Superconductivity in heavily boron-doped single crystalline and nanocrystalline diamond thin films

Our results show that the critical boron concentration $n_{c}$ for single crystalline diamond (scd) and nanocrystalline diamond (ncd) thin films is the same for the normal to superconducting and for the non-metal to metal transitions, on the order of $5 \times 10^{20}\ cm^{-3}$, in agreement with estimates derived from various theoretical approaches. In scd material, a variable range hopping behaviour was clearly observed on the insulating side of the transition, and, as expected, the characteristic temperature $T_{0}$ tended toward zero at the transition. On the metallic side, the zero temperature conductivity $\sigma_{0}$ scaled with $(n_{B}/n_{c} - 1)^{\nu}$ with $\nu \approx 1$. The critical temperature $T_{c}$ remained high in the vicinity of the metal-non metal transition, and it was rather found to scale with $(n_{B}/n_{c} - 1)^{1/2}$. These results led us to propose that the electron-phonon coupling parameter $\lambda$ remains large down to $n_{B}/n_{c} \approx 1.1$, and to examine the metal-insulator transition and the parameter set ($\lambda$, $\mu$) in terms of scaling laws. Low temperature magnetotransport measurements and the possible occurence of a superconductor-insulator transition in heavily boron-doped ncd will be discussed.