O-17 NMR ANISOTROPIC PROPERTIES OF THE PLANAR OXYGEN SITES IN AN YBA2CU3O6.5 SINGLE-CRYSTAL

We have measured the Knight shift and the spin-lattice relaxation of the O(2, 3) planar sites in a single crystal of YBa2Cu3O6.5. We report the anisotropy of these quantities, together with their striking temperature dependences. Our data are not easy to understand in any theoretical model presently available. One approach is to consider the T1 data to indicate that, for temperatures greater than T(c), some electronic fluctuations develop preferentially on the O(2p) orbitals, and not on the Cu(3d) orbitals. However, such an analysis seems incompatible at first sight with our Knight shift data, because the planar oxygen Knight shift tensor appears to result from a single electronic susceptibility and not two different susceptibilities. We, however, propose an alternative "two-fluid" model analysis which may account qualitatively for the NMR features observed. The originality of our experimental work, if compared with some other, lies to a large extent in our ability to take advantage of the possibilities offered by a O-17-enriched single crystal study.