SPIN FLUCTUATIONS IN THE METALLIC AND SUPERCONDUCTING STATES IN THE HIGH-T(C) SYSTEM YBA2CU3O6+X

The spin dynamics in single-crystals of YBa2Cu3O6+x has been successfully investigated by inelastic neutron scattering experiments, 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 becomes very short (xi/a congruent-to 0.8) as the doping increases. However, the AF coupling between the two CuO2 layers is not affected, except in the overdoped regime. At low temperatures, the spin excitation spectrum exhibits an energy gap in any superconducting sample. Its value, E(G)/kT(c) congruent-to 3.5, becomes much weaker close to the I-M transition. Moreover, a pseudo-gap persists above T(c) in the heavily doped regime. The results are compared with the magnetic excitation spectra found for the typical heavy fermion materials CeCu6 and CeRu2Si2.