Carbon-substitution dependent multiple superconducting gap of MgB$_{2}$:   a "sub-meV" resolution photoemission study
  ``Sub-meV'' resolution photoemission spectroscopy was used to study carbon-substitution dependence on the multiple superconducting gap of Mg(B$_{1 - x}$C$_{x})_{2}$. Two features corresponding to $\sigma $ and $\pi $ gaps are clearly observed in the raw spectra up to carbon concentration x = 7.5 {%}. The observed x dependence of the two gaps shows a qualitatively different behavior: a marked change of the $\sigma $ gap proportional to the $T_{c}$ variation and a negligible one of the $\pi $ gap. This as well as the temperature dependence can be explained with the two-band mean-field theory. Implications from the present study are discussed. 
S. Tsuda, T. Yokoya, T. Kiss, T. Shimojima, S. Shin, T. Togashi, S.   Watanabe, C. Chen, S. Lee, H. Uchiyama, S. Tajima, N. ...