NEUTRON-SCATTERING STUDY OF THE YBA2CU3O6+X HTC SUPERCONDUCTORS

We report an extensive neutron scattering study of the dynamical spin susceptibility chi(Q,omega) of YBa2Cu3O6+x system. Different regimes have been investigated: the pure and doped antiferromagnetic states, the weakly doped state and the metallic state up to O7. The hole doping destroys the static magnetic order, but dynamical antiferromagnetic spin fluctuations persist in the doped metallic state. In the superconducting samples, the spin excitations exhibit an anomalous temperature dependence due to the opening of a gap in the magnetic response. At large doping, the spin gap energy is close to the value predicted by the BCS value, but it is very much reduced when approaching the insulator-metal transition with x almost-equal-to 0.5. Furthermore, in the doped metallic state, a pseudo-gap opens at temperature much higher than T(c). The increase in T(c) from 60 K to 90 K can be associated with the increase in the Q-width of Imchi(Q,omega) that corresponds to the increase of the hole doping, while the characteristic energy scales of the spin fluctuations remain constant.