Metal-insulator transition and superconductivity in boron-doped diamond
Résumé
We report on a detailed analysis of the transport properties and superconducting critical temperatures of boron-doped diamond films grown along the $\{100\}$ direction. The system presents a metal-insulator transition (MIT) for a boron concentration ($n_B$) on the order of $n_c \sim 4.5 \times 10^{20}$ cm$^{-3}$ in excellent agreement with numerical calculations. The temperature dependence of the conductivity and Hall effect can be well described by variable range hopping for $n_B n_c$) present a superconducting transition at low temperature. The zero temperature conductivity $\sigma_0$ deduced from fits to the data above the critical temperature ($T_c$) using a classical quantum interference formula scales as : $\sigma_0 \propto (n_B/n_c-1)^\nu$ with $\nu \sim 1$. Large $T_c$ values ($\geq 0.4$ K) have been obtained for boron concentration down to $n_B/n_c \sim 1.1$ and $T_c$ surprisingly mimics a $(n_B/n_c-1)^{1/2}$ law. Those high $T_c$ values can be explained by a slow decrease of the electron-phonon coupling parameter $\lambda$ and a corresponding drop of the Coulomb pseudo-potential $\mu^*$ as $n_B \rightarrow n_c$.
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