LITHIUM DOPING IN (RE)BA2CU3O7 - STRUCTURAL, MAGNETIC AND NMR EFFECTS - RE=Y OR ND

Doping YBa2Cu3O7 and NdBa2Cu3O7 by lithium ions leads to compounds which keep a metallic character in the orthorhombic range up to the suppression of superconductivity. The T(c) diminution is linear with a dT(c)/dx slope in the Nd(Li) compounds stronger than in the Y(Li) ones, probably due to an instability of the Nd cuprate as regards oxygen or to an ion-size effect. The linear variation of the resistivity versus x indicates a decrease of the carrier number joined to their localization. Magnetic data show that lithium does not act as a pinning center for vortices except at a low Li content. NMR experiments carried out on Li-7 exhibit a lorentizian line. The shift, which is a Knight shift, proves that the 2s electron of lithium participates in the conduction band process. The variations of the peak width versus temperature would indicate a local paramagnetism of the charge carriers. The line shift versus temperature provides evidence for a progressive charge localization; the 1/T logarithmic law would agree with a Bok-Labbe type theoretical model.