“Flatbands in twisted Dirac materials”

Dr. Jeil Jung
Department of Physics, University of Seoul

May. 31 (Fri.), 03:45 PM
E6-2. 1st fl. #1323

Abstract:
Twisted bilayer graphene is gaining a renewed interest after recent experimental observation of superconductivity associated with the narrow bandwidth ‘flatbands’ attainable at appropriate twist angles and pressure tunable interlayer coupling. In this presentation I will introduce a method to obtain the continuum moire bands Hamiltonian of twisted layered systems using ab initio input from short period commensurate structures that goes beyond the two center approximation often used in the literature. We then analyze the electronic structure of twisted gapless and gapped Dirac materials for various combinations of graphene and hexagonal boron nitride layers in search of the optimum conditions for finding isolated flatbands where Coulomb interactions play a dominant role. We conclude that flatbands can be expected in a wide range of 2D Dirac materials parameter space, which is facilitated by the presence of intrinsic band gaps in the constituent materials. The avoided crossing of the bands in k-space give rise to well defined valley Chern numbers that can impact the character of the ground-state Hall conductivity depending on the specific configuration of the ground states.

[1] Ab initio theory of moire superlattice bands in layered two dimensional materials, J. Jung, A. Raoux, Z. Qiao, A. H. MacDonald, Phys. Rev. B 89, 205414 (2014).
[2] Gate-Tunable Topological Flat Bands in Trilayer Graphene Boron-Nitride Moiré Superlattices, B. L. Chittari, G. Chen, Y. Zhang, F. Wang, J. Jung, Physical Review Letters 122 (1), 016401 (2019).
[3] Flatbands in twisted bi-bilayer graphene, N. R. Chebrolu, B. L. Chittari, J. Jung, arXiv preprint arXiv:1901.08420 (2019).