INELASTIC NEUTRON-SCATTERING STUDY OF THE SPIN DYNAMICS IN THE YBA2CU3O6+X SYSTEM

Inelastic neutron scattering experiments have been carried out on YBa2 Cu3O6+x single crystals in order to perform a systematic investigation of the spin dynamics in the various typical regimes: the pure and dop ed AF-states (x = 0.15, 0.37), the weakly doped (x = 0.45, 0.51), the heavily doped (x = 0.69, 0.92) and the overdoped (x = 1) metallic stat es. In the insulating state, the hole doping strongly affects the AF-o rder and yields a strong renormalization of the spin wave velocity. In the metallic state, dynamical AF-correlations remain, but the in-plan e correlation length becomes very short (xi/a congruent-to 0.8) as the doping increases. However, the AF-coupling between the two CuO2-layer s is not affected, except in the overdoped regime. At low temperatures the spin excitation spectrum exhibits an energy gap in any supercondu cting samples. Its value, E(G)/kT(c) congruent-to 3.5, becomes much we aker close to the I-M transition. Moreover, a pseudo-gap persists abov e T(c) in the heavily doped regime.