Symmetry-Broken Chern Insulators in Twisted Double Bilayer Graphene

Twisted double bilayer graphene (tDBG) has emerged as a rich platform for studying strongly correlated and topological states, as its flat bands can be continuously tuned by both a perpendicular displacement field and a twist angle. Here, we construct a phase diagram representing the correlated and topological states as a function of these parameters, based on measurements of over a dozen tDBG devices encompassing two distinct stacking configurations. We find a hierarchy of symmetry-broken states that emerge sequentially as the twist angle approaches an apparent optimal value of theta approximate to 1.34(degrees). Nearby this angle, we discover a symmetry-broken Chern insulator (SBCI) state associated with a band filling of 7/2 as well as an incipient SBCI state associated with 11/3 filling. We further observe an anomalous Hall effect at zero field in all samples supporting SBCI states, indicating spontaneous time-reversal symmetry breaking and possible moire unit cell enlargement at zero magnetic field.

Automated summary

Twisted double bilayer graphene has been studied as a platform for studying strongly correlated and topological states. In this research, a phase diagram representing these states as a function of parameters was constructed based on measurements of several devices. Symmetry-broken states were observed at a specific twist angle, including a Chern insulator state with band filling of 7/2 and an incipient state with filling of 11/3. An anomalous Hall effect was also observed at zero magnetic field in samples supporting the symmetry-broken states.