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

Abstract

Copper isotope shifts of T_chave been measured in 123 and 124 cuprate superconductors with partial substitutions by Pr and La to reduce hole concentration. The 123 compositions were Pr_xY_1-xBa₂Cu₃O₇ (x=0.2, 0.3, 0.4) and YLa_0.3Ba_1.7Cu₃O₇. The 124 compounds were Pr_xY_1−xBa₂Cu₄O₈ (x=0.2, 0.3) and YLa_0.3Ba_1.7Cu₄O₈.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/αO remains 0.75±0.1 in both 123 and 124, regardless of the amount and site of the non-isovalent substitution.