The physical properties of lightly doped semiconductors are well described by electronic band-structure calculations and impurity energy levels¹. Such properties form the basis of present-day semiconductor technology. If the doping concentration n exceeds a critical value n_c, the system passes through an insulator-to-metal transition and exhibits metallic behaviour; this is widely accepted to occur as a consequence of the impurity levels merging to form energy bands². However, the electronic structure of semiconductors doped beyond n_c have not been explored in detail. Therefore, the recent observation of superconductivity emerging near the insulator-to-metal transition³ in heavily boron-doped diamond^{4, 5} has stimulated a discussion on the fundamental origin of the metallic states responsible for the superconductivity. Two approaches have been adopted for describing this metallic state: the introduction of charge carriers into either the impurity bands⁶ or the intrinsic diamond bands^{7, 8, 9}. Here we show experimentally that the doping-dependent occupied electronic structures are consistent with the diamond bands, indicating that holes in the diamond bands play an essential part in determining the metallic nature of the heavily boron-doped diamond superconductor. This supports the diamond band approach and related predictions, including the possibility of achieving dopant-induced superconductivity in silicon and germanium⁷. It should also provide a foundation for the possible development of diamond-based devices¹⁰.

1. Japan Synchrotron Radiation Research Institute (JASRI)/SPring-8, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan
2. The Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan
3. National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
4. School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
5. Department of Quantum Matter, Graduate School of Advanced Sciences of Matter (ADSM), Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530 Japan

Correspondence to: T. Yokoya^1,2 Correspondence and requests for materials should be addressed to T.Y. (Email: yokoya@cc.okayama-u.ac.jp).

Received 14 July 2005; Accepted 29 September 2005

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