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Research

We use first principles computational methods to investigate novel quantum many-body phenomena in materials and molecules. Our goal is to not only understand current experiments but also guide future experiments through quantitative prediction of properties and engineering strategies. To achieve this goal we develop theory, new computational methods and algorithms, build efficient high-performance computing software and work closely with cutting-edge experimental labs. 

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Structural reconstructions in moiré superlattices
 

Abstract Background

Studying structural reconstructions in massive moiré superlattices using density functional theory (DFT) is computationally prohibitive. We fit traditional and machine learning-based interatomic forcefields to van der Waals corrected density functional theory calculations. These forcefields have independently predicted and validated several experimental measurements. For example, in WSe2/WS2 moiré superlattices, we uncovered a surprising 3D buckling of the layers due to an inhomogeneous strain redistribution in the individual layers

(Nature Materials, 2021).

Connecting Dots

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When light shines on a material it creates exciton quasiparticles. Excitons are correlated two-particle excited states bound by Coulomb interaction between a hole in the valence band and an electron in the conduction band. Using first principles GW-Bethe Salpeter equation calculations we investigate how large-area moiré superlattices influence the creation of excitons. We recently discovered a new type of exciton in 2D moiré systems: an intralayer charge-transfer exciton.

(Nature, 2022; Science 2022)

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Novel excitons in moiré patterns

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Engineering exciton localization and character
 

Abstract Lines

We uncovered a complex interplay involving structural reconstruction, the formation of flat bands, and the ordering of excitonic states with distinct characteristics in the moiré superlattice. By applying these principles, we designed moiré superlattices in twisted bilayer WS2 with desired electronic and excitonic properties. Experimental measurements were carried out based on the proposed strategy and the predictions were validated. These studies, which involve thousands of atoms in the reconstructed moiré unit-cell, are made feasible for the first time by the PUMP approach.

(Science, 2022; Nature Materials, 2024; Nature, 2024). 

Wavy Abstract Background

The state-of-the-art method to study excited state phenomena in materials from first principles is through GW-Bethe Salpeter equation (GW-BSE) calculations. These calculations are computationally expensive compared to density functional theory calculations and often restricted to simulation cells with ten to hundred atoms. We recently pioneered a computational method, called pristine unit-cell matrix projection (PUMP), which enabled calculations on a record-breaking 4000 atoms. The application of this method to large-area moiré superlattices of 2D materials resulted in a remarkable six-orders-of-magnitude acceleration, culminating in the discovery of novel excitonic states with surprising spatial characters.

(Nature, 2022; Nature Materials, 2024)

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Pristine Unit-cell Matrix Projection (PUMP) method

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