发表机构
New York University; ELI-NP, “Horia Hulubei” National Institute for Physics and Nuclear Engineering（纽约大学; “霍里亚·胡卢贝伊”国家物理与核工程研究所ELI-NP）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本文通过第一性原理模拟，结合机器学习力场与从头计算的自旋-转动耦合张量，揭示了NH$_4^+$角动量涨落主导碘化铵转子相核自旋弛豫，并发现声子-转动耦合的贡献，为固态动力学模拟提供了定量基准。
AI 中文摘要
核自旋弛豫已被广泛用于探测固体中的分子重取向，但其解释始终需要同时假设弛豫机制和底层相关函数的形式。在转子相固体中，更多未知因素进入视野：在碘化铵中，质子T$_1$经过一个极大值后随温度降低而减小，这一行为归因于自旋-转动耦合。这里我们从头出发对固态NH$_4$I中$^1$H弛豫的偶极和自旋-转动贡献进行建模，通过将分子动力学与基于密度泛函训练的机器学习力场以及从头计算的自旋-转动耦合张量相结合。计算得到的速率重现了有序-无序转变温度以上的测量值，以及其温度依赖性和频率无关性，并表明NH$_4^+$角动量涨落主导了整个转子相中的弛豫。从模拟中直接获得的取向和角速度相关函数进一步共享一个处于晶格声子频率的振荡分量，揭示了晶格振动与阳离子重取向之间的耦合，该耦合对弛豫有可测量的贡献。分离这一分量得到的重取向势垒与中子散射结果一致。该方法使核自旋弛豫成为分子固体中模拟动力学的定量基准，并提供了直接获取固态中角动量相关函数的途径。
英文摘要
Nuclear spin relaxation has been widely used to probe molecular reorientation in solids, yet its interpretation has always required assuming both the relaxation mechanism and the form of the underlying correlation function. In rotator-phase solids more unknowns enter the picture: in ammonium iodide, the proton T$_1$ passes through a maximum and decreases with temperature, a behavior attributed to spin-rotation coupling. Here we model both the dipolar and the spin-rotation contributions to $^1$H relaxation in solid NH$_4$I from first principles, by combining molecular dynamics with machine-learned force fields trained on density functionals and ab initio spin-rotation coupling tensors. The computed rates reproduce the measured values above the order-disorder transition, together with their temperature dependence and frequency independence, and show that fluctuations of the NH$_4^+$ angular momentum dominate relaxation throughout the rotator phase. The orientational and angular-velocity correlation functions obtained directly from the simulations further share an oscillatory component at lattice-phonon frequencies, revealing a coupling between lattice vibrations and cation reorientation that contributes measurably to relaxation. Separating this component yields a reorientational barrier in agreement with neutron scattering. The approach makes nuclear spin relaxation a quantitative benchmark for simulated dynamics in molecular solids and provides direct access to angular-momentum correlations in the solid state.
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