We report the first direct observation of the oxygen-isotope
(16O/18O) effect on the in-plane penetration depth λab in
a nearly optimally doped YBa2Cu3O7−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 λab.
A substantial isotope shift Δλab/λab=2.8(7)% at 4 K is
observed, implying that the in-plane effective supercarrier mass mab∗
is oxygen-isotope dependent with Δmab∗/mab∗=5.5(1.4).Comment: 4 pages, 2 figure