Copper NMR and NQR studies of fluorine-doped electron superconductors

We report the observation of a Cu NMR signal with a pronounced quadrupole splitting (VQ approximately 13 MHz) in Nd2CuO4-yFy. This finding gives evidence that the electric-field gradient (EFG) in electron-doped superconducting compound is almost the same as for undoped parent ones. In view of this result, the mysterious absence of an EFG, reported before for similar Ce-doped compounds, is discussed. It is attributed to the appearance of spherically symmetric d10 configuration with doping. It is more likely connected with the inhomogeneities of the quadropole splitting induced by doping, rather than with the changes of electronic configuration. 9 refs., 2 figs