NMR-STUDY OF SPIN DYNAMICS AT PLANAR OXYGEN AND COPPER SITES IN YBA2CU4O8

The planar-oxygen Knight shift 17K and nuclear spin-lattice relaxation rate (17T1)-1 have been determined in the superconductor YBa2Cu4O8. T he present oxygen results have been analyzed in connection with the pl anar-copper relaxation rate (63T1)-1 data. In the normal state, both 1 7K and (17T1T)-1 decrease as the temperature is lowered towards T(c) = 81 K, while (63T1T)-1 shows a maximum around 160 K. Comparison with Y Ba2Cu3O6+x data strongly suggests that both the q and the temperature dependences of the magnetic fluctuations in YBa2Cu4O8 Present a simila rity to those for the 60-K phase of YBa2Cu3O6+x in the normal state ab ove 110 K. In the superconducting state, (63T1T)-1 decreases significa ntly with decreasing temperature, while the slope for (17T1T)-1 shows a slight change near T(c). The considerable decrease in 17T1/63T1 belo w T(c) suggests that the loss of antiferromagnetic fluctuations is dom inant in the superconducting state.