Time-Dependent Density-Functional Approaches for Exciton Dynamics

该奖项支持理论和计算研究和教育，以研究扩展系统中的光激发过程，特别强调大块半导体和有机链中的激子效应。PI将使用时变密度泛函理论。为了捕获激子效应，需要具有长空间范围的交换相关函数，而这些函数中只有少数是可用的。主要目标是开发和测试在频率相关线性响应域和非线性实时域产生激子结合的交换相关函数。PI将在线性响应域中使用随时间的密度泛函理论来计算体半导体和绝缘体中的激子结合能。一个双波段模型，证明产生激子结合的各种简单的交换相关核，将扩展到包括额外的波段。将实现和测试各种远程交换相关核，并使用自旋相关的形式计算单重态-三重态激子分裂。PI将应用实时时变密度泛函理论来模拟有机链分子中的短时间激子动力学。激子结合所需的交换相关泛函将从频率相关的线性响应区转移到实时域。一个新的计算工具，可视化激子动力学，时变跃迁密度矩阵，将被开发。将对简单聚合物链进行实时模拟，以研究局部激励如何沿着链扩散并连接到相邻单元。激子的精确时相关密度泛函理论描述将适用于广泛的材料，从块状无机半导体到聚合物和有机异质结。后一种系统将在实时计算中进行探索，以测试随时间变化的跃迁密度矩阵。这些方法的发展可能会对有机光电子学和光伏学产生影响。在先前的国家科学基金会支持下开发的本科凝聚态物理课程将扩大范围，以满足更广泛的受众。将开设一门新的理论材料科学研究生课程，目的是向学生介绍材料理论和模拟的各种主题，包括动手计算练习。该奖项支持理论和计算研究和教育，以开发新的理论和计算方法来描述材料的光学特性，特别是半导体，特别强调由称为聚合物的长链状分子制成的半导体材料。一种精确且计算效率高的方法来模拟这些材料的光学特性，将为理解和设计新型太阳能电池和光电子器件提供重要的帮助。PI将进一步发展和使用一种计算效率高的方法来描述半导体材料与光相互作用期间发生的基本过程。这种被称为时变密度泛函理论的方法已经成功地描述了分子中电子对时变电场的响应。该理论将用于模拟和实时可视化聚合物半导体中电子与光相互作用的基本步骤。在先前的国家科学基金会支持下开发的本科凝聚态物理课程将扩大范围，以满足更广泛的受众。将开设一门新的理论材料科学研究生课程，目的是向学生介绍材料理论和模拟的各种主题，包括动手计算练习。

TECHNICAL SUMMARYThis award supports theoretical and computational research and education to study optical excitation processes in extended systems, with a particular emphasis on excitonic effects in bulk semiconductors and organic chains. The PI will use time-dependent density-functional theory. Exchange-correlation functionals with a long spatial range are required to capture excitonic effects, and only a few of these are available. The primary goal is to develop and test exchange-correlation functionals which produce excitonic binding in the frequency-dependent linear-response domain and in the nonlinear real-time domain.The PI will use time-dependent density-functional theory in the linear-response domain to calculate excitonic binding energies in bulk semiconductors and insulators. A two-band model, demonstrated to produce excitonic binding for various simple exchange-correlation kernels, will be extended to include additional bands. Various long-range exchange-correlation kernels will be implemented and tested, and singlet-triplet exciton splittings will be calculated using a spin-dependent formalism.The PI will apply real-time time-dependent density-functional theory to simulate short-time exciton dynamics in organic chain molecules. The exchange-correlation functionals required for excitonic binding will be carried over from the frequency-dependent linear-response regime into the real-time domain. A new computational tool to visualize exciton dynamics, the time-dependent transition density matrix, will be developed. Real-time simulations will be carried out for simple polymer chains to study how localized excitations spread out along the chains and connect to neighboring units.An accurate time-dependent density-functional theory description of excitons will be relevant and applicable for a wide range of materials, from bulk inorganic semiconductors to polymers and organic heterojunctions. The latter systems will be explored in real-time calculations, to test the time-dependent transition density matrix. Developing these methodologies may have impact on organic optoelectronics and photovoltaics.An undergraduate condensed-matter physics course developed under prior NSF support will be broadened in scope so as to address a wider audience. A new graduate course in theoretical materials science will be developed with the goal to introduce students to a variety of topics in materials theory and simulation, including hands-on computational exercises.NON-TECHNICAL SUMMARYThis award supports theoretical and computational research and education to develop new theoretical and computational methods to describe the optical properties of materials, specifically semiconductors with particular emphasis on semiconductor materials made of long chain-like molecules called polymers. An accurate and computationally efficient approach to simulate the optical properties of these materials will provide important assistance for understanding and designing novel solar cell and optoelectronic devices.The PI will further develop and use a computationally efficient method to describe the fundamental processes that take place during the interaction of semiconductor materials with light. The method known as time-dependent density functional theory has successfully described the response of electrons in molecules to time varying electric fields. This theory will be used to simulate and visualize in real time the basic steps involving the interaction of the electrons in polymer semiconductors with light. An undergraduate condensed-matter physics course developed under prior NSF support will be broadened in scope so as to address a wider audience. A new graduate course in theoretical materials science will be developed with the goal to introduce students to a variety of topics in materials theory and simulation, including hands-on computational exercises.