Direct Observation of the Oxygen Isotope Effect on the In-Plane Magnetic Field Penetration Depth in Optimally Doped<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow><mml:mi mathvariant="normal">Y</mml:mi><mml:mi mathvariant="normal">B</mml:mi><mml:mi mathvariant="normal">a</mml:mi></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi mathvariant="normal">C</mml:mi><mml:mi mathvariant="normal">u</mml:mi></mml:mrow><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>7</mml:mn><mml:mo>−</mml:mo><mml:mi>δ</mml:mi></mml:mrow></mml:msub></mml:math>

We report the first direct observation of the oxygen-isotope ($^{16}\mathrm{O}/^{18}\mathrm{O}$) effect on the in-plane penetration depth ${\ensuremath{\lambda}}_{ab}$ in a nearly optimally doped ${\mathrm{Y}\mathrm{B}\mathrm{a}}_{2}{\mathrm{C}\mathrm{u}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ film using the novel low-energy muon-spin rotation technique. Spin-polarized low-energy muons are implanted in the film at a known depth $z$ beneath the surface and precess in the local magnetic field $B(z)$. This feature allows us to measure directly the profile $B(z)$ of the magnetic field inside the superconducting film in the Meissner state and to make a straightforward determination of ${\ensuremath{\lambda}}_{ab}$. A substantial isotope shift $\ensuremath{\Delta}{\ensuremath{\lambda}}_{ab}/{\ensuremath{\lambda}}_{ab}=2.8(1.0)%$ at 4 K is observed, implying that the in-plane effective supercarrier mass ${m}_{ab}^{*}$ is oxygen-isotope dependent with $\ensuremath{\Delta}{m}_{ab}^{*}/{m}_{ab}^{*}=5.5(2.0)%$. These results are in good agreement with magnetization measurements on powder samples.

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