Absence of a boron isotope effect in the magnetic penetration depth of ${\mathrm{MgB}}_2$

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.

Figure 1

Normalized field cooled $\left(0.5\mathrm{mT}\right)$ magnetization as a function of temperature for $\mathrm{Mg}{}^{10}\mathrm{B}_2$ and $\mathrm{Mg}{}^{11}\mathrm{B}_2$ samples.

Figure 2

Local magnetic field distribution, obtained from the Fourier transform of the muon spin precession signal, for $\mathrm{Mg}{}^{11}\mathrm{B}_2$ at $2(●)$ and $25\mathrm{K}(◻)$. Solid lines are Gaussian fits to the experimental data. The vertical solid line indicates the external field of $0.6\mathrm{T}$.

Figure 3

Temperature dependence of $\sigma$ at $B_{\mathrm{ext}}=0.6\mathrm{T}$ for the two isotope samples $\mathrm{Mg}{}^{10}\mathrm{B}_2\left(▴\right)$ and $\mathrm{Mg}{}^{11}\mathrm{B}_2(엯)$. The solid $\left(\mathrm{Mg}{}^{10}\mathrm{B}_2\right)$ and dotted $\left(\mathrm{Mg}{}^{11}\mathrm{B}_2\right)$ lines are fits using Eq. (1). Inset: low-temperature region on a larger scale. The dotted and solid horizontal lines represent the weighed average values of $\sigma \left(0\right)$ for $T<7.5\mathrm{K}$ for $\mathrm{Mg}{}^{10}\mathrm{B}_2$ and $\mathrm{Mg}{}^{11}\mathrm{B}_2$, respectively.