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

摘要

Point-contact measurements on the carbon-substituted Mg(B$_{1-x}$C$_x$)$_2$filament/powder samples directly reveal a retention of the two superconductingenergy gaps in the whole doping range from $x = 0$ to $x \approx 0.1$. Thelarge gap on the $\sigma$-band is decreased in an essentially linear fashionwith increasing the carbon concentrations. The changes in the the small gap$\Delta_{\pi}$ up to 3.8 % C are proportionally smaller and are more difficultto 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 astrong essentially linear, reduction of each gap with the transitiontemperature.