Metal-insulator transition and superconductivity in highly boron-doped nanocrystalline diamond films

The low temperature electronic transport of highly borondoped nanocrystalline diamond films is studied down to 300 mK. The films show superconducting properties with critical temperatures T _c up to 2.1 K. The metal-insulator and superconducting transitions are driven by the dopant concentration and greatly influenced by the granularity in this system, as compared to highly boron-doped single crystal diamond. The critical boron concentration for the metal-insulator transition lies in the range from 2.3×10 ²⁰ up to 2.9×10 ²⁰ cm ^-3, as determined from transport measurements at low temperatures. Insulating nanocrystalline samples follow an Efros-Shklovskii (ES) type of temperature dependence for the conductivity up to room temperature, in contrast to Mott variable range hopping (VRH) in the case of insulating single crystal diamond close to the metal-insulator transition. The electronic transport in the metallic samples not only depends on the properties of the grains (highly boron-doped single crystal diamond), but also on the intergranular coupling between the grains. The Josephson coupling between the grains plays an important role for the superconductivity in this system, leading to a superconducting transition with global phase coherence at sufficiently low temperatures. Metallic nanocrystalline samples show similarities to highly boron-doped single crystal diamond. However, metallic samples close to the metal-insulator transition show a richer behavior. In particular, a peak was observed in the low-temperature magnetoresistance measurements for samples close to the transition, which can be explained by corrections to the conductance arising from superconducting fluctuations.