Twisted bilayer materials, and in particular, twisted bilayer graphene, are created by slightly rotating the two individual crystal networks in a 2D material with respect to each other. [1-4] For small twist angles, the material undergoes a self-organized lattice reconstruction, leading to the formation of a periodically repeated domain. The resulting superlattice modulates the vibrational [1,3] and electronic structures [4] within the material, leading to changes in the behavior of electron–phonon coupling and to the observation of strong correlations and superconductivity.
In this talk, I will report on the phonon spectra of twisted bilayer graphene (tBLG) and twisted bilayer MoS2 that were computational analyzed for a series of hundreds of twisting angle values in the [0, p/6] range. The evolution of the phonon bandstructure as a function of twist angle is examined using a band unfolding scheme where the large number of phonon modes computed at the Γ point for the large moiré supercells are unfolded onto the Brillouin Zone (BZ) of one of the two constituent layers. In addition to changes to the low-frequency breathing and shear modes, a series of well-defined side-bands around high-symmetry points of the extended BZ emerge due to the twist angle-dependent structural relaxation. I will also review how these results have been confirmed experimentally in collaboration with the group of Ado Jorio: Observations of the crystallographic structure with visible light are made possible by the nano-Raman technique, which reveals the localization of lattice dynamics, with the presence of strain solitons and topological points causing detectable spectral variations.
REFERENCES
[1] Sheremetyeva, N., Lamparski, M., Daniels, C., Van Troeye, B. & Meunier, V., "Machine-learning models for Raman spectra analysis of twisted bilayer graphene," Carbon N. Y. (2020). DOI: 10.1016/j.carbon.2020.06.077
[2] Gadelha, A. C., Ohlberg, D. A. A., Rabelo, C., Neto, E. G. S., Vasconcelos, T. L., Campos, J. L., Lemos, J. S., Ornelas, V., Miranda, D., Nadas, R., Santana, F. C., Watanabe, K., Taniguchi, T., van Troeye, B., Lamparski, M., Meunier, V., Nguyen, V.-H., Paszko, D., Charlier, J.-C., Campos, L. C., Cançado, L. G., Medeiros-Ribeiro, G. & Jorio, A., "Localization of lattice dynamics in low-angle twisted bilayer graphene," Nature 590, 405–409 (2021). DOI: 10.1038/s41586-021-03252-5
[3] Lamparski, M., Van Troeye, B. & Meunier, V., "Soliton signature in the phonon spectrum of twisted bilayer graphene," 2D Mater. 7 025050 (2020).
[4] Nguyen, V. H., Paszko, D., Lamparski, M., Troeye, B. Van, Meunier, V. & Charlier, J.-C., "Electronic localization in small-angle twisted bilayer graphene," 2D Mater. 8 03504 (2021).

Figure 1: Crystal field due to reconstruction in moiré pattern (top) and unfolded phonon bandstructure with and without full atomic reconstruction. Adapted and reproduced from Ref. [3].
Lien Zoom:
https://zoom.us/j/98252972501?pwd=ZUlYbUxZU0FYR2hsbTM4YjBxbGhDdz09
code de sécurité : FNq9tU