Systematic study of two-band/two-gap superconductivity in carbon-substituted MgB₂ by point-contact spectroscopy

Point-contact measurements on the carbon-substituted Mg(B_1−xC_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≈0.1. The large gap on the σ-band is decreased in an essentially linear fashion with increasing the carbon concentrations. The changes in the small gap Δ_π up to 3.8% C are proportionally smaller and are more difficult to detect but for the heavily doped sample with x≈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.