来源：npj计算材料学
激子的结合能通常被认为与其空间位置有关。教科书上经典的 Wannier 和 Frenkel 模型分别描述了通常在普通半导体中观察到的弱束缚（离域）激子和在分子或惰性气体中观察到的紧密束缚（局域）激子的极限情况。近年来二维材料的发展，引起了人们探索与传统激子特性不一样的激子系统。二维材料的激子结合能普遍很大，且与与空间位置无关，这通常被认为与低维材料的有效屏蔽有关。然而，关于是否可以将强束缚电荷转移激子认为是 Frenkel 激子还一直存在争议。
Fig. 1 Absorption spectra of V2O5 for x, y, and z polarization (the V2O5 layers are in the xy plane).
来自法国巴黎理工学院的Gorelov教授及其同事，提出了一个激子模型，成功解释了电荷转移激子的各向异性和离域性质。他们以V2O5作为研究原型对象，结合第一性原理计算、椭偏实验和紧束缚模型，研究了局部电荷转移激发是如何形成激子的。V2O5作为平带材料，其激子具有与传统预期相矛盾的有趣特性：平带对应于局部电子态，激子结合能非常大。当混合形成激子的单粒子激发具有镜像对称性的电荷转移激发时，激子的波函数是离域的，并显示出了由电荷转移单元局部基序控制的各向异性。此外，他们还发现，明暗激子是一起出现在间隙中的，他们强烈的束缚在一起。在最低的明暗激子中，V2O5中桥氧上的空穴电子密度垂直于电子分散的原子链。这项工作为识别具有特定需求的激子的材料提供了有价值的指导。该文近期发布于npj Computational Materials 8: 94 (2022)。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. 2 Electron density distribution for dark and bright excitons.
Editorial Summary
It is common belief that the exciton binding energy is directly dependent on spatial localization. The classical Wannier and Frenkel textbook modelsdescribe, respectively, the limiting cases of weakly bound and delocalized excitons, typically found in ordinary semiconductors, and tightly bound and localized excitons, observed in molecular or noble gas solids. Recent development of two-dimensional (2D) materials have sparked the interest in exploring excitons that stand out from the conventional expectation. 2D materials usually have large exciton binding energies, irrespective of spatial extent, which can be rationalized as the consequence of reduced effective screening. However, there has been a long-standing controversy about whether strongly bound charge-transfer excitons can be considered Frenkel excitons.
Prof. Gorelov and coworkers from Institut Polytechnique de Paris and coworkers proposed an exciton model that explains the counter-intuitive anisotropy and delocalization of charge-transfer excitons. They taked V2O5 as a prototypical system to study how the localized charge-transfer excitations form an exciton by combing first-principles calculations, ellipsometry experiments and tight-binding model. As a flat-band material, V2O5 has intriguing excitons which contradict textbook expectations. Flat bands correspond to localized electronic states, and the exciton binding energy is very large. When the single-particle excitations that are mixed to form an exciton are charge-transfer excitations, the wave function of the exciton is delocalized, which shows an anisotropy determined by the local motif of the charge-transfer unit. In addition, they found that the dark and bright excitons are strongly bound in the gap. In the lowest dark and bright excitons, the electron density for a hole on a bridge oxygen extends perpendicular to the atomic chains along which electrons disperse. This work provides valuable guidelines for identifying materials where excitons exhibit properties tailored for specific needs. Thisarticle was recently published in npj Computational Materials 8: 94 (2022).
Fig. 4 Contributions to the optical absorption intensity.
原文ABstract及其翻译
Delocalization of dark and bright excitons in flat-band materials and the optical properties of V2O5 (平带材料中明暗激子的离域和V2O5的光学性质)
Vitaly Gorelov, Lucia Reining, Martin Feneberg, Rüdiger Goldhahn, André Schleife, Walter R. L. Lambrecht & Matteo Gatti
Abstract The simplest picture of excitons in materials with atomic-like localization of electrons is that of Frenkel excitons, where electrons and holes stay close together, which is associated with a large binding energy. Here, using the example of the layered oxide V2O5, we show how localized charge-transfer excitations combine to form excitons that also have a huge binding energy but, at the same time, a large electron-hole distance, and we explain this seemingly contradictory finding. The anisotropy of the exciton delocalization is determined by the local anisotropy of the structure, whereas the exciton extends orthogonally to the chains formed by the crystal structure. Moreover, we show that the bright exciton goes together with a dark exciton of even larger binding energy and more pronounced anisotropy. These findings are obtained by combining first principles many-body perturbation theory calculations, ellipsometry experiments, and tight binding modeling, leading to very good agreement and a consistent picture. Our explanation is general and can be extended to other materials.
摘要在具有类原子局域电子的材料中，激子最简单的图像是Frenkel激子，其中电子和空穴紧密靠着，这与大的结合能有关。在这里，以层状氧化物V2O5为例，我们展示了局域电荷转移激发是如何结合形成激子的，这些激子具有巨大的结合能，但电子-空穴的距离同时也很大，我们解释了这个看似矛盾的发现。激子离域的各向异性是由结构的局部各向异性决定的，而激子则正交地延伸到由晶体结构形成的链上。此外，我们发现明亮的激子是与一个具有更大结合能和更明显各向异性的暗激子一起出现的。这些发现通过结合第一性原理多体微扰理论计算、椭圆偏振实验和紧密结合模型得到，取得了非常好的一致性图像。我们的解释具有普适性且可以推广到其他材料。
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