Two-band/two-gap superconductivity in carbon-substituted ${\mathrm{MgB}}_2$ evidenced by point-contact spectroscopy

The Andreev reflection measurements of the superconducting energy gap in the carbon-substituted ${\mathrm{MgB}}_2$ are presented. Despite the strong suppression of the transition temperature by 17 K in comparison with the pure ${\mathrm{MgB}}_2,$ the same reduced value of the small superconducting energy gap with $2\Delta {/kT}_c\approx 1.7$ has been systematically observed. This indicates that the two-band/two-gap superconductivity is still preserved here.