  Resistance of a Josephson junction array consisting of randomly distributed
lead (Pb) islands on exfoliated single layer graphene shows a broad
superconducting transition to zero with an onset temperature close to the
transition temperature of bulk Pb. The transition evolves with the back-gate
voltage and exhibits two peaks in temperature derivative of resistance. The
region above the lower temperature peak is found to be well described by
Berezinskii-Kosterlitz-Thouless model of thermal unbinding of vortex
anti-vortex pairs while that below this peak fits well with the Ambegaokar-
Halperin model of thermally-activated phase slip or vortex motion in Josephson
junction arrays. Thus a gate-tunable crossover between interaction and pinning
dominated vortices is inferred as the Josephson energy, dictating the pinning
potential magnitude, increases with cooling while the effective screening
length, dictating the range of inter-vortex interaction, reduces.
