  We present the results of quantum-mechanical calculations, using the singular
gauge transformation of Franz and Tesanovic, 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 non-monotonic
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 YBCO point to antiferromagnetic spin fluctuations as a better
candidate for the elementary excitations of the superconducting state.
