This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Graphene, a material consisting of a sheet of carbon atoms arranged in a honeycomb pattern, has proved promising for various technological applications. Materials in which several layers of graphene are stacked together can exhibit different advantageous properties.
Researchers at Cornell University in the U.S., the National Institute for Materials Science in Japan and the Max Planck Institute for the Structure and Dynamics of Matter in Germany investigated the properties of rhombohedral hexalayer graphene, which includes six graphene layers shifted sideways in a repeating pattern. Their paper, published in Nature Physics, reports various quantum states in this material under different conditions. "Our discovery was somewhat unexpected," Xiaomeng Liu, senior author of the paper, told Phys.org.
"We originally set out to study twisted rhombohedral graphene, hoping to create isolated topological bands. However, in one device made by stacking two three-layer graphene sheets, the layers relaxed into alignment, forming rhombohedral six-layer graphene." The researchers' initial measurements performed on rhombohedral six-layer graphene hinted at behavior that was not reported in earlier studies focusing on five-layer graphene. This inspired them to explore the material's phase diagram, a graph describing the states that a material undergoes in different conditions.
"We were particularly intrigued by a strongly correlated semimetallic regime, where electrons and holes coexist and interact," said Liu. Liu and his colleagues first prepared material with six layers of graphene stacked following a rhombohedral pattern. This was done by separating thin flakes from graphite and then looking for this specific stacking configuration among isolated flakes.
The six-layer graphene stack was placed between insulating layers of hexagonal boron nitride, with graphite-based gates both above and below it. The researchers then collected various measurements. First, they measured how easily electricity flowed through their six-layer graphene-based device while changing its temperature, the magnetic field applied to it and the number of charge carriers within it.
They also measured the flow of electricity while they directed an electric field of varying intensity across the stack, from one side of the graphene layers to the other. "A vanishing electrical resistance revealed superconductivity, while an anomalous Hall response provided evidence of orbital magnetism arising from electrons' orbital motion," explained Liu. "We also measured quantum oscillations, variations in resistance with magnetic field that allow us to reconstruct the Fermi surface and identify the types of charge carriers present." The team ultimately identified various electronic states in their hexalayer graphene device, including interaction-driven insulating states, regions of the phase diagram with superconducting behavior and two different types of orbital magnetism.
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