SPIN DYNAMICS IN THE HIGH-TC 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 systematically investigated in five typical regimes: the pure and doped AF states (x = 0. 15, 0.37), the weakly doped (x = 0.45, 0.51, 0.53) and the heavily doped (x = 0.69, 0.92) metallic states which all exhibit a superconducting ground state. The hole doping strongly affects the AF order, dynamical AF correlations remain in the metallic state which, surprisingly, involves only in-plane spin components. An energy gap has been found in the spin excitation spectrum of any superconducting samples, however, it becomes weaker close to the I-M transition. A quite unusual T dependence of the spin excitation spectra was found.