Flux-line dynamics inYBa2Cu4O8fromY89NMR

Measurements of the $^{89}\mathrm{Y}$ NMR linewidth \ensuremath{\Delta}\ensuremath{\nu} and spin-lattice relaxation rate ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ have been performed in an oriented powder sample of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ (${\mathit{T}}_{\mathit{c}}$ = 81 K) for both ${\mathbf{H}}_{0}$\ensuremath{\parallel}c and ${\mathbf{H}}_{0}$\ensuremath{\perp}c at ${\mathit{H}}_{0}$ = 9.4 and 5.9 T. Evidence for the effects of thermal motion of flux lines (FL's) below ${\mathit{T}}_{\mathit{c}}$ 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 ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ an effective correlation time ${\mathrm{\ensuremath{\tau}}}_{\mathit{e}}$ 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 ${\mathrm{HgBa}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$. From the temperature dependence of \ensuremath{\Delta}\ensuremath{\nu} another correlation time \ensuremath{\tau}, different from ${\mathrm{\ensuremath{\tau}}}_{\mathit{e}}$, is obtained. It is argued that the correlation times derived from \ensuremath{\Delta}\ensuremath{\nu} and ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$, respectively, refer to different motional regimes below and above the irreversibility temperature ${\mathit{T}}_{\mathrm{irr}}$. \textcopyright{} 1996 The American Physical Society.