76th CEMS Colloquium

Title

Physics of twisted 2D materials

Abstract

When 2D materials having different periodicities are overlaid with each other, the interference pattern of the lattice mismatch often leads to unusual electronic properties. For instance, the twisted bilayer graphene (rotationally stacked graphene bilayer) exhibits dramatic phenomena such as the flat band formation and emergent superconductivity. In this talk, we introduce our recent theoretical works on incommensurate twisted bilayer systems and their interesting physical properties. We will discuss the twisted bilayer graphene, where we introduce a theoretical framework to reduce its complex electronic structure into an effective nanoscale tight-binding model [1]. We then discuss the lattice relaxation and the phonon vibrations in the twisted bilayer graphene. [2] We show that the original linear dispersion of graphene’s acoustic phonon is broken down into mini phonon bands separated by gaps, where the low-energy phonon modes are regarded as collective vibrations of the nanoscale moiré supercrystal (Fig. 1). Lastly we argue about the 30-degree twisted bilayer graphene, where the 12-fold quasicrystalline nature with nearly flat bands can be described in terms of the quasi-band picture.[3]

Fig. 1 Some low energy moiré vibration modes in twisted bilayer graphene with 1.05 deg. [2]

[1] M. Koshino, N. F. Q. Yuan, T. Koretsune, M. Ochi, K. Kuroki, L. Fu, Phys. Rev. X 8, 031087 (2018).
[2] M. Koshino and Y.-W. Son, Phys. Rev. B 100, 075416 (2019).
[3] P. Moon, M. Koshino and Y.-W. Son, Phys. Rev. B 99, 165430 (2019).