Systematic study of the two band/two gap superconductivity in   carbon-substituted MgB2 by point-contact spectroscopy
  Point-contact measurements on the carbon-substituted Mg(B$_{1-x}$C$_x$)$_2$ filament/powder samples directly reveal a retention of the two superconducting energy gaps in the whole doping range from $x = 0$ to $x \approx 0.1$. The large gap on the $\sigma$-band is decreased in an essentially linear fashion with increasing the carbon concentrations. The changes in the the small gap $\Delta_{\pi}$ up to 3.8 % C are proportionally smaller and are more difficult to detect but for the heavily doped sample with $x \approx 0.1$ and $T_c = 22$ K both gaps are still present, and significantly reduced, consistent with a strong essentially linear, reduction of each gap with the transition temperature. 
Z. Holanova, P. Szabo, P. Samuely, R.H.T. Wilke, S.L. Bud'ko, and P.C.   Canfield