LOW-ENERGY SPIN FLUCTUATIONS IN YBA2CU3O6.1 AND ERBA2CU3O6+X - A CU(1) T(1) NQR STUDY

Cu-63(1) NQR signals and their relaxation have been measured in YBa2Cu3O6.1 and ErBa2Cu3O6+x at temperatures between 10 K and 300 K. In YBa2Cu3O6.1 a peak is observed in the spin-lattice relaxation, T-1(-1), around 140 K. An explanation in terms of doped-hole motion is presented and alternative O(4)-O(5) diffusion mechanisms are critically discussed. In ErBa2Cu3O6+x the low-temperature T-1(-1) is dominated by magnetic fluctuations of Er; quantitative agreement is found when the correct ground state and magnetic dynamic properties of the rare earth are taken into account.