Spin-lattice relaxation in the mixed state of YBa2Cu3O7-δ and Doppler-shifted d-wave quasiparticles

We present calculations of the rate of planar Cu spin lattice relaxation in the mixed state of YBa2Cu3O7-δ due to (i) vortex vibrations and (ii) electron spin-flip scattering. We emphasize that both mechanisms give position-dependent rates due to the presence of vortices, and hence the magnetization recovery is characterized by a distribution of rates. We conclude that relaxation by vortex vibrations is too slow to be a significant factor in this material. Using a semiclassical model of Doppler shifted d-wave quasiparticles with a linear dispersion around the nodes, our calculation of the relaxation rate from electron spin-flip scattering shows partial agreement with experiment.