NEUTRON-SCATTERING INVESTIGATION OF THE SPIN DYNAMICS IN THE HIGH-TEMPERATURE SUPERCONDUCTING SYSTEM YBA2CU3O6+X

We report on inelastic neutron scattering experiments carried out on single-crystals of the YBa2Cu3O6+x system. The spin dynamics has been successfully investigated as a function of temperature in the metallic state over the whole doping range from the weakly doped (x = 0.45, 0.51) to the heavily doped (x = 0.69, 0.92) and the overdoped (x = 1) regimes. Dynamical AF-correlations remain in the metallic state but the in-plane correlation length decreases with doping whereas the AF-coupling between the two Cu(2) layers is not affected, except in the overdoped regime. At low temperatures, in any superconducting samples, an energy gap has been discovered in the spin excitation spectrum, with a value E(G)/kT(c) congruent-to 3.5 which becomes weaker close to the I-M transition. Moreover, in the heavily doped regime, a pseudo gap persists above T(c). A quite unusual T-dependence of the spin excitation spectrum was found, especially in the weakly doped regime.