Vacancy-free cubic superconducting NbN enabled by quantum anharmonicity

TL;DR

This study reveals a stable, vacancy-free cubic phase of NbN stabilized by quantum anharmonic effects, challenging previous beliefs that vacancies are necessary, and demonstrates its potential for enhanced superconductivity.

Contribution

The paper introduces a stable, vacancy-free cubic NbN phase stabilized by quantum anharmonicity, enabled by advanced first-principles calculations and machine learning techniques.

Findings

Stable cubic NbN phase without vacancies identified.

Superconducting transition temperature estimated at 20 K.

Quantum anharmonic effects crucial for phase stabilization.

Abstract

Niobium nitride (NbN) is renowned for its exceptional mechanical, electronic, magnetic, and superconducting properties. The ideal 1:1 stoichiometric $\delta$-NbN cubic phase, however, is known to be dynamically unstable, and repeated experimental observations have indicated that vacancies are necessary for its stabilization. In this work, we demonstrate that when the structure is fully relaxed and allowed to distort under quantum anharmonic effects, a previously unreported stable cubic phase with space group $P\bar{4}3m$ emerges - 65 meV/atom lower in free energy than the ideal $\delta$ phase. This discovery is enabled by state-of-the-art first-principles calculations accelerated by machine-learned interatomic potentials. To evaluate the vibrational and superconducting properties with quantum anharmonic effects accounted for, we use the stochastic self-consistent harmonic approximation (SSCHA) and molecular dynamics spectral energy density (SED) methods. Electron-phonon coupling calculations based on the SSCHA phonon dispersion yield a superconducting transition temperature of $T_\text{c}$ = 20 K, which aligns closely with experimentally reported values for near-stoichiometric NbN. These findings challenge the long-held assumption that vacancies are essential for stabilizing cubic NbN and point to the potential of synthesizing the ideal 1:1 stoichiometric phase as a route to achieving enhanced superconducting performance in this technologically significant material.