FLUX-LINE DYNAMICS IN YBA2CU4O8 FROM Y-89 NMR

Measurements of the Y-89 NMR linewidth Delta nu and spin-lattice relax ation rate T-1(-1) have been performed in an oriented powder sample of YBa2Cu4O8 (T-c = 81 K) for both H-0 parallel to c and H-0 perpendicul ar to(c) at H-0 = 9.4 and 5.9 T. Evidence for the effects of thermal m otion of flux lines (FL's) below T-c 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 T-l(-1) an effective co rrelation time tau(e) describing the pseudodiffusion of two-dimensiona l (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 HgBa2Cu O4+delta. From the temperature dependence of ay another correlation ti me tau, different from tau(e), is obtained. It is argued that the corr elation times derived from Delta nu and T-l(-1), respectively, refer t o different motional regimes below and above the irreversibility tempe rature T-irr.