Electronic structures of quantum materials - Shining new lights on the fractional quantum anomalous Hall effect

When two ultrathin materials, such as graphene (a single layer of carbon atoms arranged in honeycomb lattice), are stacked with a small twist angle, they form a large-scale “moiré” pattern (figure 1a) that can profoundly reshape their electronic behaviour. 

 

In recent years, such moiré superlattices have become a platform for discovering novel quantum phenomena. A prominent example is the fractional quantum anomalous Hall effect (FQAHE) observed in rhombohedral graphene moiré superlattice (Figure 1b).

However, a key mystery remains: although the moiré pattern exists only at the interface of the stacked materials, the FQAHE often emerges in graphene layers far from that interface. How can a nanoscale modulation confine to one-layer influence electrons several layers away?

To address this question, Shuyun Zhou’s team at Tsinghua University, together with collaborators from Shanghai Jiao Tong University and Peking University, investigated a five-layer rhombohedral graphene/boron nitride heterostructure—a system where FQAHE has been observed. Their goal was to directly visualize how the moiré potential reshapes the electronic structure throughout all five layers. Collaborating with Dr. Pavel Dudin and Dr. Jose Avila at the ANTARES beamline, the researchers employed NanoARPES, a technique that maps electronic energy-momentum relations with sub-micron spatial resolution. This level of precision was crucial because the samples are only a few micrometres in size, well below the capabilities of conventional ARPES (Figure 2).
 

The measurements revealed two key findings. First, the researchers directly imaged topological flat bands, by which electrons move with extremely low velocity and strong interactions, providing long-sought experimental evidence in this system. 

Second, they observed clear moiré bands not only at the moiré interface but also in graphene layers far from it, along with a strongly enhanced topological flat band due to the moiré potential (Figure 3). 
 

These results demonstrate that the moiré potential has a surprisingly long-range impact, modifying electronic states well beyond the interface where the moiré pattern formed. To understand this behaviour, theoretical simulations show that the strong interlayer coupling enables moiré-induced modulations of charge and electronic states to propagate through the entire stack. 
This mechanism offers a possible explanation for why the FQAHE can emerge away from the interface, providing critical information to resolve an important open question in the field.