Temperature dependent anisotropy of Cu(2) nuclear relaxation rate in YBa₂Cu₃O₇ below T_c

We have measured the anisotropy of the planar Cu nuclear spin-lattice relaxation rate in YBa2Cu3O7 in a small magnetic field (0.44 T). The anisotropy is almost temperature independent in the normal state but decreases rapidly below Tc, shows a minimum near 70 to 80 K and increases again at lower temperatures. Since the ratio of the relaxation rates at the Cu and Oxygen sites is independent of temperature below Tc, indicating that antiferromagnetic spin correlations still remain in the superconducting state and that the superconducting gap is nearly q-independent, the T-dependence of the anisotropy is difficult to understand within the framework of the models of isotropic antiferromagnetic spin fluctuations that have been proposed to analyze the normal state NMR results. © 1991.