SPIN DYNAMICS IN THE METALLIC STATE OF THE HIGH-T-C SUPERCONDUCTING SYSTEM YBA2CU3O6+X

The spin dynamics in single crystals of YBa2Cu3O6+x has been successfu lly investigated, by inelastic neutron scattering (INS) experiments, a s a function of temperature in the metallic state over the whole dopin g range from the weakly doped (x = 0.45, 0.51) to the heavily doped (x = 0.69, 0.83, 0.92) and the overdoped (x = 0.97, 1) regimes. Dynamica l AF-correlations persist in all the metallic state. Actually, the ima ginary part of the magnetic susceptibility chi '' consists of two cont ributions which have different doping and temperature dependences. At low temperature, chi '' exhibits an energy gap in any superconducting samples which becomes much weaker close to the insulating-metallic tra nsition. To emphasize the characteristic features of the spin dynamics in YBCO, INS results obtained by other groups are compared with our e xperiments. Several theoretical approaches, which intend to describe t he energy line shape of the dynamical magnetic susceptibility, are als o discussed.