We show that various factors can lead to an isotopic dependence of the penetration depth $\delta$. Nonadiabaticity (Jahn-Teller crossing) leads to the isotope effect of the charge-carrier concentration $n$ and, consequently, of $\delta$ in doped superconductors such as the cuprates. A general equation relating the isotope coefficients of $T_c$ and of $\delta$ is presented for London superconductors. We further show that the presence of magnetic impurities or a proximity contact also lead to an isotopic dependence of $\delta$; the isotope coefficient turns out to be temperature dependent, $\beta \mathrm{}\left(T\right)\mathrm{}$, in these cases. The existence of the isotope effect for the penetration depth is predicted for conventional as well as for high-temperature superconductors. Various experiments are proposed and/or discussed.

• Received 17 July 1997

DOI:https://doi.org/10.1103/PhysRevB.57.10814