Metal-insulator transition and superconductivity in boron-doped diamond

Abstract

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 $\left(n_B\right)$ on the order of $n_c\sim 4.5\times {10}^{20}{\mathrm{cm}}^{-3}$, in excellent agreement with numerical calculations. The temperature dependence of the conductivity and Hall effect can be well described by variable range hopping for $n_B<n_c$ with a characteristic hopping temperature $T_0$ strongly reduced due to the proximity of the MIT. All metallic samples (i.e., 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 $\left(T_c\right)$ 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 $(⩾0.4\mathrm{K})$ 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 pseudopotential ${\mu }^{*}$ as $n_B\to n_c$.