Researchers at the Swiss École Polytechnique Fédérale de Lausanne (EPFL) and National Institute for Materials Science in Japan have developed a new technique to directly measure energy gaps and bandwidths in multilayer graphene systems, paving the way for deeper insights into exotic quantum states and future electronic devices.
When layers of graphene are stacked on top of each other and slightly rotated, the atomic lattices create a periodic interference pattern known as a moiré pattern. This pattern significantly changes the electronic behavior of the material, sometimes leading to exotic quantum phenomena like superconductivity and magnetism. However, directly probing the fine details of these quantum states has been a challenge. Understanding how electrons behave in these stacked graphene systems is crucial for designing future electronic and quantum devices. But conventional techniques struggle to precisely measure energy gaps and bandwidth—the parameters that dictate how electrons move and interact in these systems. Without a reliable method to extract this data, researchers have been piecing together the puzzle through indirect observations.
The team of scientists, led by Mitali Banerjee at EPFL, has now developed a direct and accurate way to measure these properties. Their method uses a specialized dual-gated graphene device that allows them to extract precise energy values by monitoring how electrons respond to an applied electric field. The technique has proven particularly valuable for investigating flat-band graphene systems, where electron interactions dominate, leading to fascinating quantum behaviors.
The new method exploits a phenomenon known as Landau level spectroscopy, which can map out electronic structures in great detail. In this study, the scientists used a monolayer of graphene placed on top of a bilayer graphene sheet, carefully twisting the layers to create a moiré superlattice. By applying a dual-gate voltage, they could finely tune the electronic states and measure the energy gaps with unprecedented accuracy.
The study revealed that the bandgap in bilayer graphene changes dynamically with the applied electric field, reaching its maximum at the same field strength where the bandwidth is minimized. This behavior directly corresponds to the emergence of strongly correlated electronic phases—situations where electrons start to behave collectively rather than independently. Furthermore, the team successfully measured both integer and fractional quantum Hall states, providing critical insights into graphene’s complex quantum landscape.
Understanding and controlling the electronic properties of graphene could impact future technology, from ultra-efficient transistors to quantum computers. The ability to precisely manipulate energy gaps and bandwidths could lead to new generations of electronic and optoelectronic devices. Finally, the technique provides a new standard for investigating similar quantum materials beyond graphene, which can lead to future discoveries.
