量子非谐性实现无空位立方超导NbN
Vacancy-free cubic superconducting NbN enabled by quantum anharmonicity
AI总结：
通过第一性原理计算和量子非谐效应，发现无空位的立方相NbN（空间群P$\bar{4}3m$）是稳定的，其超导转变温度约20 K，挑战了空位稳定性的传统观点。
摘要：
氮化铌（NbN）以其优异的机械、电子、磁性和超导性能而闻名。然而，理想的1:1化学计量$\delta$-NbN立方相被认为是动态不稳定的，反复的实验观察表明空位对其稳定是必要的。在这项工作中，我们证明，当结构完全弛豫并允许在量子非谐效应下畸变时，出现了一种先前未报道的稳定立方相，空间群为$P\bar{4}3m$，其自由能比理想的$\delta$相低65 meV/原子。这一发现是通过机器学习原子间势加速的最先进的第一性原理计算实现的。为了评估考虑量子非谐效应的振动和超导性质，我们使用了随机自洽谐波近似（SSCHA）和分子动力学谱能量密度（SED）方法。基于SSCHA声子色散的电子-声子耦合计算得出超导转变温度$T_\text{c}$ = 20 K，这与近化学计量NbN的实验报告值非常吻合。这些发现挑战了空位对稳定立方NbN必不可少的长期假设，并指出了合成理想1:1化学计量相作为实现这种技术重要材料增强超导性能的途径的潜力。
英文摘要：
Niobium nitride (NbN) 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 previously unreported stable cubic phase with space group $P\bar{4}3m$ emerges - 65 meV/atom lower in free energy than the ideal $δ$ 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.
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