Locus of pairing interaction inYBa2Cu3O7by site-selective oxygen isotope shift:O18inCuO2plane layers

The oxygen isotope shift of ${\mathit{T}}_{\mathit{c}}$ in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ has been measured for samples containing ∼95% ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ in the ${\mathrm{CuO}}_2$ plane [O2,O3] sites and primarily ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ in the apical [O4] and chian [O1] sites. The ${\mathit{T}}_{\mathit{c}}$ of the site-selectively substituted samples is higher than that of the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ control samples by 0.10–0.14 K, in contrast to the decrease in ${\mathit{T}}_{\mathit{c}}$ of 0.20–0.23 K for complete ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ substitution at all sites. Our results inicate that the phonon-mediated contribution to the pairing mechanism in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ involves the apical [O4] sites, and not just the ${\mathrm{CuO}}_2$ plane [O2,O3] sites.