  The occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition is
investigated in heavily boron-doped nanocrystalline diamond films through a
combination of current-voltage and resistance measurements. We observe a robust
BKT transition in the nanocrystalline diamond films with smaller grain size
along with transport features related to vortex pinning. The vortex core energy
determined through analysis of the resistance temperature curves was found to
be anti-correlated to the BKT transition temperatures. It is also observed that
the higher BKT temperature is related to an increased vortex-antivortex binding
energy derived from the activated transport regions. Further, the magnetic
field induced superconductor insulator transition shows the possibility of the
charge glass state. The consequences of granularity such as localization and
vortex pinning can lead to tuneable BKT temperatures and strongly affects the
field induced insulating state.
