Direct view of gate-tunable miniband dispersion in graphene superlattices near the magic twist angle

TL;DR

This study uses angle-resolved photoemission spectroscopy to directly observe how the low-energy electronic bands in twisted graphene superlattices change with electron filling, revealing contrasting behaviors in different systems and highlighting their electric field tunability.

Contribution

It provides the first direct measurement of filling-dependent low-energy band structures in twisted bilayer and double bilayer graphene, showing distinct behaviors and tunability.

Findings

TBG exhibits a simple doping-dependent band shift and bandwidth change.

TDBG shows complex, non-monotonous bandwidth variations and tunable gap openings.

The work demonstrates electric field control over low-energy electronic states.

Abstract

Superlattices from twisted graphene mono- and bi-layer systems give rise to on-demand many-body states such as Mott insulators and unconventional superconductors. These phenomena are ascribed to a combination of flat bands and strong Coulomb interactions. However, a comprehensive understanding is lacking because the low-energy band structure strongly changes when the electron filling is varied. Here, we gain direct access to the filling-dependent low energy bands of twisted bilayer graphene (TBG) and twisted double bilayer graphene (TDBG) by applying micro-focused angle-resolved photoemission spectroscopy to in situ gated devices. Our findings for the two systems are in stark contrast: The doping dependent dispersion for TBG can be described in a simple model, combining a filling-dependent rigid band shift with a many-body related bandwidth change. In TDBG, on the other hand, we find a complex behaviour of the low-energy bands, combining non-monotonous bandwidth changes and tuneable gap openings. Our work establishes the extent of electric field tunability of the low energy electronic states in twisted graphene superlattices and can serve to underpin the theoretical understanding of the resulting phenomena.