SPIN DYNAMICS IN HIGH-T-C COPPER OXIDES

We analyze the spectrum of magnetic excitations as observed by neutron diffraction and NMR experiments in YBa2Cu3O6+x, in the frame of the s ingle-band t-t'-J model in which the next-nearest-neighbour hopping te rm has been introduced in order to fit the shape of the Fermi surface revealed in photoemission. Within the slave-boson approach, we have as well examined the d-wave superconducting state, and the singlet-RVB p hase appropriate to describe the normal state of heavily-doped systems . Oar calculations show a smooth evolution of the spectrum from one ph ase to the other, with the existence of a spin-gap in the frequency-de pendence of chi ''(($) over right arrow Q, omega). The valve of the th reshold of excitations E(G) as found to increase with doping, while th e characteristic temperature-scale T-m at which the spin-gap opens, ex hibits a regular decrease, reaching T-m only in overdoped regime. This very atypical combined variation of E(G) and T-m with doping results of strong-correlation effects in presence o realistic band structure. We point out the presence of a resonance in the omega-dependence of ch i ''(($) over right arrow Q,omega) in good agreement with the neutron different action results obtained at x = 0.92 and x = 1.0. This resona nce is analyzed as a dynamical Kohn anomaly of the second kind in the Cooper channel. Finally, toe examine the ($) over right arrow q-depend ence of the dynamical susceptibility allowing to study the magnetic co rrelation length xi as a function of Hoping, frequency and temperature .