Direct observation of the oxygen isotope effect on the in-plane magnetic field penetration depth in optimally doped YBa2Cu3O7-δ -: art. no. 057602

We report the first direct observation of the oxygen-isotope (O-16/O-18) effect on the in-plane penetration depth lambda(ab) in a nearly optimally doped YBa2Cu3O7-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 lambda(ab). A substantial isotope shift Deltalambda(ab)/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 Deltam(ab)*/m(ab)*=5.5(2.0)%. These results are in good agreement with magnetization measurements on powder samples.