Absence of a boron isotope effect in the magnetic penetration depth ofMgB2

The magnetic penetration depth $\lambda \left(0\right)$ in polycrystalline ${\mathrm{MgB}}_2$ for different boron isotopes $\left({}^{10}\mathrm{B}∕{}^{11}\mathrm{B}\right)$ was investigated by transverse field muon spin rotation. No boron isotope effect on the penetration depth $\lambda \left(0\right)$ was found within experimental error: $\Delta \lambda \left(0\right)∕\lambda \left(0\right)=0.8\left(8\right)\%$, suggesting that ${\mathrm{MgB}}_2$ is an adiabaic superconductor. This is in contrast to the substantial oxygen isotope effect on $\lambda \left(0\right)$ observed in cuprate high-temperature superconductors.