Date : Friday, June 23rd, 2023 4:00 pm - 5:00 pm Place : On Zoom and Lecture Room A632 , ISSP（Hybrid） Lecturer : Prof. Mikito Koshino Affiliation : Department of Physics, Osaka University Committee Chair : Takashi Oka (ext.63280)
e-mail: oka@issp.u-tokyo.ac.jpLanguage in Speech : English

Twisted 2D materials, consisting of rotationally stacked layers, can generally beregarded as quasi-periodic systems (quasicrystals)in that the systems have multipleperiodicitiesincommensurate with each other.Here we present a novel approach to characterizing energy gaps in 2D quasi-periodicsystems using a set of second Chern numbers.Theses numbers correspond to quantized Hall conductivity in a 4D system,which isconnected tothe original 2D quasicrystal through a process of dimensional reduction.We apply the theory to hBN/graphene/hBN superlattice, where we find thattheelectronic spectrum exhibits fractal minigaps,and each of them is uniquely labeled bysix second Chern numbers.We also consider the bulk-edge correspondence in this system,and discover that thenumber of edge-states bands per unit length is determined by the second Chern numbersassociated with the gap.

[1] M. Koshino, H. Oka, Phys. Rev. Research 4, 013028(2022)

[2] H. Oka and M. Koshino, Phys. Rev. B 104, 035306 (2021)