In/extrinsic granularity in superconducting boron-doped diamond

When charge carriers are introduced in diamond, e.g. by chemical doping with Boron (B), the C1-xBx diamond:B can exhibit an insulator-to-metal transition (pMott∼2×1020cm-3). Under even heavier boron doping (nB∼1021cm-3), diamond becomes superconducting. Using microwave plasma-assisted chemical vapor deposition (MPCVD) we have prepared diamond:B thin films with critical offset temperatures TC below 3 K. We have investigated the transport properties of these diamond:B thin films, which show pronounced granular effects. It turns out, that this granularity is both intrinsic as well as extrinsic. The extrinsic granularity is the effect of the growth method which needs to start from a seeding of the substrate with detonation nanodiamond, which acts as nucleation centers for further MPCVD growth of the film. In using SPM/STM techniques, we also observed intrinsic granularity, meaning that within physical grains, we observe also a strong intragrain modulation of the order parameter. As a consequence of these granularities, the transport properties show evidence of (i) strong superconducting fluctuations and (ii) Cooper pair tunneling and/or quasiparticle tunneling. The latter effects explain the observed negative magnetoresistance.