Observation of flat band in twisted double bilayer graphene using nanoARPES*

The strong correlation physics leads to observing superconductivity and insulating states in twisted bilayer graphene at the magic angle ^[1,2]. Similarly, twisted double bilayer graphene (tDBG) has been predicted to host remarkably flat bands at narrow regions around the magic angle (θ ~ 1.3 ˚) ^[3]. A direct method to understand this correlated electron physics is the evolution of band topology by tuning the density of states. We use in-operando angle-resolved photoemission spectroscopy with nanometer scale (nanoARPES) spatial resolution to study the tDBG device with the twist angle ~2.6 ± 0.2˚. A remarkable change in the band topology can be observed with the tuning of the charge carrier density using back gate voltages. Moiré miniband with flat dispersions over the whole mini-Brillouin zone is observed at higher gate voltage (Vg ≥ 6V). This flat band is separated from the dispersive Dirac bands, leading to multiple moiré hybridization. The application of external electric fields provides a control mechanism over the bandwidth and flat-band structure and unveils the electron−electron interaction phenomena at different filling factors with in situ electrostatic gating.

[1] Cao, Y. et al. Nature 556, 80–84 (2018).

[2] Cao, Y. et al. Nature 556, 43–50 (2018).

[3] Shen, C., Chu, Y., Wu, Q. et al. Nat. Phys. 16, 520–525 (2020).