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

TL;DR

This study combines experiments and simulations to reveal how atomic-scale disorder, especially N-interstitial defects, suppresses superconductivity in $ ext{δ}$-NbN thin films by altering their electronic structure.

Contribution

It provides a detailed atomic-level understanding of how disorder affects superconductivity in $ ext{δ}$-NbN, linking experimental observations with ab-initio simulations.

Findings

Atomic and molecular N-interstitial defects form spontaneously under N-rich conditions.

These defects cause electronic density of states smearing near the Fermi level.

Disorder-induced electronic reconstruction correlates with Tc suppression.

Abstract

Rock-salt type niobium nitride ($\delta$-NbN) is a well-known superconductor having superconducting transition temperature (Tc) $\approx$ 18\,K and a large superconducting gap $\approx$3\,meV. The Tc of $\delta$-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 $\delta$-NbN thin film by employing combined methods of experiments and ab-initio simulations. Sputtered $\delta$-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 $\delta$-NbN by smearing its electronic densities of states around Fermi level.