Evolution of Kosterlitz-Thouless-Berezinskii (BKT) Transition in Ultra-Thin NbN Films

We explore the Berezinskii-Kosterlitz-Thouless (BKT) transition in ultrathin NbN superconducting films. We have measured the super fluid density (ns∝1/λ2), normal carrier density and resistivity (ρ) of a set of NbN thin films. Our results show that while the ground state is well described by BCS theory, at elevated temperatures, ultra-thin films show sudden drop in superfluid density associated with the BKT transition close to superconducting transition temperature (Tc). The sudden drop starts at higher superfluid density than expected from 2D XY model, which can be understood by considering the low vortex core energy and slight inhomogeneity in the system. Resistivity data is explained using effective medium theory (EMT) including both Aslamazov-Larkin and KT fluctuations. This is also in good agreement with the corresponding superfluid density.