The superconducting transition temperature (T C ) of rock-salt type niobium nitride (δ − NbN) has been reported to vary in a large range (between 9 to 17 K) and the theoretically predicted value of 18 K has not achieved hitherto. Such a variation in the T C has been assigned to disorder present in δ − NbN irrespective of microstructure (polycrystalline or epitaxial), methods or conditions applied during the growth of NbN thin films. In this work, we investigate the atomic origin such suppression of T C in δ − NbN thin film by employing combined methods of experiments and abinitio simulations. Sputtered δ − NbN thin films with different disorder were studied using N and Nb K-edge x-ray absorption spectroscopy. A strong correlation between the superconductivity and the atomic distortion induced electronic reconstruction was observed. The theoretical analysis revealed that under N-rich conditions, atomic and molecular N-interstitial defects assisted by cation vacancies form spontaneously. As a result, the electronic densities of states around the Fermi level get smeared leading to a suppression of the T C in δ − NbN.
Niobium nitride (NbN) has attracted scientific interest due to its diverse physical properties and a variety of structural phases. The structure and superconductivity of the cubic [Formula: see text]-NbN phase are well established, but its hexagonal phases are not explored hitherto. In the present work, we report a simple synthesis route and a detailed study of hexagonal [Formula: see text]-Nb2N thin films. Thermal annealing of sputtered grown [Formula: see text]-NbN leads to a single phase [Formula: see text]-Nb2N at 973 K as confirmed by x-ray diffraction and absorption spectroscopy. The electrical transport measurements revealed a dominance of electron–phonon interactions with a superconducting transition around 4.74 K and an upper critical field [[Formula: see text]] of 3.99 T. The estimated [Formula: see text] is well below the calculated Pauli limit, and the Maki parameter value ([Formula: see text] [Formula: see text] 1) indicates that [Formula: see text] is dominated by an orbital pair breaking effect. Finally, the obtained value of electron–phonon coupling constant ([Formula: see text]) is in excellent agreement with a weak coupling Bardeen–Cooper–Schrieffer value of conventional superconducting materials.
Chromium nitride (CrN) spurred enormous interest due to its coupled magnetostructural and unique metal-insulator transition. The underneath electronic structure of CrN remains elusive. Herein, the electronic structure of epitaxial CrN thin film has been explored by employing resonant photoemission spectroscopy (RPES) and x-ray absorption near edge spectroscopy study in combination with the first-principle calculations. The RPES study indicates the presence of a charge-transfer screened 3dn L (L: hole in the N-2p) and 3dn−1 final-states in the valence band regime. The combined experimental electronic band structure along with the orbital resolved electronic density of states from the first-principle calculations reveals the presence of Cr(3d)-N(2p) hybridized (3dn L) states between lower Hubbard (3dn−1) and upper Hubbard (3dn+1) bands with onsite Coulomb repulsion energy (U) and charge-transfer energy (Δ) estimated as ≈ 4.5 and 3.6 eV, respectively. It verifies the participation of ligand (N-2p) states in low energy charge fluctuations and provides concrete evidence for the charge-transfer (Δ<U) insulating nature of CrN thin film.
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