Ab Initio Downfolding Approach to Exciton-Continuum

该奖项支持研究、教育和推广活动，重点是了解材料中光的吸收所产生的激发。理解这些激发对于依赖于光吸收的广泛过程至关重要，包括太阳能电池的光收集，植物的光合作用以及由光催化的化学反应。在所有这些过程中，光的吸收产生激子，激子是由一个带负电的电子和一个带正电的空穴通过库仑相互作用结合在一起组成的激发态。计算这些激子的能级和特征是可能的，因此，可以被材料吸收的能量和光的量，但是目前，由于计算的计算成本，这种性质的精确计算仅限于只有几十个原子的系统和在小能量范围内的激发。该项目旨在开发、实施和应用新的计算技术，这些技术将允许计算包含多达数千个原子的更大的系统，其能量跨越从红外到x射线范围的电磁波谱。其主要思想是将大型系统划分为较小的子系统。只要激子可以很好地近似于单个子系统，那么每个子系统都可以单独计算，这大大降低了计算成本。子系统之间相互作用的影响可以通过改变每个子系统内部相互作用的强度来近似，这反过来揭示了子系统之间的散射如何影响激子能量和寿命。该方法将应用于研究x射线吸收光谱和块状材料表面吸附的分子。该项目将加强对更高能量和更快时间尺度激子的理解，并为开发改进的光收集和分子传感设备铺平道路。这个项目支持一名研究生的教育。此外，该项目将把研究、教育和指导与一个框架式的推广计划结合起来，旨在扩大在科学和工程领域历来代表性不足的群体的参与。PI将举办公开讲座，开展暑期材料科学研讨会，并为纽黑文公立学校系统的学生提供每年一次的本科暑期实习机会。拟议的外展包括一个严格的数据收集部分，将允许评估项目的影响。该奖项支持研究、教育和推广活动，重点是理解纳米结构和异质系统中子空间之间的激子动力学和散射。目前，在多体微扰理论（MBPT）中，激子的从头计算受到相当大的计算成本的限制，只能在具有数十个原子和小于10 eV的能量范围的小系统上进行静态计算，这使得许多当前物理感兴趣的系统，如异质结构、缺陷和复杂纳米结构，超出了传统计算的范围。该项目旨在开发、实施和应用MBPT内的从头算形式，描述由能量尺度或空间定位定义的不同物理子系统中激子之间的动态相互作用。目标是扩展这些计算技术的当前能力，以研究更大，更复杂的系统和数百到数千eV的能量尺度中的光-物质相互作用。该项目同时开发新技术，包括激子谱计算中物理子系统之间耦合的动态和寿命效应。所提出的方法是基于GW + Bethe Salpeter方程（GW- bse）形式主义的扩展。而不是截断希尔伯特空间，就像在传统计算中所做的那样，物理子系统到更大的希尔伯特空间的耦合将通过包含耦合动力学的矩阵下折叠方法来解释。所开发的方法将应用于二维层状范德华材料、氧化物和非晶系统的x射线吸收光谱中与激子相关的光谱和寿命的计算。所开发的方法也将应用于研究分子和二维材料之间异质界面上的激子能量传递。这个项目支持一名研究生的教育。此外，该项目将把研究、教育和指导与一个框架式的推广计划结合起来，旨在扩大在科学和工程领域历来代表性不足的群体的参与。PI将举办公开讲座，开展暑期材料科学研讨会，并为纽黑文公立学校系统的学生提供每年一次的本科暑期实习机会。拟议的外展包括一个严格的数据收集部分，将允许评估项目的影响。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NONTECHNICAL SUMMARYThis award supports research, education, and outreach activities focused on understanding excitations created by the absorption of light in materials. Understanding these excitations is of fundamental importance to a wide range of processes that rely on light absorption, including light harvesting by solar cells, photosynthesis in plants, and chemical reactions catalyzed by light. In all these processes, the absorption of light creates excitons, which are excited states composed of a negatively charged electron and a positively charged hole bound together by the Coulomb interaction. It is possible to calculate the energy levels and character of these excitons, and consequently, the energy and amount of light that can be absorbed by a material, but currently, accurate calculations of this nature are limited to systems with only tens of atoms and excitations over a small energy range due to the computational cost of the calculation. This project aims to develop, implement and apply new computational techniques that will allow for the calculation of much larger systems containing up to thousands of atoms and with energies spanning the electromagnetic spectrum from the infrared to x-ray ranges. The main idea is to divide the largescale system into smaller subsystems. As long as excitons can be well-approximated as being confined to a single subsystem, then each subsystem can be calculated separately, which dramatically reduces the computational cost. The effect of interactions between the subsystems is approximated by changing the strength of interactions inside each subsystem, which in turn reveals how scattering between subsystems can influence the exciton energies and lifetimes. This method will be applied to study x-ray absorption spectra and molecules adsorbed on the surface of bulk materials. This project will enhance understanding of excitons at higher energy and faster timescales, and pave the way for the development of improved devices for light harvesting and molecular sensing.This project supports the education of a graduate student. Additionally, this project will integrate research, education, and mentoring with a scaffolded outreach program designed to broaden the participation of groups that are historically underrepresented in science and engineering. The PI will give public lectures, develop a summer materials science workshop, and offer a yearly undergraduate summer internship for students from the New Haven public school system. The proposed outreach includes a rigorous data collection component that will allow the impact of the program to be assessed.TECHNICAL SUMMARYThis award supports research, education, and outreach activities focused on understanding exciton dynamics and scattering between subspaces in nanostructured and heterogeneous systems. Currently, ab initio calculations of excitons within many-body perturbation theory (MBPT) are limited by their considerable computational cost to static calculations on small systems with tens of atoms and energy ranges of less than 10 eV, leaving many systems of current physical interest, such as heterostructures, defects, and complex nanostructures, beyond the reach of conventional calculations. This project aims to develop, implement and apply an ab initio formalism within MBPT that describes the dynamical interactions between excitons in different physical subsystems, defined by energy scales or spatial localization. The objective is to extend the current capabilities of these computational techniques to study light-matter interactions in larger, more complex systems and over energy scales of hundreds to thousands of eV. This project concurrently develops new techniques to include dynamical and lifetime effects due to coupling between physical subsystems in calculations of exciton spectra.The proposed methods are based on an extension of the GW plus Bethe Salpeter equation (GW-BSE) formalism. Instead of truncating the Hilbert space, as is done in conventional calculations, the coupling of a physical subsystem to a larger Hilbert space will be accounted for through a matrix downfolding approach that includes the dynamics of the coupling. The developed methods will be applied to the computation of spectra and lifetimes associated with excitons in x-ray absorption spectroscopy of two-dimensional layered van der Waals materials, oxides, and amorphous systems. The developed methods will also be applied to study excitonic energy transfer across heterointerfaces between molecules and two-dimensional materials. This project supports the education of a graduate student. Additionally, this project will integrate research, education, and mentoring with a scaffolded outreach program designed to broaden the participation of groups that are historically underrepresented in science and engineering. The PI will give public lectures, develop a summer materials science workshop, and offer a yearly undergraduate summer internship for students from the New Haven public school system. The proposed outreach includes a rigorous data collection component that will allow the impact of the program to be assessed.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.