Nuclear spin-lattice relaxation induced by thermally fluctuating flux lines

Thermal motion of the flux lines (FL’s) gives rise to fluctuating magnetic fields. These dynamic fields couple to the nuclei in the sample and relax the nuclear spins. We provide a theoretical description of the nuclear spin-lattice relaxation process due to the fluctuating FL’s in clean type-II superconductors. At low fields, the calculated longitudinal relaxation rate ${\mathit{T}}_1^{\mathrm{-}1}$ is enormously enhanced at temperatures just below ${\mathit{T}}_{\mathit{c}}$. At intermediate fields, the resulting ${\mathit{T}}_1^{\mathrm{-}1}$ exhibits a peak structure as a function of temperature, which is eventually suppressed as the field is increased. The vibrational modes that have components propagating along the FL’s play an essential role in the ${\mathit{T}}_1^{\mathrm{-}1}$ process.