Direct observation of the oxygen isotope effect on the in-plane magnetic field penetration depth in optimally doped YBa$_2$Cu$_3$O$_{7-δ}$
We report the first direct observation of the oxygen-isotope ($^{16}$O/$^{18}$O) effect on the in-plane penetration depth $λ_{ab}$ in a nearly optimally doped YBa$_2$Cu$_3$O$_{7-δ}$ 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 model independent determination of $λ_{ab}$. A substantial isotope shift $Δλ_{ab}/λ_{ab}=2.8(7)$% at 4 K is observed, implying that the in-plane effective supercarrier mass $m_{ab}^\ast$ is oxygen-isotope dependent with $Δm_{ab}^\ast/m_{ab}^\ast = 5.5(1.4)%$.