Role of the vortex-core energy on the Beresinkii-Kosterlitz-Thouless   transition in thin films of NbN
  We analyze the occurrence of the Beresinkii-Kosterlitz-Thouless transition in thin films of NbN at various film thickness, by probing the effect of vortex fluctuations on the temperature dependence of the superfluid density below $T_{BKT}$ and of the resistivity above $T_{BKT}$. By direct comparison between the experimental data and the theory we show the crucial role played by the vortex-core energy in determining the characteristic signatures of the BKT physics, and we estimate its dependence on the disorder level. Our work provides a paradigmatic example of BKT physics in a quasi-two-dimensional superconductor. 
Mintu Mondal, Sanjeev Kumar, Madhavi Chand, Anand Kamlapure, Garima   Saraswat, G. Seibold, L. Benfatto and Pratap Raychaudhuri
cond-mat.supr-con cond-mat.dis-nn cond-mat.stat-mech