Flux-line dynamics in YBa2Cu4O8 from 89Y NMR

Measurements of the 89Y NMR linewidth Δν and spin-lattice relaxation rate T1-1 have been performed in an oriented powder sample of YBa2Cu4O8 (Tc = 81 K) for both H0 parallel c and H0 perpendicular c at H0 = 9.4 and 5.9 T. Evidence for the effects of thermal motion of flux lines (FL close-quote s) below Tc is presented and the microscopic correlation times of the FL motion are derived by using a simple model. From the temperature and field dependence of T1-1 an effective correlation time τe describing the pseudodiffusion of two-dimensional (2D) pancake vortices is extracted. The effective activation energy of the FL motion is found to be inversely proportional to the magnetic field, supporting the similar conclusion recently obtained in HgBa2CuO4+δ. From the temperature dependence of Δν another correlation time τ, different from τe, is obtained. It is argued that the correlation times derived from Δν and T1-1, respectively, refer to different motional regimes below and above the irreversibility temperature Tirr. copyright 1996 The American Physical Society