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 (nB) on the order of nc∼4.5×1020cm−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 nB<nc with a characteristic hopping temperature T0 strongly reduced due to the proximity of the MIT. All metallic samples (i.e., for nB>nc) present a superconducting transition at low temperature. The zero-temperature conductivity σ0 deduced from fits to the data above the critical temperature (Tc) using a classical quantum interference formula scales as σ0∝(nB/nc−1)ν with ν∼1. Large Tc values (⩾0.4K) have been obtained for boron concentration down to nB/nc∼1.1 and Tc surprisingly mimics a (nB/nc−1)1/2 law. Those high Tc values can be explained by a slow decrease of the electron-phonon coupling parameter λ and a corresponding drop of the Coulomb pseudopotential μ∗ as nB→nc.