Evolution of Berensinkii-Kosterlitz-Thouless Transition in Ultra thin NbN films

We explore the role of phase fluctuations in epitaxial thin films of the conventional superconductor NbN. The epitaxial NbN thin films are grown on single crystalline MgO substrates using reactive dc magnetron sputtering by sputtering a Nb target in Ar/N2 (80:20) gas mixture at a substrate temperature of 600 °C with varying thickness. The thickness (d) (ranging from d ~ 3-50 nm) of the films is controlled by controlling the time of deposition keeping all other parameters constant. 

The combination of super fluid density (nsµ1/2), normal state carrier density and resistivity () measurements on set of NbN thin films 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 Beresinkii-Kosterlitz-Thouless (BKT) transition close to superconducting transition temperature (Tc). The sudden drop starts at higher superfluid density than expected from 2-D XY model, which can be understood by considering the low vortex core energy and slight inhomogeneity in the system.