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 (Delta) highly modulated over a length scale on the order of similar to 30 nm, which is much shorter than the typical diamond grain size. The sample local and macroscopic behavior favors for the Delta modulation as being an intrinsic property of the NCD granules. On the other hand, Delta shows its temperature dependence [Delta(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.