CAREER: Investigating Strong Electron-Phonon Interactions in Semiconducting Crystals Using Reciprocal-Space Quantum-Classical Modeling

Northwestern University, Evanston IL

Investigators

Abstract

With support from the Chemical Theory, Models and Computational Methods (CTMC) program in the Division of Chemistry, Roel Tempelaar of Northwestern University will study strong interactions between electrons and nuclear vibrations (phonons) in semiconducting materials for use in optoelectronic devices and information technologies. Various transformative materials feature electron-phonon coupling strengths exceeding those seen for commonly-used semiconductors, enabling radically new functionalities, but also prompting the need for new predictive theoretical methodologies that combine accuracy with scalability. Tempelaar and his team aim to address this need by developing methods where electrons are described quantum-mechanically while nuclei are described classically. By adopting a momentum representation of the involved coordinates, materials of exceedingly large sizes can be simulated. This opens ways to quantitatively understand and control material behaviors, with implications for the efficient harvesting of solar energy and the use of quantum states for energy-efficient and secure information technologies. This research is integrated with an educational component addressing code illiteracy among economically-disadvantaged nearby highschoolers. This component aims to foster coding exposure beyond a classroom environment by constructing an engaging sequence of coding exercises based on abundant smartphone technology. In various emerging classes of crystalline semiconductors, including metal-halide perovskites and transition-metal dichalcogenides, electron-phonon couplings are stronger than typically found for inorganic materials. While such couplings strengths give rise to unique photophysical properties, they pose the need for non-perturbative modeling in order to unravel their mechanistic principles. Tempelaar and his team seek to address this need by reformulating quantum-classical dynamical methods in terms of Bloch states commonly used to describe crystal excitations. Within the Bloch representation, basis truncations can be performed that will significantly reduce the cost of simulations. This should enable the team to address the evolution of photo-excitations, the photophysics emerging from chiral phonon modes, and the dynamics of coupled spin-momentum states in hexagonal lattices under realistic conditions. Through this research program, graduate students and postdoctoral researchers in the Tempelaar team will be exposed to a combination of techniques from theoretical chemistry, physics, and materials science, fostering their development as cross-disciplinary scientists. 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.