发表机构
University of Delaware; Universidad Nacional de Colombia（特拉华大学; 哥伦比亚国立大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
该研究对比广义Kadanoff-Baym近似与二阶绝热展开在自旋泵浦、自旋转移力矩和磁惯性中的表现，发现二阶绝热展开能准确描述前两者，但无法捕捉磁惯性特征。
AI 中文摘要
广义Kadanoff-Baym近似（GKBA）[P. Lipavský等人，Phys. Rev. B 34, 6933 (1986)]已成为一种流行且数值高效的算法，用于简化基于非平衡格林函数（NEGF）的时间相关量子输运计算。对于可分解为经典和量子自由度的系统，另一种流行的简化策略是基于经典自由度速度的NEGF绝热展开（AE）[N. Bode等人，Phys. Rev. Lett. 107, 036804 (2011)；S. Deghi等人，Phys. Rev. B 110, 115409 (2024)]，其中经典自由度包括自旋电子学中的局域磁矩（LMMs）或纳米电子学中的核坐标。在此，我们将GKBA和二阶AE与数值精确基准进行比较，针对两端结系统，其中心区域包含量子电子和经典LMMs，并连接到两个半无限正常金属引线。我们采用三个简单模型来展示自旋电子学中的基石性时间相关效应——自旋泵浦和自旋转移力矩（STT），以及作为近期探索现象的磁惯性。我们发现GKBA无法描述由进动LMMs引起的自旋电流泵浦或STT矢量，以及由此诱导的LMM动力学。相反，二阶AE对这两种效应与数值精确基准的匹配都非常出色，从而也揭示了自旋泵浦本质上具有非绝热性。因此，AE为准确描述STT驱动的磁化动力学开辟了道路，包括与第一性原理哈密顿量的结合，同时仅需全NEGF时间演化成本的一小部分。然而，尽管AE包含了LMMs的二阶时间导数项，该方法仍无法捕捉LMMs进动运动之上的快速章动振荡，而这正是磁惯性的标志性特征。
英文摘要
The generalized Kadanoff-Baym ansatz (GKBA) [P. Lipavský {\em et al.}, Phys. Rev. B {\bf 34}, 6933 (1986)] has emerged as a popular and numerically efficient algorithm for simplification of nonequilibrium Green's function (NEGF)-based calculations of time-dependent quantum transport. For systems that can be split into classical and quantum degrees of freedom, another popular simplifying strategy is adiabatic expansion (AE) of NEGF [N. Bode {\em et al.}, Phys. Rev. Lett. {\bf 107}, 036804 (2011); S. Deghi {\em et al.}, Phys. Rev. B {\bf 110}, 115409 (2024)] in terms of the velocity of classical degrees of freedom, such as localized magnetic moments (LMMs) in spintronics or coordinates of nuclei in nanoelectronics. Here we compare GKBA and second-order AE with numerically exact benchmarks for two-terminal junctions whose central region hosting quantum electrons and classical LMMs is attached to two semi-infinite normal metal leads. Three simple models are employed to exhibit cornerstone time-dependent effects in spintronics---spin pumping and spin-transfer torque (STT), as well as magnetic inertia as a recently explored phenomenon. We find that GKBA fails to describe pumping of spin current by precessing LMMs, or STT vectors, and thereby induced LMM dynamics. Conversely, the second-order AE matches numerically exact benchmarks for both effects remarkably well, thereby also revealing the essentially {\em nonadiabatic} nature of spin pumping. Thus, AE opens a path toward an accurate description of STT-driven magnetization dynamics, including combination with first-principles Hamiltonians, while incurring a fraction of the cost of time evolution of full NEGF. However, despite including terms up to the second time derivatives of LMMs into AE, this approach fails to capture fast nutational oscillations on top of the precessional motion of LMMs as the hallmark of magnetic inertia.
Comments13 pages, 6 figures, 106 references
