Atomically thin van der Waals materials stacked with an interlayer twist have proven to be an excellent platform toward achieving gate-tunable correlated phenomena linked to the formation of flat electronic bands. In this work we demonstrate the formation of emergent correlated phases in multilayer rhombohedral graphene––a simple material that also exhibits a flat electronic band edge but without the need of having a moiré superlattice induced by twisted van der Waals layers. We show that two layers of bilayer graphene that are twisted by an arbitrary tiny angle host large (micrometer-scale) regions of uniform rhombohedral four-layer (ABCA) graphene that can be independently studied. Scanning tunneling spectroscopy reveals that ABCA graphene hosts an unprecedentedly sharp van Hove singularity of 3–5-meV half-width. We demonstrate that when this van Hove singularity straddles the Fermi level, a correlated many-body gap emerges with peak-to-peak value of 9.5 meV at charge neutrality. Mean-field theoretical calculations for model with short-ranged interactions indicate that two primary candidates for the appearance of this broken symmetry state are a charge-transfer excitonic insulator and a ferrimagnet. Finally, 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.

原子级薄范德华材料与层间扭曲堆叠已被证明是实现与平坦电子能带形成相关的栅极可调相关现象的绝佳平台。在这项工作中，我们展示了多层菱面体石墨烯中出现的相关相的形成——一种简单的材料，它也表现出平坦的电子能带边缘，但不需要由扭曲的范德华层引起的莫尔超晶格。我们表明，以任意微小角度扭曲的两层双层石墨烯承载了可以独立研究的均匀菱面体四层 (ABCA) 石墨烯的大（微米级）区域。扫描隧道光谱显示ABCA石墨烯具有前所未有的尖锐范霍夫奇点，半宽为3-5meV。我们证明，当这个范霍夫奇点跨越费米能级时，会出现一个相关的多体间隙，在电荷中性时峰峰值为 9.5 meV。具有短程相互作用的模型的平均场理论计算表明，出现这种破坏对称状态的两个主要候选者是电荷转移激子绝缘体和铁磁体。最后，我们表明 ABCA 石墨烯在与 ABAB 石墨烯的自然界面处具有表面拓扑螺旋边缘态，可以通过栅极电压打开和关闭，这意味着小角度扭曲双层石墨烯是一种理想的可编程拓扑量子材料。具有短程相互作用的模型的平均场理论计算表明，出现这种破坏对称状态的两个主要候选者是电荷转移激子绝缘体和铁磁体。最后，我们表明 ABCA 石墨烯在与 ABAB 石墨烯的自然界面处具有表面拓扑螺旋边缘态，可以通过栅极电压打开和关闭，这意味着小角度扭曲双层石墨烯是一种理想的可编程拓扑量子材料。具有短程相互作用的模型的平均场理论计算表明，出现这种破坏对称状态的两个主要候选者是电荷转移激子绝缘体和铁磁体。最后，我们表明 ABCA 石墨烯在与 ABAB 石墨烯的自然界面处具有表面拓扑螺旋边缘态，可以通过栅极电压打开和关闭，这意味着小角度扭曲双层石墨烯是一种理想的可编程拓扑量子材料。