Holes in the valence band of superconducting boron-doped diamond film   studied by soft X-ray absorption and emission spectroscopy
  Carbon- and boron-2$p$ states of superconducting and non-superconducting boron-doped diamond samples are measured using soft X-ray emission and absorption spectroscopy. For the superconducting sample, a large density of hole states is observed in the valence band in addition to the states in the impurity band. The hole states in the valence band is located at about 1.3 eV below the valence band maximum regardless of the doping level, which cannot be interpreted within a simple rigid band model. Present experimental results, combined with the first principles calculations, suggest that superconductivity is to be attributed to the holes in the valence band. 
Jin Nakamura, Tamio Oguchi, Nobuyoshi Yamada, Kazuhiko Kuroki, Kozo   Okada, Yoshihiko Takano, Masanori Nagao, Isao Sakaguch...