We report measurements of the oxygen-isotope effect (OIE) on the in-plane penetration depth ${\lambda }_{\mathrm{ab}}\left(0\right)$ in underdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ single crystals. A highly sensitive magnetic torque sensor with a resolution of $\Delta \tau \simeq {10}^{-12\mathrm{}}\mathrm{N}\mathrm{m}$ was used for the magnetic measurements on microcrystals with a mass of $\approx 10\mu \mathrm{g}$. The OIE on ${\lambda }_{\mathrm{ab}}^{-2\mathrm{}}\left(0\right)$ is found to be $-10\mathrm{}\left(2\right)\%$ for $x=0.080\mathrm{}$ and $-8\mathrm{}\left(1\right)\%$ for $x=0.086\mathrm{}$. It arises mainly from the oxygen-mass dependence of the in-plane effective mass $m_{\mathrm{ab}}^{*}$. The present results suggest that lattice vibrations are important for the occurrence of high temperature superconductivity.

• Received 20 December 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.4192