Excitons are electron-hole pairs appearing below the band gap in insulators and semiconductors. They are vital to photovoltaics, but are hard to obtain with time-dependent density-functional theory (TDDFT), since most standard exchange-correlation (xc) functionals lack the proper long-range behavior. Furthermore, optical spectra of bulk solids calculated with TDDFT often lack the required resolution to distinguish discrete, weakly bound excitons from the continuum. We adapt the Casida equation formalism for molecular excitations to periodic solids, which allows us to obtain exciton binding energies directly. We calculate exciton binding energies for both small- and large-gap semiconductors and insulators, study the recently proposed bootstrap xc kernel [S. Sharma et al., Phys. Rev. Lett. 107, 186401 (2011)], and extend the formalism to triplet excitons.
Assessment of long-range-corrected exchange-correlation kernels for solids: accurate exciton binding energies via an empirically scaled Bootstrap kernel
Stable excitons in semiconductor monolayers such as transition-metal dichalcogenides (TMDCs) enable and motivate fundamental research as well as the development of room-temperature optoelectronics applications. The newly discovered layered magnetic materials present a unique opportunity to integrate optical functionalities with magnetism. We predict that a large class of antiferromagnetic semiconducting monolayers of the MPX$_3$ family exhibit giant excitonic binding energies, making them suitable platforms for magneto-optical investigations and optospintronics applications. Indeed, our invest…
The electronic and optical properties of monolayer transition-metal dichalcogenides (TMDs) and van der Waals heterostructures are strongly subject to their dielectric environment. In each layer the field lines of the Coulomb interaction are screened by the adjacent material, which reduces the single-particle band gap as well as exciton and trion binding energies. By combining an electrostatic model for a dielectric hetero-multi-layered environment with semiconductor many-particle methods, we demonstrate that the electronic and optical properties are sensitive to the interlayer distances on the…
Diameter and Chirality Dependence of Exciton Properties in Carbon Nanotubes
我们计算了半导体单壁碳纳米管（SWNTs）中激子的结合能、尺寸和亮暗分裂随直径和手性的变化。利用{ \it ab initio}计算的结果和见解，我们采用了基于有效质量和密度函数近似的变分方法，使用紧束缚波函数。结合能和空间范围显示出与直径的主要依赖性，分别为$1/d$和$d$，手性校正提供了大约20%的扩散，具有强的家族行为。暗激子分裂显示出$1/d^2$领先的依赖性。我们提供的结合能，大小和分裂，应该是有用的，以指导未来的实验的解析表达式。
Direct-gap materials hold promises for excitonic insulator. In contrast to indirect-gap materials, here the difficulty to distinguish from a Peierls charge density wave is circumvented. However, direct-gap materials still suffer from the divergence of polarizability when the band gap approaches zero, leading to diminishing exciton binding energy. We propose that one can decouple the exciton binding energy from the band gap in materials where band-edge states have the same parity. First-principles calculations of two-dimensional GaAs and experimentally mechanically exfoliated single-layer TiS 3…
Wigner-Seitz truncated TDDFT approach for the calculation of exciton binding energies in solids
Centro de Física de Materiales - Materials Physics Center (CFM-MPC)（材料物理中心）；Donostia International Physics Center (DIPC)（多诺西亚国际物理中心）；EHU Quantum Center, Universidad del País Vasco/Euskal Herriko Unibertsitatea UPV/EHU（巴斯克大学量子中心）
Spectral ordering between dark and bright excitons in transition metal dichalcogenides is of increasing interest for optoelectronic applications. However, little is known about dark exciton energies and their binding energies. We report the exciton landscape including momentum-forbidden dark excitons of MoS$_{2}$ monolayer using single shot GW-Bethe Salpeter equation (G$_{0}$W$_{0}$-BSE) calculations. We find the lowest-energy exciton to be indirect at ($\textrm K'_{v} \rightarrow \textrm K_{c}$) in agreement with recent GdW-BSE calculations [2D Mater. 6, 035003 (2019)]. We also find that by l…
Pressure effects on neutral and charged excitons in self-assembled InGaAs/GaAs quantum dots
通过结合原子赝势方法和组态相互作用方法，我们预测了透镜形自组装In $_{0.6}$ Ga $_{0.4}$ As/GaAs量子点中中性和带电激子：$X^0$（中性单激子），$X^{-}$和$X^{+}$（带电三子），以及$XX^0$（双激子）的结合能随压力的变化.结果表明：（1）随着压力的增加，$X^0$和$X^+$的结合能增加，$X^-$的结合能减小，而$XX^0$的结合能几乎不受压力的影响。(ii)相关性对$X^0$的结合能影响很小，而它们在很大程度上决定了$X^-$、$X^+$和$XX^0$的结合能。(iii)相关性弱依赖于压力;因此，结合能的压力依赖性可以在Hartree-Fock近似中理解，并且它由直接库仑积分$J$的压力依赖性控制。因此，我们在（i）中的结果可以通过注意到空穴比电子更局域化来解释，因此库仑能量服从$J^{(hh)}>J^{(eh)}>J^{(ee)}$。
Influence Functional Approach to Non-Perturbative Exciton Binding Renormalization from Phonons
University of California, Berkeley（加州大学伯克利分校）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；The University of Texas at Austin（德克萨斯大学奥斯汀分校）；Oden Institute for Computational Engineering and Sciences, The University of Texas at Austin（德克萨斯大学奥斯汀分校奥登计算工程与科学研究所）；Kavli Energy NanoScience Institute（卡弗里能源纳米科学研究所）
First-principles density functional theory plus Bethe-Salpeter equation calculations are employed to investigate the electronic and excitonic properties of monolayer titanium trichalcogenide alloys TiS$_{3-x}$Se$_x$ ($x$=1 and 2). It is found that bandgap and exciton binding energy display asymmetric dependence on the substitution of Se for S. While the bandgap can be significantly decreased as compared to that of pristine TiS$_3$, the exciton binding energy just varies a little, regardless of position and concentration of the Se substitution. A negative exciton formation energy is found when …
Accurately described excitonic properties of transition metal dichalcogenide heterobilayers (HBLs) are crucial to comprehend the optical response and the charge carrier dynamics of them. Excitons in multilayer systems posses inter or intralayer character whose spectral positions depend on their binding energy and the band alignment of the constituent single-layers. In this study, we report the electronic structure and the absorption spectra of MoS$_2$/WS$_2$ and MoSe$_2$/WSe$_2$ HBLs from first-principles calculations. We explore the spectral positions, binding energies and the origins of inte…
Absorption and emission of pristine-like semiconducting monolayers of BN, AlN, GaN, and InN are here systematically studied by ab-initio methods. We calculate the absorption spectra for in-plane and out-of-plane light polarization including quasiparticle and excitonic effects. Chemical trends with the cation of the absorption edge and the exciton binding are discussed in terms of the band structures. Exciton binding energies and localization radii are explained within the Keldysh model for excitons in two dimensions. The strong excitonic effects are due to the interplay of low dimensionality, …
Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (SWCNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100~meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells. Here, we have conducted terahertz emission and photocurrent …
Photoexcited intralayer excitons in van der Waals heterostructures (vdWHs) with type-II band alignment have been observed to tunnel into interlayer excitons on ultrafast timescales. Such interlayer excitons have sufficiently long lifetimes that inducing dissociation with external in-plane electric fields becomes an attractive option of improving efficiency of photocurrent devices. In the present paper, we calculate interlayer exciton binding energies, Stark shifts, and dissociation rates for six different transition metal dichalcogenide (TMD) vdWHs using a numerical procedure based on exterior…
XPANCEO；Moscow Center for Advanced Studies（莫斯科高等研究中心）；Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences（中国科学院福建物质结构研究所）；Quantum Science Center of Guangdong–Hong Kong–Macao Greater Bay Area (Guangdong)（粤港澳大湾区量子科学中心（广东））；University of Manchester（曼彻斯特大学）；National University of Singapore（新加坡国立大学）
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 V$_2$O$_5$ , 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…
A theoretical study of the exciton binding energy in the two-dimensional hexagonal boron nitride monolayer is presented within the tight-binding approximation (TBA). A self-consistent equation for the interband electron-hole propagators is derived and in the long wavelength limit reduced to the standard hydrogen atom like Schrodinger equation. It is shown that inclusion of dynamically screened Coulomb interaction in ladder term is of crucial importance for proper description of exciton binding energy. This leads to the self-consistent eigenvalue problem with dynamical screening. The dependence…
Exciton problem is solved in the two-dimensional Dirac model with allowance for strong electron-hole attraction. The exciton binding energy is assumed smaller than but comparable to the band gap. The exciton wavefunction is found in the momentum space as a superposition of all four two-particle states including electron and hole states with both positive and negative energies. The matrix element of exciton generation is shown to depend on the additional components of the exciton wavefunction. Both the Coulomb and the Rytova-Keldysh potentials are considered. The dependence of the binding energ…
