Thermal depinning and motions of the flux lines inYBa 2 Cu 4 O 8 from NMR experiments

Abstract

The spin-lattice relaxation time T1, the spin echo dephasing time T2 and the NMR linewidth δv are used to study the spontaneous fluctuations under thermal excitation of the flux lines (FL) in YBa2Cu4O8. It is shown how the relaxation rate T1-1 is sensitive to the fluctuations of the transverse components of the local field at a frequency around ω = γH0, while T2-1 and δv probe the longitudinal components at frequencies smaller by a factor 103-105. Thus, from a combination of T1 T2 andδv measurements one derives microscopic insights on the degree of correlation in the motion of 2D pancakes belonging to adjacent superconducting planes, on the barriers for the pseudo-diffusion of the FL’s and on their field and temperature dependence. 2D NMR spectroscopy can also be used to evidence the diffusion of the FL’s and a comparison between YBa2Cu3O7 (YBCO123) and YBa2Cu48 (YBCO124) is briefly discussed. Finally for temperatures below Tirr(H the evolution of the amplitude of the63Cu NQR echo after sudden application and/or rotation of a magnetic field is used to investigate the FL flow in the critical state.