Direct observation of the oxygen isotope effect on the in-plane magnetic field penetration depth in optimally doped YBa${}_2$Cu${}_3$O${}_{7-\hat{I}´}$

Abstract

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

4 pages, 2 figures