Effect of disorder on superconductivity of NbN thin films studied using x-ray absorption spectroscopy.

The superconducting transition temperature (Tc) of rock-salt type niobium nitride (\dNbN) typically varies between 9 to 17 K and the theoretically predicted value of 18 K has not been achieved hitherto. The low Tc in \dNbN has been assigned to some structural disorder which is always present irrespective of the microstructure (polycrystalline or epitaxial), methods or conditions adopted during the growth of NbN thin films. In this work, we investigate the atomic origin of such suppression of the Tc in \dNbN thin films by employing combined methods of experiments and ab-initio simulations. Sputtered \dNbN thin films with different disorder were studied using the synchrotron-based N and Nb K-edge x-ray absorption spectroscopy techniques. A strong correlation between the superconductivity and the electronic structure reconstruction was observed. The theoretical analysis revealed that under N-rich growth conditions, atomic and molecular N-interstitial defects assisted by cation vacancies form spontaneously and results into a smeared electronic structure around Fermi-level. The role of electronic smearing on the Tc is thoroughly discussed.