顺磁相变过程中电子结构的第一性原理研究

研究磁性量子材料的关键之一是磁相变过程，其中最常见的现象是低温磁有序相向高温顺磁相的转变。对顺磁相的透彻理解有助于我们加深对量子磁体的认识，尤其是随机分布的磁矩和不同局部环境所带来的对称性破缺效应。也正因为如此，顺磁相的理论处理是不简单的；更重要的问题是，这类含有局域电子的“关联固体”在理论上能否用单电子平均场近似合理描述。目前对这类材料的绝大多数计算模拟是通过选取非磁构型下的最小单胞，然后赋以无磁或磁有序构型来完成的，因此一个位点与其近邻单胞的相应位点是完全等价的（即一种“单基元描述”）。虽然这样的近似捕捉到了顺磁相的一些部分特征，但是在许多绝缘材料中，实验确认了顺磁相变前后不发生电子能隙闭合，而无磁单基元描述却未能重现它。其中的关键因素是，局域磁矩的存在改变了局部对称性，从而移除简并同时打开了能隙。在单基元描述下，由于受限单胞引起的高对称性导致了电子轨道半填充，从而产生了与实验严重冲突的金属态。因此，为了正确描述顺磁相，局部自旋导致的对称性破缺必须被严格考虑在内。 在本工作中，我们应用密度泛函理论框架内的两种不同方法，即“单基元描述”和近期由 Zunger 等人开发的“多基元描述”，来研究由温度驱动的顺磁相变中电子结构的变化。通过综合对比体系总能量、对称性、能带色散和带隙，我们解释了正确处理顺磁相的必要性。本文所介绍的例子主要集中在近期受到广泛关注的拓扑材料和低维磁体，包括 Co3Sn2S2、 MnBi2Te4、 MnTe、 RuCl3 和 EuIn2As2等。对于泡利顺磁相，自旋磁矩的消失使得非磁性的单基元描述可以给出合理的结果；而对于局域磁矩无序分布的顺磁相，基于足够大超晶胞结构的多基元描述，可以将不同局域环境对电子结构的影响纳入考虑，例如几何无序导致的对称性降低。这样的多自由度表示可以提供合理的顺磁能带特征和总能量，与实验测量结果进行直接对比。我们的工作在原子水平上为局部磁矩诱导的顺磁相提供了可靠的电子结构描述，为理解新兴量子材料中磁相变或磁诱导拓扑相变过程铺平了道路。

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