Friedrich Christoph, Saşıoğlu Ersoy, Müller Mathias, Schindlmayr Arno, Blügel Stefan
Peter Grünberg Institut and Institute for Advanced Simulation, Forschungszentrum Jülich and JARA, 52425, Jülich, Germany,
Top Curr Chem. 2014;347:259-301. doi: 10.1007/128_2013_518.
Collective spin excitations form a fundamental class of excitations in magnetic materials. As their energy reaches down to only a few meV, they are present at all temperatures and substantially influence the properties of magnetic systems. To study the spin excitations in solids from first principles, we have developed a computational scheme based on many-body perturbation theory within the full-potential linearized augmented plane-wave (FLAPW) method. The main quantity of interest is the dynamical transverse spin susceptibility or magnetic response function, from which magnetic excitations, including single-particle spin-flip Stoner excitations and collective spin-wave modes as well as their lifetimes, can be obtained. In order to describe spin waves we include appropriate vertex corrections in the form of a multiple-scattering T matrix, which describes the coupling of electrons and holes with different spins. The electron-hole interaction incorporates the screening of the many-body system within the random-phase approximation. To reduce the numerical cost in evaluating the four-point T matrix, we exploit a transformation to maximally localized Wannier functions that takes advantage of the short spatial range of electronic correlation in the partially filled d or f orbitals of magnetic materials. The theory and the implementation are discussed in detail. In particular, we show how the magnetic response function can be evaluated for arbitrary k points. This enables the calculation of smooth dispersion curves, allowing one to study fine details in the k dependence of the spin-wave spectra. We also demonstrate how spatial and time-reversal symmetry can be exploited to accelerate substantially the computation of the four-point quantities. As an illustration, we present spin-wave spectra and dispersions for the elementary ferromagnet bcc Fe, B2-type tetragonal FeCo, and CrO₂ calculated with our scheme. The results are in good agreement with available experimental data.
集体自旋激发是磁性材料中一类基本的激发态。由于其能量低至仅几毫电子伏特，它们在所有温度下都存在，并对磁系统的性质产生重大影响。为了从第一性原理研究固体中的自旋激发，我们基于多体微扰理论，在全势线性缀加平面波（FLAPW）方法的基础上开发了一种计算方案。主要关注的量是动态横向自旋磁化率或磁响应函数，从中可以获得磁激发，包括单粒子自旋翻转斯托纳激发和集体自旋波模式及其寿命。为了描述自旋波，我们以多重散射T矩阵的形式包含适当的顶点修正，该矩阵描述了不同自旋的电子和空穴的耦合。电子 - 空穴相互作用在随机相位近似内纳入了多体系统的屏蔽。为了降低评估四点T矩阵的数值成本，我们利用了一种变换，转换到最大局域化的万尼尔函数，该函数利用了磁性材料部分填充的d或f轨道中电子关联的短空间范围。详细讨论了该理论和实现方法。特别是，我们展示了如何针对任意k点评估磁响应函数。这使得能够计算平滑的色散曲线，从而可以研究自旋波谱在k依赖性方面的精细细节。我们还展示了如何利用空间和时间反演对称性来大幅加速四点量的计算。作为示例，我们展示了用我们的方案计算的体心立方铁、B2型四方铁钴和二氧化铬等基本铁磁体的自旋波谱和色散。结果与现有实验数据吻合良好。
