Observation of an oxygen isotope effect in YBa 2 Cu 3 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}$.