CAREER: Stochastic Methods for Electronic Excitations in Complex Nanoscale System

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NONTECHNICAL SUMMARYA quantitative theoretical understanding of electronic excitations is necessary for searching and designing new electronic devices. Properties of these systems are dominated by quantum mechanical interactions that extend over nanometer scales. In these cases, simulations of electronic excitations pose a considerable challenge for conventional theoretical approaches, and such calculations are often deemed untractable. This CAREER award supports three focus areas: (i) development of new methods that break the current computational limitations; (ii) application advanced quantum many-body theory to complex nanoscale systems to elucidate new strategies for design of electronic devices; and (iii) educational activities in physics, chemistry, and materials science focusing on electronic and optical properties in condensed systems.The research team combines new numerical techniques to describe the dynamics of excitations and electron-electron interactions. The methodology is based on a non-deterministic quantum-statistical approach. It is computationally inexpensive, and it allows treating exceedingly large systems with high accuracy. The project also provides a new methodology to account for finite temperature effects. Hence, it is possible to establish a direct correspondence between computational results and experimental observations. The theoretical tools developed and used in this project will be made available to the community. The new developments will be applied to photovoltaic systems to elucidate how structural and chemical modifications affect the electronic excitations in photovoltaic devices and their interfaces.The educational goals of the project aim to provide tools and training for graduate, undergraduate, as well as K-12 students. The PI will provide graduate students with research experience in theory and computation. Multiple research and training opportunities for undergraduate students will be established through this proposal in collaboration with the Califonia Alliance for Minority Participation. This award also supports a new educational K-12 outreach activity on relations between molecular and electronic structures. TECHNICAL SUMMARYThis CAREER award supports research and education in the theory of electronic excitations. The research team will develop a stochastic form of the many-body perturbation theory based on an efficient numerical sampling of wavefunctions and expectations values of operators. The representation of the electron-electron interactions is derived in the Green’s function formalism, which includes high order correlation (vertex) terms. This step is combined with a new stochastic formalism for computing electron-phonon couplings, which need to be considered for quantitative comparison with experiments. The research team will determine the most accurate and efficient stochastic implementations of the many-body perturbation theory for predicting electronic excitations. The new developments will be applied to organic photovoltaic systems with strong charge transfer and excitonic effects.This project will be carried out in three steps: (i) development of an accurate stochastic many-body framework beyond the commonly used approximations, including the treatment of strong electron-hole interactions (vertex terms); (ii) development of new stochastic methodology to study vibrational effects on excitation energies in large systems; and (iii) applications of the new stochastic framework to large molecular systems and their assemblies in the condensed phase. The new computational framework will enable simulations of quantum many-body interactions in systems with hundreds and thousands of atoms. The results will provide a detailed understanding of how composition, environment, and the degree of intermolecular interactions affect the properties of organic (opto)electronic systems. Effectively, this research bridges theoretical chemistry and the condensed-matter physics research areas. This multifaceted approach is integrated into graduate and undergraduate training, as well as newly developed K-12 outreach activities.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.