Second-order superlattices form when moiré superlattices with similar periodicities interfere with each other, leading to larger superlattice periodicities. These crystalline structures are engineered using two-dimensional materials such as graphene and hexagonal boron nitride, and the specific alignment plays a crucial role in facilitating correlation-driven topological phases. Signatures of second-order superlattices have been identified in magnetotransport experiments; however, real-space visualization is still lacking. Here we reveal the second-order superlattice in magic-angle twisted bilayer graphene closely aligned with hexagonal boron nitride through electronic transport measurements and cryogenic nanoscale photovoltage measurements and evidenced by long-range periodic photovoltage modulations. Our results show that even minuscule strain and twist-angle variations as small as 0.01° can lead to drastic changes in the second-order superlattice structure. Our real-space observations, therefore, serve as a ‘magnifying glass’ for strain and twist angle and can elucidate the mechanisms responsible for the breaking of spatial symmetries in twisted bilayer graphene.

当具有相似周期性的莫尔超晶格相互干扰时，就会形成二阶超晶格，从而导致更大的超晶格周期性。这些晶体结构是使用石墨烯和六方氮化硼等二维材料设计的，并且特定的排列在促进相关驱动的拓扑相方面起着至关重要的作用。二阶超晶格的特征已在磁输运实验中得到证实；然而，真实空间可视化仍然缺乏。在这里，我们通过电子输运测量和低温纳米级光电压测量揭示了与六方氮化硼紧密排列的魔角扭曲双层石墨烯中的二阶超晶格，并通过长程周期性光电压调制来证明。我们的结果表明，即使是小至 0.01° 的微小应变和扭转角变化也会导致二阶超晶格结构发生巨大变化。因此，我们的真实空间观察可以作为应变和扭曲角的“放大镜”，可以阐明扭曲双层石墨烯空间对称性被打破的机制。