Intrinsic granularity in nanocrystalline boron-doped diamond films measured by scanning tunneling microscopy

We report on low-temperature scanning tunneling microscopy/spectroscopy experiments performed on superconducting boron-doped nanocrystalline diamond (NCD) thin films prepared by chemical-vapor deposition methods. The most representative sample reveals the observed superconducting gap (Δ) highly modulated over a length scale on the order of ∼30nm, which is much shorter than the typical diamond grain size. The sample local and macroscopic behavior favors for the Δ modulation as being an intrinsic property of the NCD granules. On the other hand, Δ shows its temperature dependence [Δ (T)] consistent with the results obtained by Fominov and Feigel'man [Phys. Rev. B 63, 094518 (2001)], who studied theoretically the behavior of the superconducting gap of a BCS superconductor in contact with a normal layer by solving the one-dimensional Usadel equations on the superconducting side of the superconducting to normal interface.