Electronic Cascades and Chern Insulators in Twisted Bilayer Graphene

Magic-angle twisted bilayer graphene (MATBG) is a remarkable platform for studying strongly correlated electrons due to its incredibly vibrant phase diagram. To understand this phase diagram, we employ gate-tunable scanning tunneling microscopy (STM) and spectroscopy (STS) to probe the energy- and density-dependent electronic structure of MATBG. We find, at a temperature above the transition temperatures for superconductivity and correlated insulators, a cascade of transitions in which the electronic excitation spectrum and chemical potential reset at integer fillings of MATBG’s flat bands [1]. We explain this cascade of transitions in terms of an interplay between strong electronic correlations and the degeneracy of the flat bands, and we extract a measure of the on-site Coulomb repulsion strength between electrons. Upon cooling MATBG to millikelvin temperatures and applying a moderate out-of-plane magnetic field, we observe the formation of insulating gaps that occur at carrier densities that shift with magnetic-field strength [2]. We identify Chern numbers associated with these insulating gaps, showing that strong electron-electron interactions can lead to novel topological states in MATBG.

[1] D. Wong et al. Nature 582, 198-202 (2020)
[2] K. P. Nuckolls et al. arXiv:2007.0381