THERMAL DEPINNING AND MOTIONS OF THE FLUX LINES IN YBA2CU4O8 FROM NMREXPERIMENTS

The spin-lattice relaxation time T-1, the spin echo dephasing time T-2 and the NMR linewidth delta nu are used to study the spontaneous fluc tuations under thermal excitation of the flux lines (FL) in YBa2Cu4O8. It is shown how the relaxation rate T-1(-1) is sensitive to the fluct uations of the transverse components of the local field at a frequency around omega(L) = gamma H-0, while T-2(-1) and delta nu probe the lon gitudinal components at frequencies smaller by a factor 10(3)-10(5). T hus, from a combination of T-1, T-2 and delta nu measurements one deri ves microscopic insights on the degree of correlation in the motion of 2D pancakes belonging to adjacent superconducting planes, on the barr iers for the pseudo-diffusion of the FL's and on their field and tempe rature dependence. 2D NMR spectroscopy can also be used tio evidence t he diffusion of the FL's and a comparison between YBa2CU3O7 (YBCO123) and YBa2Cu4O8 (YBCO124) is briefly discussed. Finally for temperatures below T-irr(H) the evolution of the amplitude of the Cu-63 NQR echo a fter sudden application and/or rotation of a magnetic field is used to investigate the FL flow in the critical state.