Structure and superconductivity of isotope-enriched boron-doped diamond

Superconducting boron-doped diamond samples were synthesized with isotopes of ¹⁰B, ¹¹B, ¹³C and ¹²C. We claim the presence of a carbon isotope effect on the superconducting transition temperature, which supports the 'diamond-carbon'-related nature of superconductivity and the importance of the electron-phonon interaction as the mechanism of superconductivity in diamond. Isotope substitution permits us to relate almost all bands in the Raman spectra of heavily boron-doped diamond to the vibrations of carbon atoms. The 500 cm^-1 Raman band shifts with either carbon or boron isotope substitution and may be associated with vibrations of paired or clustered boron. The absence of a superconducting transition (down to 1.6 K) in diamonds synthesized in the Co-C-B system at 1900 K correlates with the small boron concentration deduced from lattice parameters.