Revisiting the instability and superconductivity of 𝛿-NbN from First-Principles

Niobium nitride has emerged as a promising candidate material for superconducting qubits. There are several polymorphs of NbN, while 𝛿-NbN possesses high critical temperature (Tc = 15.3 -18 K) among its crystalline phases. Experimental studies have successfully grown 𝛿-NbN, however, density functional theory calculations report it to be dynamically unstable. In this work, we investigate the origin of this theoretical-experimental discrepancy, and how its structural properties relate to its Tc. We studied the role of strain, hydrostatic pressure,  off-stoichiometry and structural disorder on the stability of various polymorphs of NbN with DFT and their corresponding influence on electron-phonon coupling and Tc, comparing closely with experimental structural characterization and Tc measurements. These results suggest that the higher Tc superconductivity in NbN thin films likely originates from a disorder, bridging a gap between computational predictions and experiment