Absence of a boron isotope effect in the magnetic penetration depth of MgB 2

The magnetic penetration depth $\ensuremath{\lambda}(0)$ in polycrystalline ${\mathrm{MgB}}_{2}$ for different boron isotopes $(^{10}\mathrm{B}∕^{11}\mathrm{B})$ was investigated by transverse field muon spin rotation. No boron isotope effect on the penetration depth $\ensuremath{\lambda}(0)$ was found within experimental error: $\ensuremath{\Delta}\ensuremath{\lambda}(0)∕\ensuremath{\lambda}(0)=0.8(8)%$, suggesting that ${\mathrm{MgB}}_{2}$ is an adiabaic superconductor. This is in contrast to the substantial oxygen isotope effect on $\ensuremath{\lambda}(0)$ observed in cuprate high-temperature superconductors.