Probing band topology in ABAB and ABBA stacked twisted double bilayer graphene

TL;DR

This study investigates the stacking-dependent topological properties of twisted double bilayer graphene by analyzing Landau level spectra, revealing non-trivial topology in ABBA stacking and trivial topology in ABAB stacking, with implications for future topological state research.

Contribution

It provides the first experimental demonstration of stacking-dependent topology in TDBG through Landau level spectroscopy, distinguishing between ABBA and ABAB configurations.

Findings

ABBA-TDBG exhibits non-trivial topology at zero electric displacement.

ABAB-TDBG remains topologically trivial under similar conditions.

Topological transition occurs at finite displacement fields, evidenced by Landau fan evolution.

Abstract

Twisted graphene moire superlattice has been demonstrated as an exotic platform for investigating correlated states and nontrivial topology. Among the moire family, twisted double bilayer graphene (TDBG) is a tunable flat band system expected to show stacking-dependent topological properties. However, electron correlations and the band topology are usually intertwined in the flat band limit, rendering the unique topological property due to stacking still elusive. Focusing on a large-angle TDBG with weak electron correlations, here we probe the Landau level (LL) spectra in two differently stacked TDBG, i.e. ABBA- and ABAB-TDBG, to unveil their distinct topological properties. For ABBA-TDBG, we observe non-trivial topology at zero electric displacement filed, evident from both the emergence of Chern bands from half fillings and the closure of gap at CNP above a critical magnetic field. For ABAB-TDBG, by contrast, we find that the moire band is topologically trivial, supported by the absence of LLs from half fillings and the persistence of the gap at CNP above the critical magnetic fields. In addition, we also observe an evolution of the trivial-to-nontrivial topological transition at finite D fields, confirmed by the emerged Landau fans originating from quarter filling v = 1. Our result demonstrates, for the first time, the unique stacking-dependent topology in TDBG, offering a promising avenue for future investigations on topological states in correlated systems.