Observation of an oxygen isotope effect in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

A small decrease in ${\mathrm{T}}_{\mathrm{c}}$ of 0.3 K to 0.5 K is observed when as much as 90% of the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ is substituted with ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$. This result is consistent with our observation that there is an oxygen isotope effect in ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_4$, but in contrast with previous reports that there is no isotope effect for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. This new result suggests that phonons play an important role in the electron-pairing mechanism in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$.