Site-selective oxygen isotope effect in optimally doped YBa₂Cu₃O_{6 + x}

Abstract

IN CONVENTIONAL superconductors, the large dependence of the superconducting transition temperature (T_c) on the isotope mass (T_c∝m^−α, with α≈0.5) gives strong evidence for electron pairing by a phonon-mediated mechanism. The copper oxide superconductors, on the other hand, exhibit a small oxygen isotope effect (α≈0.02; see, for example, refs 1–14), suggesting a mechanism more complex than simple phonon-mediated pairing¹⁵. To obtain further insight into this mechanism, it is important to determine the contribution from different oxygen sites in the crystal lattice to the total oxygen isotope effect. Recently, a negative isotope effect associated with oxygen atoms in the copper oxide planes of the crystal structure was reported¹³ for highly doped YBa₂Cu₃O_{6 + x}, implying that the apical/chain oxygens make a substantial contribution to the total oxygen isotope shift. Here we investigate this effect in optimally doped YBa₂Cu₃O_{6 + x}, by measuring separately the contributions to the total isotope shift of the planar and apical/chain oxygen sites. The sum of the site-selective isotope shifts equals that of the total measured shift, illustrating the self-consistency of our results. In contrast to the previous work¹³, we find that more than 80% of the total (positive) isotope effect is associated with the copper oxide planes.