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.
Niobium mononitride (NbN) has gained considerable attention because of its superconducting and refectory properties making it a promising material in devices like spin filters, Josephson junctions, superconducting qubits, and transistors especially when combined with other semiconductors and/or magnetic materials. [1][2][3][4][5][6][7] The ease of formation along with a relatively higher superconducting transition temperature (T C > 10 K) and a lower superconducting energy gap (Δ(0) % 3 meV) has made NbN to be one of the most studied superconducting materials for a wide range of applications. [8] Additionally, it has been widely used as an oxidation, wear-resistant layer, and diffusion barrier in protective and hard coatings. [9,10] NbN thin films can be synthesized using different growth techniques such as chemical vapor deposition, [11] ion beam-assisted deposition, [12] reactive direct current magnetron sputtering (dcMS), molecular beam epitaxy, [1] atomic layer deposition, [13] pulsed laser deposition, [14] etc.Among these, the dcMS has been widely used to grow thin films due to its cost effectiveness and the ability to grow uniform thin films. In a typical dcMS process, the plasma is mostly dominated by neutrals and the fraction of ionized species is very small (typically less than 5%). [15] The structural properties of the sputtered films strongly depend on the kinetic energies and ionization state of the impinging atomic and molecular species on the surface of growing films. It is well-known that the low ionization in dcMS process results into a porous and loosely packed microstructure and forms a large density of defects in the grown films. [16,17] To overcome these issues high power impulse magnetron sputtering (HiPIMS) has been developed. [18,19] In the HiPIMS technique, high power pulses are applied to a sputtering target contrary to a constant DC voltage in the dcMS. The use of high power pulses onto the sputtering target results in a large fraction of ionized-sputtered species. In addition, gases that are present in the vicinity of target also get ionized. [20] Earlier experimental and theoretical reports suggest that high ion to neutral ratios during HiPIMS process results into better thin-films' qualities in terms of denser microstructure, smoother surface, and interfaces. [21][22][23] Recent experimental and theoretical results [24,25] suggest that Nb vacancies are the most dominant
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