Local-field dependence of the 17O spin-lattice relaxation and echo decay rates in the mixed state of YBa2Cu3O7

We find that the planar and apical O-17 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 YBa2Cu3O7 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 less than or similar to 25K. 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.