Direct visualization of gate-tunable flat bands in twisted double bilayer graphene

TL;DR

This study uses angle-resolved photoemission spectroscopy to directly visualize and analyze how flat bands in twisted double bilayer graphene evolve with external parameters, revealing conditions conducive to correlated electronic states.

Contribution

It provides the first direct momentum-resolved visualization of flat bands in TDBG and elucidates how they are influenced by displacement field and carrier density.

Findings

Multiple flat moiré minibands observed near charge neutrality

A flat remote valence band identified below the flat-band manifold

Coulomb energy exceeds flat-band bandwidth, favoring correlated phenomena

Abstract

The symmetry-broken correlated states in twisted double bilayer graphene (TDBG) can be tuned via several external knobs, including twist angle, displacement field, and carrier density. However, a direct, momentum-resolved characterization of how these parameters reshape the flat-band structure remains limited. In this study, we employ micro focused angle-resolved photoemission spectroscopy to investigate the flat-band dispersion of TDBG at a twist angle of 1.6, systematically varying the displacement field and carrier density via electrostatic gating. We directly observe multiple flat moir'e minibands near charge neutrality, including a flat remote valence band residing below the low-energy flat-band manifold. Furthermore, the dominant Coulomb repulsive energy over the flat- band bandwidth suggests favorable conditions for the emergence of interaction-driven correlated phenomena in TDBG. These findings establish that the formation and evolution of flat bands in TDBG arises from the interplay between the electron filling and the displacement field.