DMFT：二维磁性材料的准确计算

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近年来，二维范德华材料因其丰富性质受到广泛关注。Fe_nGeTe₂（FGT）是最近发现的一类二维范德华材料，具有室温下的磁性。然而，FGT类材料的子系统显示出与位点相关的磁性行为。

Fig. 1 Crystal structure of FenGeTe2 monolayers.

因此，对这类材料的磁性和电子关联效应进行准确评估是至关重要的。FGT系统的主要特性与其磁性和电子特性密切相关。

Fig. 2 Ji interactions, MAE and TC for FenGeTe2 (n = 3, 4, 5) monolayers computed with different methods.

然而，由于不同Fe亚晶格的磁矩不同，可能会导致某些铁原子的电子流动性更强。针对FGT的理论计算，目前的研究主要使用标准密度泛函理论（GGA）、包含静态电子相关性的GGA + U（即DMFT ）方法以及包含动态电子相关性效应的GGA + DMFT方法来研究，但哪种方法最为可靠目前仍未定论。

Fig. 3 Distance dependent isotropic symmetric exchange parameters.

来自瑞典乌普萨拉大学物理与天文系的Sukanya Ghosh等，通过第一性原理计算来分析了不同计算方法对于FGT系统的电子结构和磁学性能。

Fig. 4 Distance dependent anisotropic exchange parameters.

他们的研究结果表明，具有静态电子相关性的GGA + U得到的晶格参数、磁矩和居里温度均与实验有较大偏差，故不适用于FGT系统。而标准GGA得到的磁矩与实验一致，但居里温度则被高估许多。

Fig. 5 Comparison of spectral properties of Fe3GeTe2 monolayer.

在GGA + DMFT方法中，加入动态电子相关性以及修改交换关联作用中的各向同性对称交换参数后，磁矩几乎保持不变，而居里温度却降低到与实验结果一致，这说明动力学参数在计算FGT系统的电子结构和磁性性能上是非常有必要的。

Fig. 6 Crystal structure and Jij interactions of bulk Fe3GeTe2

该文近期发布于npj Computational Materials 9: 85(2023)。手机阅读原文，请点击本文底部左下角“阅读原文”，进入后亦可下载全文PDF文件。

Fig. 7 Density of states D(ϵ) of Fe3GeTe2 bulk.

Editorial Summary

DMFT: Accurate calculation of 2D magnetic materials

In recent years, two-dimensional van der Waals materials have attracted extensive attention due to their intriguing properties. Fe_nGeTe₂(FGT), a recently discovered class of two-dimensional van der Waals material, exhibits magnetic behavior at room temperature. However, subsystems belonging to FGT class display site-dependent magnetic properties, necessitating an accurate assessment of their magnetic and electronic effects. The main peculiarity related to all the FGT systems is their site-dependent magnetic and electronic properties. In other words, based on the magnetic moments associated with different Fe sublattices, electrons belonging to some Fe atoms are expected to have more itinerant character than others. Up to this date, researchers have used DFT, DFT + U or DMFT approaches, leaving behind the most crucial question unanswered, i.e., which method is the most reliable one to correctly treat the FGT systems.

Fig. 8 Spectral functi on S(k, ϵ) for bulk Fe3GeTe2.

Sukanya Ghosh et al from the Department of Physics and Astronomy, Uppsala University, performed a first-principles study on the electronic structures and magnetic properties of Fe_nGeTe₂ (n = 3, 4, and 5) systems by GGA, LSDA, GGA + U and GGA + DMFT methods. The results indicated that GGA + U with static electronic correlation yields substantial deviations in lattice parameters, magnetic moments, and Curie temperatures compared to experimental values, rendering it unsuitable for FGT systems. Standard GGA reproduces magnetic moments consistent with experiment, but overestimates Curie temperatures significantly. Upon the inclusion of dynamical electronic correlations within the GGA + DMFT approach, the magnetic moments remain almost unchanged, while the exchange interactions, especially the isotropic symmetric exchange parameters get significantly modified to decrease the Curie temperature substantially to have a nice agreement with the experimental results. This study demonstrates that the consideration of dynamical correlations is necessary to capture the correct electronic structure and magnetic behavior of FGT systems. This article was recently published in npj Computational Materials 9: 86 (2023).

原文Abstract及其翻译

Unraveling effects of electron correlation in two-dimensional Fe_nGeTe₂(n = 3, 4, 5) by dynamical mean field theory（用动态平均场论阐明二维Fe_nGeTe₂（n = 3,4,5）中的电子关联效应）

Sukanya Ghosh, Soheil Ershadrad, Vladislav Borisov & Biplab Sanyal

Abstract The Fe_nGeTe₂systems are recently discovered two-dimensional van-der-Waals materials, exhibiting magnetism at room temperature. The sub-systems belonging to Fe_nGeTe₂class are special because they show site-dependent magnetic behavior. We focus on the critical evaluation of magnetic properties and electron correlation effects in Fe_nGeTe₂ (n = 3, 4, 5) (FGT) systems performing first-principles calculations. Three different ab initio approaches have been used primarily, viz., (i) standard density functional theory (GGA), (ii) incorporating static electron correlation (GGA + U) and (iii) inclusion of dynamic electron correlation effect (GGA + DMFT). Our results show that GGA + DMFT is the more accurate technique to correctly reproduce the magnetic interactions, experimentally observed transition temperatures and electronic properties. The inaccurate values of magnetic moments, exchange interactions obtained from GGA + U make this method inapplicable for the FGT family. Correct determination of magnetic properties for this class of materials is important since they are promising candidates for spin transport and spintronic applications at room temperature.

摘要 Fe_nGeTe₂系统是最近发现的二维范德华材料，在室温下表现出磁性。然而，从属于Fe_nGeTe₂类的子系统却显示出与位点相关的磁性行为。本文着重于对Fe_nGeTe₂（n=3、4、5）（FGT）体系中的磁性能和电子的关联效应进行研究，研究手段为第一性原理计算。主要采用了三种不同的从头计算方法，即（i）标准密度泛函理论（GGA）、（ii）包含静态电子相关性（GGA+U）和（iii）考虑动态电子相关效应（GGA+DMFT）。我们的结果表明，GGA+DMFT的计算结果更加准确，能够正确重现磁相互作用、转变温度和电子性质。而GGA+U计算得出的磁矩、交换相互作用数值不准确，使得该方法不适用于FGT家族。FGT体系材料在室温下的自旋输运和自旋器件应用这一领域非常有应用前景，正确计算其磁性性能非常重要。

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DMFT：二维磁性材料的准确计算

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