Boron Isotope Effect in Superconducting ${\mathrm{MgB}}_2$

We report the preparation method of and boron isotope effect for ${\mathrm{MgB}}_2$, a new binary intermetallic superconductor with a remarkably high superconducting transition temperature $T_c{(}^{10}\mathrm{B})=40.2\mathrm{}\mathrm{K}$. Measurements of both temperature dependent magnetization and specific heat reveal a 1.0 K shift in $T_c$ between ${\mathrm{Mg}}^{11}{\mathrm{B}}_2$ and ${\mathrm{Mg}}^{10}{\mathrm{B}}_2$. Whereas such a high transition temperature might imply exotic coupling mechanisms, the boron isotope effect in ${\mathrm{MgB}}_2$ is consistent with the material being a phonon-mediated BCS superconductor.