17O NMR in YBa2Cu3O7-δ Superconductors

We present the first 17O NMR data obtained in YBa2Cu3O7-δ powder with Tc = 90 K in which oxygen sites on Cu-O chains had been selectively populated by a gas exchange process performed at low temperature. A comparison between fully exchanged and chain only exchanged samples suggests that magnetic-field-swept spectra cannot discriminate between both kinds of samples. Fourier-transformed spectra do however provide a very clear assignment between plane and chain-bridge sites. The spin-lattice relaxation of chain sites measured between 4.2 and 300 K is dramatically nonexponential, as expected for nuclei in the presence of an important quadrupolar interaction. Our data show the existence of a shortening of the chain relaxation in a limited temperature domain slightly below Tc. This hump of relaxation supports the absence of zeros of the superconducting order parameter over the Fermi surface and the s-symmetry for the pair wave function.