Microscopic Origin of Structural Disorder in $δ$-NbN: Correlation of Superconductivity and Electronic Structure

Abstract

Rock-salt type niobium nitride (δ − NbN) is a well-known superconductor having superconducting transition temperature (TC) ≈ 18 K and a large superconducting gap ≈ 3 meV. The TC of δ − NbN thin film exhibits a large scattering irrespective of the growth conditions and lattice parameter. In this work, we investigate the atomic origin of suppression of TC in δ − NbN thin film by employing combined methods of experiments and ab-initio simulations. Sputtered δ − NbN thin films with different disorder were analyzed through electrical resistivity and 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 growth conditions, atomic and molecular N-interstitial defects assisted by cation vacancies form spontaneously and are responsible for the suppression of TC in δ − NbN by smearing its electronic densities of states around Fermi level.

Superconducting niobium nitride: a perspective from processing, microstructure, and superconducting property for single photon detectors

“…In a comparative study by Sanjinés et al [15], both β-NbN and δ ′ -NbN hexagonal structures are notably more covalent than the δ-NbN phase, where the phase stability of hexagonal nitrides increases on account of the empty antibonding levels in the Fermi level [15]. It is also noted that cation and anion vacancies in the δ-NbN phase can modify the density of states (DOS) below the Fermi level, leading to the suppression of T c [16,17].…”

Understanding the Structural–Chemical Evolution of Epitaxial NbN/Al2O3/NbN Trilayers with Varying NbN Thickness

“…Apart from the strain induced by lattice mismatching, the observed N/Nb inhomogeneity might also be associated with N:Nb stoichiometric deviations introduced by the presence of a considerable amount of N vacancies, which is typical in NbN films. ,,, Alongside the formation of secondary NbO x phases, O atoms that are incorporated within NbN films as local substitutional or interstitial point defects typically play a pivotal role in tuning the superconducting properties of NbN layers and, hence, the quantum behavior of NbN/Al 2 O 3 /NbN trilayers. , Because of the detection limit of correlative TEM and APT, it is extremely difficult to probe the formation of local point defects, such as N vacancies and substitutional/interstitial O impurities, and their effects on the local structure of NbN films. To address this information void, a theoretical investigation using first-principles DFT calculations was adapted in the next part of this work to provide a complete understanding of the effect of local point defects on the NbN crystal structure and its properties.…”