NUCLEAR-SPIN-LATTICE RELAXATION RATE OF PLANAR OXYGEN IN YBA2CU3O6.52 AND YBA1.92SR0.08CU3O7 SINGLE-CRYSTALS

A full account is given of an NMR investigation of planar oxygen in the normal state (T > T(c)) of two characteristic superconducting Y-Ba-Cu-O single CrYstalS: YBa2Cu3O6.52 representing the underdoped ''60-K phase,'' and YBa1.92Sr0.08Cu3O7 representing the overdoped ''90-K phase.'' (We argue that Sr impurities in the latter crystal do not significantly change the behavior of the system.) Data are analyzed within the framework of the Mila-Rice spin Hamiltonian and using inelastic-neutron-scattering results, in order to obtain detailed and consistent information on the antiferromagnetic fluctuations (AFF) in this system. Within the model, the filtering of the AFF at the oxygen site is always found to be imperfect, even for the AFF that are localized to a maximum degree in the q space around Q(AF). In our overdoped sample this is also confirmed experimentally, in contradiction to other published data. Any shape of chi''(q-Q(AF)) that falls off slower than Gaussian is found to be incompatible with experimental data. In the underdoped regime, the observed temperature dependence of the T1 anisotropy is in clear contradiction with any single-component model. A possible explanation could involve an extra contribution to relaxation coming from spinless quasiparticles.