Rise and fall of Mott insulating gaps in YNiO3 paramagnets as a reflection of symmetry breaking and remaking
Rise and fall of Mott insulating gaps in YNiO3 paramagnets as a reflection of symmetry breaking and remaking
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YNiO3 顺磁体中莫特绝缘间隙的上升和下降反映了对称性破缺和重构
DOI：
10.1103/physrevmaterials.7.044409
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发表时间：
2023
影响因子：
3.4
通讯作者：
Zunger, Alex
中科院分区：
文献类型：
--
作者：
Malyi, Oleksandr I.;Zunger, Alex
镍酸盐是一种范式电子氧化物，表现出从(i)磁性有序绝缘体到（ii）顺磁性（PM）绝缘体再到（iii） PM金属的三个有趣的温度介导的相变序列。这种现象提出了磁性和结构对称性破缺与绝缘带隙在(i)和（ii）中出现和在（iii）中消失之间联系的本质问题。本文证明了由分子动力学温度演化驱动的第一性原理类平均场密度泛函理论（DFT）不仅可以描述磁性长程有序绝缘相(i)的起源，还可以描述缺乏自旋长程有序绝缘顺磁体（ii）的产生，以及温度升高时金属顺磁体（iii）的产生。该方法提供了不同温度下结构和磁对称破缺的模式，并与将演化几何用作DFT电子带结构计算输入时获得的带隙平行。这就解开了自旋、电荷和轨道自由度之间复杂的相互作用。分析表明，通过允许足够的灵活性来描述作为DFT输入的不同局部结构和磁性基元，可以成功地描述沿相变序列的绝缘带隙的上升和下降。这需要使用足够大的超级细胞来表达八面体的结构歧化，以及将PM相描述为局部磁矩的分布（而不是使用单个平均矩）。历史上对类平均场DFT的摒弃似乎是不成熟的，因为它是基于平均晶体学单位细胞的考虑，这种描述洗掉了局部对称性破坏的基元。磁有序绝缘相(i)和PM绝缘相（ii）由于允许对称性破缺而导致DFT，考虑非热内能已经很明显了。相反，PM金属相（iii）是通过涂抹形成的，从而削弱了对称破缺。对不同形式的结构与磁对称破缺的快照分析表明，只有（ii）中存在的磁矩的多态分布的损失才会导致带隙的下降，从而导致（iii）中的金属态。有趣的结论是，对绝缘间隙的上升[在(i)和（ii）阶段]和下降[在（iii）阶段]的这种描述并不依赖于传统的莫特式强相关理解，而是依赖于能量降低中反映的磁性和结构对称性的破坏和重塑。
Thenickelate is a paradigm-electron oxide that manifests the intriguing temperature-mediated sequence of three phases transitions from (i) magnetically ordered insulator to (ii) paramagnetic (PM) insulator and then to (iii) PM metal. Such phenomena raised the question of the nature of the association of magnetism and structural symmetry breaking with the appearance in (i) and (ii) and disappearance in (iii) of insulating band gaps. It is demonstrated here that first-principles mean-field–like density-functional theory (DFT), driven by molecular dynamics temperature evolution, can describe not only the origin of the magnetically long-range ordered insulating phase (i), but also the creation of aninsulating paramagnet(ii) that lacks spin- long-range order, and of a metallic paramagnet (iii) as temperature rises. This approach provides the patterns of structural and magnetic symmetry breaking at different temperatures, in parallel with band gaps obtained when the evolving geometries are used as input to DFT electronic band-structure calculations. This disentangles the complex interplay among spin, charge, and orbital degrees of freedom. Analysis shows that the success in describing the rise and fall of the insulating band gaps along the phase transition sequence is enabled by allowing sufficient flexibility in describing diverse local structural and magnetic motifs as input to DFT. This entails the use of sufficiently large supercells that allow expressing structural disproportionation of octahedra, as well as a description of PM phases as a distribution of local magnetic moments (rather than using a single averaged moment). It appears that the historic dismissal of mean-field–like DFT as being unable to describe such Mott-like transitions was premature, as it was based on consideration of averaged crystallographic unit cells, a description that washes out local symmetry-breaking motifs. The magnetically ordered insulatingphase (i) and the PM insulating phase (ii) result in DFT from allowing symmetry breaking, evident already by considering the athermal internal energy. In contrast, the PM metallic phase (iii) is formed thermally by smearing out thus weakening symmetry breaking. Analysis of snapshots of the different forms of structural vs magnetic symmetry breaking shows that only the loss of the polymorphous distribution of magnetic moments existing in (ii) causes the fall of the band gap, resulting in the metallic state in (iii). The interesting conclusion is that such a description of the rise [in phases (i) and (ii)] and fall [in phase (iii)] of the insulating gap does not rely on the traditional Mott-like strong correlation understanding, but on breaking and remaking of magnetic and structural symmetries reflected in energy lowering.