Mn₅Si₃中的朗道理论与交换不稳定性：反对交替磁性的案例
Landau theory and exchange instabilities in Mn$_5$Si$_3$: A case against altermagnetism
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中文总结 AI 辅助
本研究通过朗道理论、第一性原理计算与蒙特卡洛模拟，分析Mn₅Si₃的磁相，反对其存在假定的交替磁性相，认为其M星处的不稳定性及3M相更合理。
中文摘要 AI 辅助
薄膜Mn₅Si₃是研究最广泛的交替磁性候选材料之一，原因在于其金属性、已证实的反常输运性质，以及被认为能实现自旋极化输运和各类自旋电子学应用的d波交换分裂模式。其假定的交替磁性结构具有零传播矢量，与在M星处有序的共线反铁磁体相（AFM2）形成对比。本研究采用朗道理论、顺磁不稳定性的第一性原理计算以及蒙特卡洛模拟对这两个相进行分析。在朗道理论中，AFM2表现为M星单个臂处受对称性保护的偶宇称、奇置换模式；在Γ点，相同的晶胞内有序模式属于E₂g序参量的共线分支，两种情况均需高阶项进行相选择。针对顺磁无序局域矩态的第一性原理计算可准确识别M星处的主导交换不稳定性，由此得到的经典海森堡模型在合理温度下有序为单格点熵所青睐的正交3M相。Γ点的E₂g模式（其朗道理论包含交替磁性部分）强度显著更弱，且会被代表Mn₅Si₃薄膜的外延应变进一步抑制，这类薄膜表现出反常输运特性；相同的应变还会降低主导磁交换尺度。这些结果为体材料M点不稳定性提供了自然解释，但强烈反对在中等应变类体Mn₅Si₃薄膜中传播矢量从M到Γ的假定迁移，表明若无额外物理机制，相应的交替磁性相不太可能稳定存在。
英文摘要
Thin-film Mn$_5$Si$_3$ is one of the most studied altermagnetic candidates thanks to its metallicity, demonstrated anomalous transport properties, and assumed $d$-wave exchange splitting pattern enabling spin-polarized transport and various spintronic applications. Its postulated altermagnetic structure has zero propagation vector, in contrast to the collinear antiferromagnetic bulk phase (AFM2) which orders at the $M$ star. In this work, the two phases are analyzed using Landau theories, first-principles calculations of the paramagnetic instabilities, and Monte Carlo simulations. AFM2 appears in a Landau theory as a symmetry-protected inversion-even, permutation-odd mode at a single arm of the $M$ star. At $Γ$, the same intracell ordering pattern belongs to the collinear branch of an $E_{2g}$ order parameter. In both cases, higher-order terms are required for the phase selection. First-principles calculations for the paramagnetic, disordered-local-moment state correctly identify the leading exchange instability at the $M$ star, and the resulting classical Heisenberg model orders at a reasonable temperature into the orthogonal $3M$ phase favored by single-site entropy. The $Γ$-point $E_{2g}$ mode, whose Landau theory contains the altermagnetic sector, is substantially weaker and further suppressed by epitaxial strain representative of Mn$_5$Si$_3$ films exhibiting anomalous transport. The same strain reduces the leading magnetic exchange scale. These results provide a natural explanation for the bulk $M$-point instability but strongly disfavor the postulated relocation of the propagation vector from $M$ to $Γ$ in a moderately strained bulklike Mn$_5$Si$_3$ film, suggesting that the corresponding altermagnetic phase is unlikely to be stabilized without additional physics.
