Dirac quasiparticles and spin-lattice relaxation in the mixed state

We present the results of quantum-mechanical calculations, using the singular gauge transformation of Franz and Tes̆anović [Phys. Rev. Lett. 80, 4763 (1998)], of the rate of planar Cu spin-lattice relaxation due to electron spin-flip scattering in the mixed state of high-Tc cuprate superconductors. The results show a nonmonotonic temperature and frequency dependence that differs markedly from semiclassical Doppler-shifted results, and challenges the assertion that recent experimental observations of the rate of planar Cu and O spin-lattice relaxation in the mixed state of YBa2Cu3O7-δ point to antiferromagnetic spin fluctuations as a better candidate for the elementary excitations of the superconducting state.