17O NMR in YBa2Cu3O6.65. Discrimination between t-J and two-band models

17 O NMR has been performed between 10 and 360 K on an oriented powder of YBa 2 Cu 3 O 6.65 ( T c =60 K). The temperature dependence of the magnetic hyperfine shift (MHS) tensors has been followed for the O(2,3) and O(4) sites. For O(4) nuclei, the MHS stays constant above T c , whereas for the O(2,3) sites it scales linearly with the macroscopic susceptibility χ m . This demonstrates that the origin of the temperature dependence of χ m is to be found within the CuO 2 plane. Two methods are presented to discriminate between the t - J model and two-band models: (a) the comparison of the temperature dependence of the isotropic K iso and the axial part K ax of the O(2,3) MHS, (b) a scaling between the experimentally measured products K 2 iso T 1 T in 17 O and 89 Y. From the second method, our results indicate the existence of an oxygen 2p-band with a density of states per spin direction of 0.11 to 0.19 eV -1 at -1 .