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 ¹⁷O NMR spin-lattice relaxation rate (1/T₁) and the spin-echo decay rate (1/T₂) are spatially dependent in the mixed state of YBa₂Cu₃O₇. For both sites, 1/T₁ and 1/T₂ 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/T₁ for both sites below T≲25K. Above this temperature, the planar 1/T₁ 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.