Atomically thin van der Waals materials stacked with an interlayer twist have proven to be an excellent platform towards achieving gate-tunable correlated phenomena linked to the formation offlat electronic bands. In this work we demonstrate the formation of emergent correlated phases inmultilayer rhombohedral graphene – a simple material that also exhibits a flat electronic band butwithout the need of having a moiré superlattice induced by twisted van der Waals layers. We showthat two layers of bilayer graphene that are twisted by an arbitrary tiny angle host large (micron-scale) regions of uniform rhombohedral four-layer (ABCA) graphene that can be independentlystudied. Scanning tunneling spectroscopy reveals that ABCA graphene hosts an unprecedentedly sharp flat band of 3-5 meV half-width. We demonstrate that when this flat band straddles the Fermi level, a correlated many-body gap emerges with peak-to-peak value of 9.5 meV at charge neutrality. We show that ABCA graphene hosts surface topological helical edge states at natural interfaces with ABAB graphene which can be turned on and off with gate voltage, implying that small angle twisted double bilayer graphene is an ideal programmable topological quantum material.

Carmen Rubio-Verdú

Columbia University