Nuclear magnetic relaxation of ¹H in high-T_c superconductor YBa₂Cu₃O_6.9H_x (x = 0.18 and 0.63)

Spin-lattice relaxation time T₁ (at 29 and 58 MHz) and T_1ρ{variant} of ¹H have been measured in YBa₂Cu₃O_6.9H_x (x = 0.18 and 0.63) between 300 and 77 K. In the fresh sample with x = 0.18, T₁ has a minimum near 260 K, which is attributed to the diffusion of hydrogen in the oxygen vacancies in the Cu(1)-O plane with an activation energy of E_a = 0.29 eV. For the sample with x = 0.63, another diffusion mode with E_a = 0.25 eV exists at lower temperature around 167 K. This mode might be due to the diffusion in the interstitial site in the Y-plane. But all of these modes disappeared in the temperature dependence of T₁ after keeping the samples in ampoules in three months, and no evidence of diffusion was found either in T_1ρ{variant} measurements. The temperature dependence of T₁ for x = 0.18 showed an abrupt change at T_c ≊ 90 K, suggesting the effect of superconductivity.