Local-field dependence of the17Ospin-lattice relaxation and echo decay rates in the mixed state ofYBa2Cu3O7

We find that the planar and apical ${}^{17}\mathrm{O}$ NMR spin-lattice relaxation rate ${(1/T}_1)$ and the spin-echo decay rate ${(1/T}_2)$ are spatially dependent in the mixed state of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7.$ For both sites, ${1/T}_1$ and ${1/T}_2$ increase by a factor of 2 to 3 for nuclei close to the vortex cores. The data suggest that fluctuating fields from the low-frequency vibrations of the vortex lattice give rise to the similar echo decay observed at both sites. These vibrations also dominate ${1/T}_1$ for both sites below $T\lesssim 25\mathrm{K}.$ Above this temperature, the planar ${1/T}_1$ is dominated by spin-flip scattering with delocalized quasiparticles associated with the nodes in the superconducting gap showing that the position dependence arises from the effect of the supercurrents on the quasiparticle energy spectrum.