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

Cu-63(1) NQR signals and their relaxation have been measured in YBa2Cu 3O6.1 and ErBa2Cu3O6+x at temperatures between 10 K and 300 K. In YBa2 Cu3O6.1 a peak is observed in the spin-lattice relaxation, T-1(-1), ar ound 140 K. An explanation in terms of doped-hole motion is presented and alternative O(4)-O(5) diffusion mechanisms are critically discusse d. In ErBa2Cu3O6+x the low-temperature T-1(-1) is dominated by magneti c fluctuations of Er; quantitative agreement is found when the correct ground state and magnetic dynamic properties of the rare earth are ta ken into account.