In the first 12 months, we primarily focused on RO1. Using accurate classical force-field based simulations, we have discovered valley phonons in moiré materials could be chiral. Additionally, we discovered two sets of emergent chiral valley phonon modes that originate from an inversion symmetry breaking at the moiré scale. Moreover, we also discovered the formation of flat phonon bands in these materials. We demonstrated the existence of chiral and flat phonon modes in twisted bilayer WSe2, MoSe2/WSe2 heterobilayer, and a strain engineered moiré pattern created from WSe2 bilayer. These results were published as a Letter in Phys. Rev. B. After the completion of this project, we focused on understanding the temperature-dependent electronic properties of moiré materials as a direct signature of electron-phonon coupling. We have combined extensive large-scale ab-initio electronic structure calculations and molecular dynamics simulations to demonstrate that the moiré potential in a MoSe2/WSe2 heterobilayer is dynamic at finite temperature. The origin of the dynamic moiré potential can be ascribed to two factors: a moiré magnification, where the atomic displacements are magnified at the moiré scale, and the presence of special low-energy phonon modes called phasons which correspond to the relative sliding of the two layers. We show that electrons and holes follow the movements of the dynamic moiré potential and surf the phason waves. These results are under review at ACS Nano Letters.
In the last 12 months, we primarily focused on addressing RO2. A major challenge associated with RO2 was both the development of new algorithms to solve the Bethe-Salpeter-Equation (BSE) and the implementation of those algorithms into a computer package. In particular, we have used a Wannierization procedure to generate an ab-initio tight-binding model of the moiré materials and then exploit the localization of the Wannier functions to efficiently evaluate the electron-hole interaction matrix elements required to solve the BSE. The implementation of the new algorithms for solving the BSE resulted in the new PyMEX package (A Python package for Moiré EXcitons) with both MPI and OpenMP support. Using this package, we were able to compute the interlayer and intralayer excitons in WS2/WSe2 heterobilayers. Our calculations were in excellent agreement with previous experiments. We are finalizing our manuscript for publication. Our work on excitons and phonons has resulted in nice international collaborations with experimentalists at Lawrence Berkeley National Lab with Prof. Archana Raja and Prof. Alex Weber-Bargioni’s group and at Cornell University with Prof. Jared Maxson’s group. In these works, we studied how phonons couple with electrons and excitons in WS2/WSe2 (under review at Nature Materials) and MoSe2/WSe2 heterobilayers (to be submitted to Nature Materials).
Most of these works are available on the arXiv for free. We have presented our works in multiple national and international conferences, such as American Physical Society March meeting 2022 and 2023 (United States), Psi-k 2022 (Switzerland), ETSF-Young-Researchers-Meet-2021 (Italy), Moiré twistronics workshop 2021 (United Kingdom), Materials Chemistry Consortium meeting 2022 (United Kingdom), and multiple in-house seminars at Imperial College London. To ensure a broader public engagement, a summary was also posted on social media platforms, such as Facebook and Twitter at the time of the publication. We plan to publish summaries of the work done during the fellowship through Imperial College’s news and views website as well (upon the acceptance of the rest of the publications).
