXCITATIONS IN THE CUPRATE SUPERCONDUCTORS

We have analysed the spin-excitation spectrum as observed by neutron a nd NMR experiments in YBa2Cu3O6+x in the frame of a bidimensional t-t' -J model. Next-nearest neighbour hopping term t' is introduced in orde r to fit the shape of the Fermi surface revealed by photoemission. Wit hin the slave-boson approach, we study as well the d-wave superconduct ing state, as the singlet-RVB state which is expected to describe the normal phase of heavily-doped systems. In the superconducting state, d -wave pairing leads to the prediction of a resonance and a gap-structu re in the spin-excitation spectrum, in good agrement with neutron diff raction results for x = 0.92 and x = 1.0. Our calculations show a smoo th evolution of the spectrum from one phase to the other: the value of the threshold of excitations E(G) increases with doping, while the te mperature T-m at which the spin-gap opens regularly decreases, reachin g T-c only in the overdoped regime. This very atypical combined variat ion of E(G) and T-m with doping results of strong-correlation effects in presence of the realistic band-structure considered here.