Copper isotope shifts in Pr and La substituted 123 and 124 cuprate superconductors; comparison with oxygen isotope shifts

Abstract

Copper isotope shifts of T c have been measured in 123 and 124 cuprate superconductors with partial substitutions by Pr and La to reduce hole concentration. The 123 compositions were Pr x Y 1− x Ba 2Cu 3O 7 ( x=0.2, 0.3, 0.4) and YLa 0.3Ba 1.7Cu 3O 7. The 124 compounds were Pr x Y 1− x Ba 2Cu 4O 8 ( x=0.2, 0.3) and YLa 0.3Ba 1.7Cu 4O 8. Oxygen isotope experiments in materials with these compositions have shown substantial isotope shifts. We find that the Cu isotope shifts are also substantial, but somewhat smaller. Other findings are: (1) the Cu isotope exponent α Cu is positive, (2) α Cu increases rapidly as T c is reduced by substitution, and rises to a substantial fraction of the BCS value, behavior which is similar to that of the oxygen isotope exponent α O, and (3) when T c is reduced by non-isovalent substitution, both O and Cu isotope shifts increase greatly, but the ratio α Cu/ α O remains 0.75±0.1 in both 123 and 124, regardless of the amount and site of the non-isovalent substitution.