Vacancy-free cubic superconducting NbN enabled by quantum anharmonicity

Abstract Niobium nitride is renowned for its exceptional mechanical, electronic, magnetic, and superconducting properties. The ideal 1:1 stoichiometric δ -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 stable cubic phase with space group $$P\bar{4}3m$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>P</mml:mi> <mml:mover> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> <mml:mn>3</mml:mn> <mml:mi>m</mml:mi> </mml:math> emerges — 65 meV/atom lower in free energy than the δ phase. This discovery is enabled by state-of-the-art first-principles calculations accelerated by machine-learned interatomic potentials. To evaluate the vibrational properties with quantum anharmonic effects accounted for, we use the stochastic self-consistent harmonic approximation and molecular dynamics spectral energy density methods. Electron-phonon coupling calculations based on the anharmonic phonon dispersion yield a superconducting transition temperature of 20 K, which aligns 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.