Strongly correlated impurity band superconductivity in diamond: X-ray spectroscopic evidence

In a recent X–ray absorption study in boron doped diamond, Nakamura et al. have seen a wellisolated narrow boron impurity band in non-superconducting samples and an additional narrow band at the chemical potential in a superconducting sample. We interpret the beautiful spectra as evidence forupper Hubbard band of a Mott insulating impurity band and an additional metallic ‘mid–gap band’ of a conducting ‘self–doped’ Mott insulator. This supports the basic framework of a recent theory of the present author of strongly correlated impurity band superconductivity (impurity band resonating valence bond, IBRVB theory) in a template of a wide-gap insulator, with no direct involvement of valence band states.