TEMPERATURE-DEPENDENCE OF THE ANISOTROPY OF THE PLANAR OXYGEN NUCLEAR-SPIN-LATTICE RELAXATION RATE IN YBA2CU3OY

We report measurements of the full planar O-17 nuclear spin-lattice re laxation rate tensor in optimally and underdoped YBa2Cu3Oy (y=6.96 and 6.63). The anisotropy varies strongly with temperature in both sample s: for the y = 6.63 sample, the ratio of the relaxation rate measured with the applied field in the ab plane and oriented parallel to the co pper-oxygen bond axis to that measured with the applied field in the a b plane and oriented perpendicular to the bond axis changes by over 25 % from room temperature to T-c. The consequences of these results for theoretical models are discussed. They suggest that a second spin degr ee of freedom independent of the Cu(3d(x2-y2))-O(2p sigma) band exists in the CuO2 planes of YBa2Cu3Oy.