COPPER ISOTOPE SHIFTS IN PR AND LA SUBSTITUTED 123-CUPRATE AND 124-CUPRATE SUPERCONDUCTORS - COMPARISON WITH OXYGEN-ISOTOPE SHIFTS

Copper isotope shifts of T-c have been measured in 123 and 124 cuprate superconductors with partial substitutions by Pr and La to reduce hol e concentration. The 123 compositions were PrxY1-xBa2Cu3O7 (x=0.2, 0.3 , 0.4) and YLa0.3Ba1.7Cu3O7. The 124 compounds were PrxY1-xBa2Cu4O8 (x = 0.2, 0.3) and YLa0.3Ba1.7Cu4O8. Oxygen isotope experiments in mater ials with these compositions have shown substantial isotope shifts. We find that the Cu isotope shifts are also substantial, but somewhat sm aller. Other findings are: (1) the Cu isotope exponent alpha(Cu) is po sitive, (2) alpha(Cu) increases rapidly as T-c is reduced by substitut ion, and rises to a substantial fraction of the BCS value, behavior wh ich is similar to that of the oxygen isotope exponent alpha(O) and (3) when T-c is reduced by non-isovalent substitution, both O and Cu isot ope shifts increase greatly, but the ratio alpha(Cu)/alpha(O) remains 0.75 +/- 0.1 in both 123 and 124, regardless of the amount and site of the non-isovalent substitution. (C) 1998 Published by Elsevier Scienc e B.V.