磁性第二声的第一性原理透视
First-principles perspective on magnetic second sound
摘要：
从理论上研究了铁磁Fe和Ni以及磷属元素化物超导体FeSe的反铁磁相的磁序参量的涨落或纵向自旋激发。这些系统的电荷和自旋动力学的描述通过评估的广义电荷和自旋密度响应函数计算的第一性原理线性响应时间依赖密度泛函理论在绝热局域自旋密度近似。我们观察到，正式的非相互作用的科恩-沙姆系统的功能之间的强耦合的纵向通道中的磁化和电荷动力学，耦合被有效地删除后，包括库仑相互作用的电荷通道和由此产生的外观等离子体激元。纵向自旋涨落在没有戈德斯通玻色子出现的情况下获得了集体性质，类似于非磁性金属如Pd中的顺磁子激发。在铁磁Fe和Ni的纵向自旋动力学是由低能量带内电子空穴对之间的相互作用，而在准二维反铁磁FeSe，它是由带间跃迁与交换分裂的顺序的能量为主。在后一种材料中，集体纵向磁化波动具有定义明确的能量和小动量的长寿命，并出现在粒子-空穴连续体下方。模式成为强烈的朗道阻尼增长的波矢量。我们将我们的理论研究结果与现有的实验自旋极化电子能量损失谱的结果。在体心立方铁，纵向磁模式出现以上的横向自旋波的典型能量，具有可比的斯通纳自旋翻转激发连续的能量，和数量级少的能量比电荷动力学。
英文摘要：
The fluctuations of the magnetic order parameter, or longitudinal spin excitations, are investigated theoretically in the ferromagnetic Fe and Ni as well as in the antiferromagnetic phase of the pnictide superconductor FeSe. The charge and spin dynamics of these systems is described by evaluating the generalized charge and spin density response function calculated from first-principles linear response time dependent density functional theory within adiabatic local spin density approximation. We observe that the formally non-interacting Kohn-Sham system features strong coupling between the magnetization and charge dynamics in the longitudinal channel and that the coupling is effectively removed upon the inclusion of the Coulomb interaction in the charge channel and the resulting appearance of plasmons. The longitudinal spin fluctuations acquire a collective character without the emergence of the Goldstone boson, similar to the case of paramagnon excitations in non-magnetic metals like Pd. In ferromagnetic Fe and Ni the longitudinal spin dynamics is governed by interactions between low-energy intraband electron-hole pairs while in quasi two dimensional antiferromagnet FeSe it is dominated by the interband transitions with energies of the order of exchange splitting. In the later material, the collective longitudinal magnetization fluctuations feature well defined energies and long life times for small momenta and appear below the particle-hole continuum. The modes become strongly Landau-damped for growing wave-vectors. We relate our theoretical findings to existing experimental spin-polarized electron energy loss spectroscopy results. In bulk bcc Fe, the longitudinal magnetic modes appear above the typical energies of transverse spin-waves, have energies comparable with the Stoner spin-flip excitation continuum, and are order of magnitude less energetic than the charge dynamics.
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