Cascade of electronic transitions in magic-angle twisted bilayer graphene

TL;DR

This study uses high-resolution STM to reveal a cascade of electronic transitions in magic-angle twisted bilayer graphene, driven by Coulomb interactions that split flat bands and are sensitive to magnetic fields, shedding light on its correlated phases.

Contribution

It provides the first direct spectroscopic evidence of Coulomb interaction-induced band splitting and cascade transitions in MATBG, elucidating the microscopic mechanisms of its correlated states.

Findings

Observation of a cascade of spectroscopic transitions at integer fillings.

Identification of Coulomb interactions splitting flat bands into Hubbard sub-bands.

Sensitivity of electronic transitions to perpendicular magnetic fields.

Abstract

Magic-angle twisted bilayer graphene (MATBG) exhibits a rich variety of electronic states, including correlated insulators, superconductors, and topological phases. Understanding the microscopic mechanisms responsible for these phases requires determining the interplay between electron-electron interactions and quantum degeneracy due to spin and valley degrees of freedom. Signatures of strong electron-electron correlations have been observed at partial fillings of the flat electronic bands in recent spectroscopic measurements. Transport experiments have shown changes in the Landau level degeneracy at fillings corresponding to an integer number of electrons per moir\'e unit cell. However, the interplay between interaction effects and the degeneracy of the system is currently unclear. Using high-resolution scanning tunneling microscopy (STM), we observed a cascade of transitions in the spectroscopic properties of MATBG as a function of electron filling. We find distinct changes in the chemical potential and a rearrangement of the low-energy excitations at each integer filling of the moir\'e flat bands. These spectroscopic features are a direct consequence of Coulomb interactions, which split the degenerate flat bands into Hubbard sub-bands. We find these interactions, the strength of which we can extract experimentally, to be surprisingly sensitive to the presence of a perpendicular magnetic field, which strongly modifies the spectroscopic transitions. The cascade of transitions we report here characterizes the correlated high-temperature parent phase from which various insulating and superconducting ground-state phases emerge at low temperatures in MATBG.